Underground temperatures as indicators of surface temperatures – part 1

This article was originally written and published by me in Finnish in Ilmastotieto-blog and this is just an English version of it.

Temperatures measured from deep undergound can be used to indicate the surface temperatures in different times. Eventhough the measurements are made with thermometers, they can’t be said to be direct temperature measurements at least for the surface temperature, because there the effects of surface temperature are being measured, not directly the surface temperature. When surface temperature changes, the change is seen immediately in the soil that is in direct contact with the surface. The temperature changes in surface soil are being transferred deeper by heat conduction. Therefore the deeper ground shows the same temperature change as the surface, but in later time. The penetration of a temperature change to the deeper ground takes long time; it takes hundreds of years for a temperature change to show in the depth of few hundred meters. Because of this the deep ground has saved a long time record of Earth’s surface temperature. It just has to be measured from down there. The major source for this article is the review article of Pollack & Huang (2000) of which structure and content is being closely followed in this article. A lot was left unsaid so I urge those interested to read their article.

Historical development

Surface temperature showing in the depths of ground was understood already in early 20th century. At first measurements were taken from mines in order to establish the timings relating to ice ages. Hotchkiss & Ingersoll (1934) write in their abstract:

The retreat of the glacial sheet a number of thousand years ago and the subsequent long-period surface temperature variations must almost certainly have left their impress on the geothermal curve. An attempt has been made to find such an effect and to interpret it in terms of past surface temperatures by subjecting to mathematical analysis a series of geothermal measurements recently made in the Calumet and Hecla conglomerate mine. The results indicate that the glacial epoch ended for this region 20,000-30,000 years ago, and that it was followed by a period with ground temperatures distinctly warmer than the present, succeeded in turn by one cooler, and lasting until comparatively recent times.

Since then also boreholes were being used, as in Beck & Judge (1969):

Heat flow data from a 600-m deep diamond drilled borehole has been used to estimate how short a section of borehole will give a valid heat flow value, to test for recent and ancient climatic changes, underground water-flows and the variation of terrestrial heat flow with depth. Temperatures were repeatedly measured at 3-m intervals; measurements of thermal conductivity, density and porosity were made on specimens sampled at approximately 4-m intervals along the length of the hole. The mean heat flow for the whole borehole before applying any corrections is 0.76 h.f.u. while after correcting for the Wisconsin glaciation the mean value is 1.17 h.f.u., but in both cases some 30 to 100-m sections of the borehole differ by ±20 per cent from the mean values. The differences cannot be entirely explained as being due to structure, topography, climatic changes or underground water-flows.

(In the abstract above the “h.f.u.” is “heat flow unit” and 1 h.f.u = 41.8 mW/m².)

Cermak (1971) already made a thorough surface temperature reconstruction from the measurements of two boreholes:

There is considerable evidence from different fields of investigation that the world climate has undergone significant variations, even during the last 1,000 years. The effect of the change of temperature on the earth’s surface in the past may be preserved at depths of several hundred feet below the surface. The relation between underground and surface temperature is the reaction of the internal field in a semi-infinite medium to the boundary conditions. Any change at the surface is propagated downwards, and it is shown that the detailed record of temperature with depth can be used to trace the past climatic history. The theory of climatic correction of heat flow is used, and the data is obtained from two boreholes in northeastern Ontario. After analysis the measured underground temperature clearly confirmed the notably warm climate that lasted a few hundred years around A.D. 1000–1200 and the following cold period after 1500.

Both these recent climatic extremes, for which the terms “Little Climatic Optimum” and “Little Ice Age” were coined, are well substantiated, but the magnitude of the temperature variations is uncertain. The relation between mean annual air temperature and surface (ground) temperature depends very much on the precipitation character and the duration of snow cover. The calculated magnitudes of the surface temperature changes probably correspond to the minimum changes of the annual air temperatures, which might have been more pronounced. The results presented indicate for the Kapuskasing area a surface temperature during the Little Climatic Optimum at least 1.5°C higher than the reference value; the mean temperature during the Little Ice Age was about 1°C below this reference value. A remarkable increase since about 1850 reaches value in excess of 3°C.

Finally Lachenbruch & Marshall (1986) suggested that the recent climate change might already be evident in subsurface temperature measurements:

Temperature profiles measured in permafrost in northernmost Alaska usually have anomalous curvature in the upper 100 meters or so. When analyzed by heat-conduction theory, the profiles indicate a variable but widespread secular warming of the permafrost surface, generally in the range of 2 to 4 Celsius degrees during the last few decades to a century. Although details of the climatic change cannot be resolved with existing data, there is little doubt of its general magnitude and timing; alternative explanations are limited by the fact that heat transfer in cold permafrost is exclusively by conduction. Since models of greenhouse warming predict climatic change will be greatest in the Arctic and might already be in progress, it is prudent to attempt to understand the rapidly changing thermal regime in this region.

Figure 1. The measurement sites for subsurface temperature profiles. The map is from NOAA Paleoclimatology website.

Subsurface temperature measurements

There are many methods to measure the subsurface temperatures. One thermometer can be lowered to the borehole and readings are taken from it at different heights. By taking enough readings, the borehole temperature profile can be determined. Another method is to lower a cable with lots of temperature sensors to the borehole. By this method the borehole temperature profile can also be determined. By using this latter method also more can be achieved; by leaving the cable with its many sensors to the borehole for a long time, the changes in the temperature profile can be monitored through time. Nearer surface the sensors can also be buried to the soil. The mines can also be utilized by drilling the sensors deeper into the rock from the walls of the mines. Currently the accuracy of individual measurements is better than one hundreth of a Celsius-degree.

Temperature measurements have been done for thousands of boreholes. However, the data from them is not very well compatible because different methods have been used, measurements have been done with different intervals, and the conditions in measurements sites are sometimes not known well enough. There is however enough of good quality measurements for making temperature reconstructions in many places all over the world. In addition to borehole measurements there is lot of underground temperature data from other fields of study (soil studies, etc.) where measurements have usually been done closer to the surface. Typically the depth of these studies vary from few centimeters to few tens of meters (whereas the depth of boreholes usually is hundreds of meters). The good aspect of these other studies is that usually there’s also lots of other things measured with temperature (surface temperature, soil moisture, etc.) so the conditions of the site are well known.

From the measurements it has been found out that daily variation of surface temperature can be seen in the depth of 2 meters and annual variation of surface temperature can be seen in the depth of 20 meters. Rapid temperature changes are therefore not conveyed very deep so the temperature reconstructions from boreholes don’t show rapid temperature changes, but they show how the temperature has varied during decades and centuries.

There are lot of disturbing factors affecting measurements. Surface topography, vegetation and hydrological conditions affect also to the subsurface temperature. Below surface the temperature profile is being disturbed by changes in groundwater conditions. Historical variations in any of these factors can make the temperature profile to show fallacious climate change. There has been lot of studies on the disturbing factors, but especially in a single measurement site temperature reconstruction the disturing factors can cause a lot of uncertainty.

Continued in part 2.

References

Beck & Judge (1969), “Analysis of Heat Flow Data—I Detailed Observations in a Single Borehole”, Geophysical Journal of the Royal Astronomical Society, Volume 18 Issue 2, Pages 145 – 158, doi: 10.1111/j.1365-246X.1969.tb03558.x, [abstract]

Cermak (1971), “Analysis of Heat Flow Data—I Detailed Observations in a Single Borehole”, Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 10, Issue 1, July 1971, Pages 1-19, doi:10.1016/0031-0182(71)90043-5, [abstract]

Hotchkiss & Ingersoll (1934), “Postglacial Time Calculations from Recent Geothermal Measurements in the Calumet Copper Mines”, The Journal of Geology, Vol. 42, No. 2 (Feb. – Mar., 1934), pp. 113-122, [abstract]

Lachenbruch & Marshall (1986), “Geothermal Evidence from Permafrost in the Alaskan Arctic”, Science 7 November 1986:
Vol. 234. no. 4777, pp. 689 – 696, DOI: 10.1126/science.234.4777.689, [abstract]

Pollack & Huang (2000), “Climate Reconstruction from Subsurface Temperatures”, Annual Review of Earth and Planetary Sciences, Vol. 28: 339-365, doi:10.1146/annurev.earth.28.1.339, [abstract, full text]

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Papers on temperature reconstructions from boreholes

This is a list of papers on temperature reconstructions from boreholes. Emphasis is on global analysis so there’s not much papers on single locations. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.

A late Quaternary climate reconstruction based on borehole heat flux data, borehole temperature data, and the instrumental record – Huang et al. (2008) “We present a suite of new 20,000 year reconstructions that integrate three types of geothermal information: a global database of terrestrial heat flux measurements, another database of temperature versus depth observations, and the 20th century instrumental record of temperature, all referenced to the 1961–1990 mean of the instrumental record. These reconstructions show the warming from the last glacial maximum, the occurrence of a mid-Holocene warm episode, a Medieval Warm Period (MWP), a Little Ice Age (LIA), and the rapid warming of the 20th century. The reconstructions show the temperatures of the mid-Holocene warm episode some 1–2 K above the reference level, the maximum of the MWP at or slightly below the reference level, the minimum of the LIA about 1 K below the reference level, and end-of-20th century temperatures about 0.5 K above the reference level.” [Full text]

Borehole climate reconstructions: Spatial structure and hemispheric averages – Pollack & Smerdon (2004) “We demonstrate the consistency of GST warming estimates by showing that over a wide range of grid element area and occupancy weighting schemes, the five-century GST change falls in the range of 0.89–1.05 K. We examine the subhemispheric spatial correlation of GST and SAT trends at various spatial scales. In the 5-degree grid employed for optimal detection, we find that the majority of grid element means are determined from three or fewer boreholes, a number that is insufficient to suppress site-specific noise via ensemble averaging. Significant spatial correlation between SAT and GST emerges in a 5-degree grid if low-occupancy grid elements are excluded, and also in a 30-degree grid in which grid element means are better determined through higher occupancy. Reconstructions assembled after excluding low-occupancy grid elements show a five-century GST change in the range of 1.02–1.06 K.” [Full text]

Optimal surface temperature reconstructions using terrestrial borehole data – Mann et al. (2003) “We derive an optimal Northern Hemisphere mean surface temperature reconstruction from terrestrial borehole temperature profiles spanning the past five centuries. The pattern of borehole ground surface temperature (GST) reconstructions displays prominent discrepancies with instrumental surface air temperature (SAT) estimates during the 20th century, suggesting the presence of a considerable amount of noise and/or bias in any underlying spatial SAT signal. The vast majority of variance in the borehole dataset is efficiently retained by its two leading eigenvectors. A sizable share of the variance in the first eigenvector appears to be associated with non-SAT related bias in the borehole data. A weak but detectable SAT signal appears to be described by a combination of the first two eigenvectors. Exploiting this eigendecomposition, application of optimal signal estimation methods yields a hemispheric borehole SAT reconstruction that is largely consistent with instrumental data available in past centuries, and is indistinguishable in its major features from several published long-term temperature estimates based on both climate proxy data and model simulations.” [Full text]

Climate from Borehole Data: Energy Fluxes and Temperatures since 1500 – Beltrami (2002) “Here I apply singular value decomposition (SVD) Here I apply singular value decomposition (SVD) inversion methods to 826 temperature-depth profiles distributed world wide, in order to reconstruct ground surface temperature histories (GSTH) and surface heat flux histories (SHFH) from the temperature and heat flux anomalies detected in the shallow subsurface. Inversions yielded a mean ground surface temperature and surface heat flux histories flux histories for the Earth’s continents for the last 500 years. Results indicate that the global average ground temperature and ground heat flux have increased an average of 0.45 ° K and 18.0 $mWm^{2}$ respectively over the last 200 years, and 0.9 ° K in the last five centuries.” [Full text]

Earth’s Long-Term Memory – Beltrami (2002) “Two methods-multiproxy and geothermal-are commonly used to reconstruct Northern Hemisphere climate of the last 500 to 1000 years. Both show warming in the 20th century, but in earlier centuries the temperature curves diverge strongly. In his Perspective, Beltrami investigates the reasons for these discrepancies. He explains the difficulties that arise in trying to compare the two types of records and calls for integrated analyses in which all models are interpreted jointly.”

Mid-latitude (30°–60° N) climatic warming inferred by combining borehole temperatures with surface air temperatures – Harris & Chapman (2001) “We construct a mid-latitude (30°–60° N) reduced temperature-depth profile from a global borehole temperature database compiled for climate reconstruction. This reduced temperature profile is interpreted in terms of past surface ground temperature change and indicates warming on the order of 1°C over the past 100 to 200 years. The combination of an initial temperature (the primary free parameter) with the last 140 years of gridded surface air temperature (SAT) data yields a synthetic temperature profile that is an excellent fit to observations, accounting for 99% of the observed variance and a RMS misfit of only 12 mK. The good correlation suggests that this reduced temperature profile shares much information with the mean SAT record over large areas and long time-scales. Our analysis indicates 0.7°±0.1°C of ground warming between pre-industrial time and the 1961–1990 mean SAT.”

Climate Reconstruction from Subsurface Temperatures – Pollack & Huang (2000) A review article. “Temperature changes at the Earth’s surface propagate downward into the subsurface and impart a thermal signature to the rocks. This signature can be measured in boreholes and then analyzed to reconstruct the surface temperature history over the past several centuries. The ability to resolve surface temperature history from subsurface temperatures diminishes with time. Microclimatic effects associated with the topography and vegetation patterns at the site of a borehole, along with local anthropogenic perturbations associated with land use change, can obscure the regional climate change signal. Regional and global ensembles of boreholes reveal the broader patterns of temperature changes at the Earth’s surface. The average surface temperature of the continents has increased by about 1.0 K over the past 5 centuries; half of this increase has occurred in the twentieth century alone.” [Full text]

Temperature trends over the past five centuries reconstructed from borehole temperatures – Huang et al. (2000) “Here we use present-day temperatures in 616 boreholes from all continents except Antarctica to reconstruct century-long trends in temperatures over the past 500 years at global, hemispheric and continental scales. The results confirm the unusual warming of the twentieth century revealed by the instrumental record, but suggest that the cumulative change over the past five centuries amounts to about 1 K, exceeding recent estimates from conventional climate proxies.” [Full text]

Climate Change Record in Subsurface Temperatures: A Global Perspective – Pollack et al. (1998) “Analyses of underground temperature measurements from 358 boreholes in eastern North America, central Europe, southern Africa, and Australia indicate that, in the 20th century, the average surface temperature of Earth has increased by about 0.5°C and that the 20th century has been the warmest of the past five centuries. The subsurface temperatures also indicate that Earth’s mean surface temperature has increased by about 1.0°C over the past five centuries. The geothermal data offer an independent confirmation of the unusual character of 20th-century climate that has emerged from recent multiproxy studies.” [Full text]

Late Quaternary temperature changes seen in world-wide continental heat flow measurements – Huang et al. (1997) “Analysis of more than six thousand continental heat flow measurements as a function of depth has yielded a reconstruction of a global average ground surface temperature history over the last 20,000 years. The early to mid-Holocene appears as a relatively long warm interval some 0.2–0.6 K above present-day temperatures, the culmination of the warming that followed the end of the last glaciation. Temperatures were also warmer than present 500–1,000 years ago, but then cooled to a minimum some 0.2–0.7 K below present about 200 years ago. Although temperature variations in this type of reconstruction are highly smoothed, the results clearly resemble the broad outlines of late Quaternary climate changes suggested by proxies.” [Full text]

Geothermal Evidence from Permafrost in the Alaskan Arctic – Lachenbruch & Marshall (1986) “Temperature profiles measured in permafrost in northernmost Alaska usually have anomalous curvature in the upper 100 meters or so. When analyzed by heat-conduction theory, the profiles indicate a variable but widespread secular warming of the permafrost surface, generally in the range of 2 to 4 Celsius degrees during the last few decades to a century. Although details of the climatic change cannot be resolved with existing data, there is little doubt of its general magnitude and timing; alternative explanations are limited by the fact that heat transfer in cold permafrost is exclusively by conduction. Since models of greenhouse warming predict climatic change will be greatest in the Arctic and might already be in progress, it is prudent to attempt to understand the rapidly changing thermal regime in this region.”

Underground temperature and inferred climatic temperature of the past millenium – Cermak (1971) “Any change at the surface is propagated downwards, and it is shown that the detailed record of temperature with depth can be used to trace the past climatic history. The theory of climatic correction of heat flow is used, and the data is obtained from two boreholes in northeastern Ontario. After analysis the measured underground temperature clearly confirmed the notably warm climate that lasted a few hundred years around A.D. 1000–1200 and the following cold period after 1500. … The results presented indicate for the Kapuskasing area a surface temperature during the Little Climatic Optimum at least 1.5°C higher than the reference value; the mean temperature during the Little Ice Age was about 1°C below this reference value. A remarkable increase since about 1850 reaches value in excess of 3°C.”

Analysis of Heat Flow Data—I Detailed Observations in a Single Borehole – Beck & Judge (1969) “Heat flow data from a 600-m deep diamond drilled borehole has been used to estimate how short a section of borehole will give a valid heat flow value, to test for recent and ancient climatic changes, underground water-flows and the variation of terrestrial heat flow with depth. Temperatures were repeatedly measured at 3-m intervals; measurements of thermal conductivity, density and porosity were made on specimens sampled at approximately 4-m intervals along the length of the hole. The mean heat flow for the whole borehole before applying any corrections is 0.76 h.f.u. while after correcting for the Wisconsin glaciation the mean value is 1.17 h.f.u., but in both cases some 30 to 100-m sections of the borehole differ by ±20 per cent from the mean values. The differences cannot be entirely explained as being due to structure, topography, climatic changes or underground water-flows.”

Postglacial Time Calculations from Recent Geothermal Measurements in the Calumet Copper Mines – Hotchkiss & Ingersoll (1934) “The retreat of the glacial sheet a number of thousand years ago and the subsequent long-period surface temperature variations must almost certainly have left their impress on the geothermal curve. An attempt has been made to find such an effect and to interpret it in terms of past surface temperatures by subjecting to mathematical analysis a series of geothermal measurements recently made in the Calumet and Hecla conglomerate mine. The results indicate that the glacial epoch ended for this region 20,000-30,000 years ago, and that it was followed by a period with ground temperatures distinctly warmer than the present, succeeded in turn by one cooler, and lasting until comparatively recent times.”

Papers on CO2-temperature correlation

This is a list of papers on the correlation between carbon dioxide concentration and temperature. This subject was suggested by Brad Carpenter in the paperlist suggestion thread. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.

UPDATE (February 25, 2016): Stips et al. (2016) added. Thanks to Keith Pickering for pointing it out.
UPDATE (January 9, 2014): Kang & Larsson (2013) and Attanasio et al. (2013) added.
UPDATE (March 29, 2012): Attanasio (2012) and Attanasio et al. (2012)added.
UPDATE (October 31, 2010): Sun & Wong (1996), Stern & Kaufmann (1999), Verdes (2005), Smirnov & Mokhov (2009) and Kodra et al. (2010) added.

Modern climate

On the causal structure between CO2 and global temperature – Stips et al. (2016)
Abstract: We use a newly developed technique that is based on the information flow concept to investigate the causal structure between the global radiative forcing and the annual global mean surface temperature anomalies (GMTA) since 1850. Our study unambiguously shows one-way causality between the total Greenhouse Gases and GMTA. Specifically, it is confirmed that the former, especially CO2, are the main causal drivers of the recent warming. A significant but smaller information flow comes from aerosol direct and indirect forcing, and on short time periods, volcanic forcings. In contrast the causality contribution from natural forcings (solar irradiance and volcanic forcing) to the long term trend is not significant. The spatial explicit analysis reveals that the anthropogenic forcing fingerprint is significantly regionally varying in both hemispheres. On paleoclimate time scales, however, the cause-effect direction is reversed: temperature changes cause subsequent CO2/CH4 changes.
Citation: Adolf Stips, Diego Macias, Clare Coughlan, Elisa Garcia-Gorriz & X. San Liang (2016), Scientific Reports 6, Article number: 21691, doi:10.1038/srep21691.

Granger Causality Analyses for Climatic Attribution – Attanasio et al. (2013) “This review paper focuses on the application of the Granger causality technique to the study of the causes of recent global warming (a case of climatic attribution). A concise but comprehensive review is performed and particular attention is paid to the direct role of anthropogenic and natural forcings, and to the influence of patterns of natural variability. By analyzing both in-sample and out-of-sample results, clear evidences are obtained (e.g., the major role of greenhousegases radiative forcing in driving temperature, a recent causal decoupling between solar irradiance and temperature itself) together with interesting prospects of further research.” Alessandro Attanasio, Antonello Pasini, Umberto Triacca, Atmospheric and Climate Sciences, Vol. 3 No. 4, 2013, pp. 515-522. doi: 10.4236/acs.2013.34054. [Full text]

Testing for linear Granger causality from natural/anthropogenic forcings to global temperature anomalies – Attanasio (2012) “In this paper, we analyze the Granger causality from natural or anthropogenic forcings to global temperature anomalies. The lag-augmented Wald test is performed, and its robustness is also evaluated considering bootstrap method. The results show there is no-evidence of Granger causality from natural forcings to global temperature. On the contrary, a detectable Granger causality is found from anthropogenic forcings to global temperature confirming that greenhouse gases have an important role on recent global warming.” Alessandro Attanasio, Theoretical and Applied Climatology, DOI: 10.1007/s00704-012-0634-x.

A contribution to attribution of recent global warming by out-of-sample Granger causality analysis – Attanasio et al. (2012) “The topic of attribution of recent global warming is usually faced by studies performed through global climate models (GCMs). Even simpler econometric models have been applied to this problem, but they led to contrasting results. In this article, we show that a genuine predictive approach of Granger analysis leads to overcome problems shown by these models and to obtain a clear signal of linear Granger causality from greenhouse gases (GHGs) to the global temperature of the second half of the 20th century. In contrast, Granger causality is not evident using time series of natural forcing.” Alessandro Attanasio, Antonello Pasini, Umberto Triacca, Atmospheric Science Letters, Volume 13, Issue 1, pages 67–72, January/March 2012, DOI: 10.1002/asl.365. [Full text]

Exploring Granger causality between global average observed time series of carbon dioxide and temperature – Kodra et al. (2010) “Detection and attribution methodologies have been developed over the years to delineate anthropogenic from natural drivers of climate change and impacts. A majority of prior attribution studies, which have used climate model simulations and observations or reanalysis datasets, have found evidence for human-induced climate change. This papers tests the hypothesis that Granger causality can be extracted from the bivariate series of globally averaged land surface temperature (GT) observations and observed CO₂ in the atmosphere using a reverse cumulative Granger causality test. This proposed extension of the classic Granger causality test is better suited to handle the multisource nature of the data and provides further statistical rigor. The results from this modified test show evidence for Granger causality from a proxy of total radiative forcing (RC), which in this case is a transformation of atmospheric CO₂, to GT. Prior literature failed to extract these results via the standard Granger causality test. A forecasting test shows that a holdout set of GT can be better predicted with the addition of lagged RC as a predictor, lending further credibility to the Granger test results. However, since second-order-differenced RC is neither normally distributed nor variance stationary, caution should be exercised in the interpretation of our results.” [Full text]

From Granger causality to long-term causality: Application to climatic data – Smirnov & Mokhov (2009) “Quantitative characterization of interaction between processes from time series is often required in different fields of natural science including geophysics and biophysics. Typically, one estimates “short-term” influences, e.g., the widely used Granger causality is defined via one-step-ahead predictions. Such an approach does not reveal how strongly the “long-term” behavior of one process under study is affected by the others. To overcome this problem, we introduce the concept of long-term causality, which extends the concept of Granger causality. The long-term causality is estimated from data via empirical modeling and analysis of model dynamics under different conditions. Apart from mathematical examples, we apply both approaches to find out how strongly the global surface temperature (GST) is affected by variations in carbon dioxide atmospheric content, solar activity, and volcanic activity during the last 150 years. Influences of all the three factors on GST are detected with the Granger causality. However, the long-term causality shows that the rise in GST during the last decades can be explained only if the anthropogenic factor (CO₂) is taken into account in a model.” [Full text]

Correlation Analysis between Global Temperature Anomaly and two main factors (CO₂ and aa index) – Moon (2008) “We have made the correlation analysis between gloabl temperature anomaly and two main factos: geomagnetic activity (aa index) and CO₂ content. … These results imply that the CO₂ effect become very important since at least 1990.”

Is Granger causality analysis appropriate to investigate the relationship between atmospheric concentration of carbon dioxide and global surface air temperature? – Triacca (2005) “Many time series based studies use Granger causality analysis in order to investigate the connection between atmospheric carbon-dioxide concentrations and global mean temperature. This note re-examines the causal relationship between these variables and shows the inappropriateness of the Granger test to the problem under investigation.”

Assessing causality from multivariate time series – Verdes (2005) “In this work we propose a general nonparametric test of causality for weakly dependent time series. More precisely, we study the problem of attribution, i.e., the proper comparison of the relative influence that two or more external dynamics trigger on a given system of interest. We illustrate the possible applications of the proposed methodology in very different fields like physiology and climate science.”

Econometric analysis of global climate change – Stern & Kaufmann (1999) “This paper reports on research that applies econometric time series methods to the analysis of global climate change. The aim of this research was to test hypotheses concerning the causes of the historically observed rise in global temperatures. Longer term applications include quantification of the contribution of different forcing variables to historic warming and use of the model as a module in integrated assessment. Research to date has comprised three stages. In the first stage we used the concept of Granger causality and differences between the temperature record in the northern and southern hemispheres to investigate the causes of temperature increase. In the second stage we tested various global change time series for the presence of stochastic trends. We found that most series contain a stochastic trend with the greenhouse gas series containing I(2) stochastic trends. In the third stage we developed a structural time series to investigate some of the hypotheses suggested by the earlier stages and further tested for the presence of an I(2) trend in hemispheric temperature series. We found that the two temperature series share a common I(2) stochastic trend that may have its source in radiative forcing due to greenhouse gases. There is a second non-stationary component that appears only in the northern hemisphere and appears to be related to radiative forcing due to anthropogenic sulphur emissions.”

A Bayesian Statistical Analysis of the Enhanced Greenhouse Effect – Tol & De Vos (1998) “This paper demonstrates that there is a robust statistical relationship between the records of the global mean surface air temperature and the atmospheric concentration of carbon dioxide over the period 1870–1991. As such, the enhanced greenhouse effect is a plausible explanation for the observed global warming. Long term natural variability is another prime candidate for explaining the temperature rise of the last century. Analysis of natural variability from paleo-reconstructions, however, shows that human activity is so much more likely an explanation that the earlier conclusion is not refuted.” [Full text]

Global-scale temperature patterns and climate forcing over the past six centuries – Mann et al. (1998) “Time-dependent correlations of the reconstructions with time-series records representing changes in greenhouse-gas concentrations, solar irradiance, and volcanic aerosols suggest that each of these factors has contributed to the climate variability of the past 400 years, with greenhouse gases emerging as the dominant forcing during the twentieth century.” [Full text]

Dependence of global temperatures on atmospheric CO₂ and solar irradiance – Thomson (1997) “Changes in global average temperatures and of the seasonal cycle are strongly coupled to the concentration of atmospheric CO₂. I estimate transfer functions from changes in atmospheric CO₂ and from changes in solar irradiance to hemispheric temperatures that have been corrected for the effects of precession. They show that changes from CO₂ over the last century are about three times larger than those from changes in solar irradiance.” [Full text]

Global Warming and Global Dioxide Emission: An Empirical Study – Sun & Wong (1996) “In this paper, the dynamic relationship between global surface temperature (global warming) and global carbon dioxide emission (CO₂) is modelled and analyzed by causality and spectral analysis in the time domain and frequency domain, respectively. Historical data of global CO₂ emission and global surface temperature anomalies over 129 years from 1860–1988 are used in this study. The causal relationship between the two phenomena is first examined using the Sim and Granger causality test in the time domain after the data series are filtered by ARIMA models. The Granger causal relationship is further scrutinized and confirmed by cross-spectral and multichannel spectral analysis in the frequency domain. The evidence found from both analyses proves that there is a positive causal relationship between the two variables. The time domain analysis suggests that Granger causality exists between global surface temperature and global CO₂ emission. Further, CO₂ emission causes the change in temperature. The conclusions are further confirmed by the frequency domain analysis, which indicates that the increase in CO₂ emission causes climate warming because a high coherence exists between the two variables. Furthermore, it is proved that climate changes happen after an increase in CO₂ emission, which confirms that the increase in CO₂ emission does cause global warming.”

Interannual extremes in the rate of rise of atmospheric carbon dioxide since 1980 – Keeling et al. (1995) Abstract doesn’t say it, but they make an interesting comparison with carbon dioxide record and temperature record. “The marked discrepancy between the predicted and observed anomalies on the decadal timescale after 1980, and differences throughout the record on shorter timescales, may be related to climate forcing involving air temperature, because anomalous variations in CO₂ and in temperature have tended to occur coherently, as suggested by comparing Fig. 2a with Fig. 2b. Many of these coherent anomalies are associated with El Niño events (arrows in Fig. 2a), but they also occur on the decadal timescale (solid curves) as previously noted by Keeling et al. (see p. 211 of ref. 1), and confirmed by rigorous analysis of coherence.” [Full text]

Coherence established between atmospheric carbon dioxide and global temperature – Kuo et al. (1990) “The hypothesis that the increase in atmospheric carbon dioxide is related to observable changes in the climate is tested using modern methods of time-series analysis. The results confirm that average global temperature is increasing, and that temperature and atmospheric carbon dioxide are significantly correlated over the past thirty years. Changes in carbon dioxide content lag those in temperature by five months.”

Past climate

What is the link between temperature and carbon dioxide levels? A Granger causality analysis based on ice core data – Kang & Larsson (2013) “We use statistical methods to analyze whether there exists long-term causality between temperature and carbon dioxide concentration. The analysis is based on a the Vostok Ice Core data from 400,000 to 6,000 years ago, extended by the EPICA Dome C data which go back to 800,000 years ago. At first, to make the data equidistant, we reconstruct it by linear interpolation. Then, using an approximation of a piecewise exponential function, we adjust for a deterministic trend. Finally, we employ the Granger causality test. We are able to strongly reject the null hypothesis that carbon dioxide concentration does not Granger cause temperature as well as the reverse hypothesis that temperature does not Granger cause carbon dioxide concentration.” Jian Kang, Rolf Larsson, Theoretical and Applied Climatology, July 2013, DOI: 10.1007/s00704-013-0960-7.

Stable Carbon Cycle–Climate Relationship During the Late Pleistocene – Siegenthaler et al. (2005) “A record of atmospheric carbon dioxide (CO₂) concentrations measured on the EPICA (European Project for Ice Coring in Antarctica) Dome Concordia ice core extends the Vostok CO₂ record back to 650,000 years before the present (yr B.P.). Before 430,000 yr B.P., partial pressure of atmospheric CO₂ lies within the range of 260 and 180 parts per million by volume. This range is almost 30% smaller than that of the last four glacial cycles; however, the apparent sensitivity between deuterium and CO₂ remains stable throughout the six glacial cycles, suggesting that the relationship between CO₂ and Antarctic climate remained rather constant over this interval.” [Full text]

Timing of Atmospheric CO₂ and Antarctic Temperature Changes Across Termination III – Caillon et al. (2003) “We have measured the isotopic composition of argon in air bubbles in the Vostok core during Termination III (~240,000 years before the present). This record most likely reflects the temperature and accumulation change, although the mechanism remains unclear. The sequence of events during Termination III suggests that the CO₂ increase lagged Antarctic deglacial warming by 800 ± 200 years and preceded the Northern Hemisphere deglaciation.”

Carbon dioxide and climate over the past 300Myr – Retallack (2002) “Large and growing databases on these proxy indicators support the idea that atmospheric CO₂ and temperature are coupled. In contrast, CO₂–temperature uncoupling has been proposed from geological time-series of carbon isotopic composition of palaeosols and of marine phytoplankton compared with foraminifera, which fail to indicate high CO₂ at known times of high palaeotemperature. Failure of carbon isotopic palaeobarometers may be due to episodic release of CH₄, which has an unusually light isotopic value (down to −110[promille], and typically −60[promille]δ13C) and which oxidizes rapidly (within 7–24 yr) to isotopically light CO₂.” [Full text]

The phase relations among atmospheric CO₂ content, temperature and global ice volume over the past 420 ka – Mudelsee (2001) “Over the full 420 ka of the Vostok record, CO2 variations lag behind atmospheric temperature changes in the Southern Hemisphere by 1.3±1.0 ka, and lead over global ice-volume variations by 2.7±1.3 ka. However, significant short-term changes in the lag of CO₂ relative to temperature, subsequent to Terminations II and III, are also detected.” [Full text]

Covariation of carbon dioxide and temperature from the Vostok ice core after deuterium-excess correction – Cuffey & Vimeux (2001) “Here we incorporate measurements of deuterium excess from Vostok in the temperature reconstruction and show that much of the mismatch is an artefact caused by variations of climate in the water vapour source regions. Using a model that corrects for this effect, we derive a new estimate for the covariation of CO₂ and temperature, of r² = 0.89 for the past 150 kyr and r² = 0.84 for the period 350–150 kyr ago. Given the complexity of the biogeochemical systems involved, this close relationship strongly supports the importance of carbon dioxide as a forcing factor of climate. Our results also suggest that the mechanisms responsible for the drawdown of CO₂ may be more responsive to temperature than previously thought.”

Atmospheric CO₂ Concentrations over the Last Glacial Termination – Monnin et al. (2001) “A record of atmospheric carbon dioxide (CO₂) concentration during the transition from the Last Glacial Maximum to the Holocene, obtained from the Dome Concordia, Antarctica, ice core, reveals that an increase of 76 parts per million by volume occurred over a period of 6000 years in four clearly distinguishable intervals. The close correlation between CO₂ concentration and Antarctic temperature indicates that the Southern Ocean played an important role in causing the CO₂ increase. However, the similarity of changes in CO₂ concentration and variations of atmospheric methane concentration suggests that processes in the tropics and in the Northern Hemisphere, where the main sources for methane are located, also had substantial effects on atmospheric CO₂ concentrations.”

Atmospheric CO₂ concentration and millennial-scale climate change during the last glacial period – Stauffer et al. (1998) “To compare the rapid climate changes recorded in the Greenland ice with the global trends in atmospheric CO₂ concentrations as recorded in the Antarctic ice, an accurate common timescale is needed. Here we provide such a timescale for the last glacial period using the records of global atmospheric methane concentrations from both Greenland and Antarctic ice. We find that the atmospheric concentration of CO₂ generally varied little with Dansgaard–Oeschger events (<10 parts per million by volume, p.p.m.v.) but varied significantly with Heinrich iceberg-discharge events (20 p.p.m.v.), especially those starting with a long-lasting Dansgaard–Oeschger event.” [Full text]

Closely related

Papers on GHG role in historical climate changes is very closely related list of papers, some papers exist in both lists.

Revisiting Svensmark & Friis-Christensen (1997)

Lately I have been putting together stuff with intention to write a thorough review of the situation with the role of the cosmic rays in the climate. For that, I read for the first time the original Svensmark & Friis-Christensen (1997, “SF97” from hereafter). To my surprise, I found out that their whole thing seems to be based on false cloud trends. Some of you might have read my previous writing of ISCCP problems. It turns out that SF97 use ISCCP data to show a correlation between clouds and cosmic rays.

At one point, they say this about cloud cover:

“In Fig. 2 it is seen that a pronounced variation (corresponding to 3%-4%) takes place during this period with a maximum around 1986-1987, close to the minimum in solar activity.”

Their Figure 2 then shows that situation with the changes in cosmic rays and there indeed seems to be good correlation between the two in the short interval they are presenting. Both increase roughly from 1984 to 1987 and then decrease from 1987 to 1990.

Among others, Evan et al. (2007) have studied the problems in ISCCP cloud data. They noted that addition of satellites to the measurement network causes decrease in the measured cloud cover due to the changes in the satellite viewing angle. They listed some points in time when there had been remarkable changes in the satellite network. One of the changes is the launch of satellite GOES 7. It happened in February 1987 and fits well to the strong decrease in the ISCCP cloud data at that time. Evan et al. also showed how the trends change if one uses only the regions where viewing angle problem does not affect the data much. They presented it in their Figure 3. Figure 1 here shows the Fig. 2 of SF97 and Fig. 3 of Evan et al. Note that the cloud cover changes are little different in these two because SF97 only used data taken over oceans.

Figure 1. Figure 2 from SF97 (top panel) and Figure 3 from Evan et al. (bottom panel). The data from regions affected less by the viewing angle problem has been highlighted with red between the time period with ISCCP data (about 1984-1990) used in SF97.

When we look at the data highlighted with red that presents the time period where SF97 showed the correlation (shown in top panel of Figure 1 above), we notice that the cloud trends between that time have disappeared almost completely. What follows is that the correlation presented in SF97 is most likely not real but a result of ISCCP viewing angle problem.

After this, SF97 show the situation with other cloud data in their Figure 4, but that image is very unclear. It is difficult to estimate if the other cloud data supports the correlation or not. I first thought that this was just due to scanning the original paper to electronic format, but Laut (2003) has also noted the blurred appearance of the figure, so the problem was already in the original publication. However, it can be seen from Laut’s discussion that the other cloud data don’t support the correlation (putting the blurred appearance of SF97 figure and its presentation as if it would support the correlation in not so pleasing light), so the correlation presented in SF97 seems to rest on the ISCCP data alone.

SF97 also show that the correlation is less near the equator and they think it supports their view. If the apparent correlation would be due to ISCCP viewing angle problem, then the geometry of the problem would also cause the correlation to appear less near the equator because at the equator there is generally less affected areas than elsewhere.

So, there you have it. It seems in a strange way quite amusing to think that the whole of this cosmic ray issue might have been resting on the ISCCP viewing angle problem in the original paper and the later works have just been stamp collecting the apparent correlations here and there. In this light it is not surprising that we keep finding lot of problems with the cosmic ray hypothesis.

References

Evan et al. (2007), “Arguments against a physical long-term trend in global ISCCP cloud amounts”, Geophys. Res. Lett., 34, L04701, doi:10.1029/2006GL028083, [abstract, full text]

Laut (2003), “Solar activity and terrestrial climate: an analysis of some purported correlations”, Journal of Atmospheric and Solar-Terrestrial Physics, Volume 65, Issue 7, May 2003, Pages 801-812, [abstract, full text]

Svensmark & Friis-Christensen (1997), “Variation of cosmic ray flux and global cloud coverage—a missing link in solar-climate relationships”, Journal of Atmospheric and Solar-Terrestrial Physics, Volume 59, Issue 11, July 1997, Pages 1225-1232, [abstract, full text]

The Skeptical Scientist

Recently, I made an announcement of our translations of Skeptical Science articles and even more recently, I announced that we have started a new climate science blog in Finnish. In that new blog, as it is the home also of the translator team, the Skeptical Science translations were announced. To accompany that announcement, I interviewed John Cook of Skeptical Science briefly about relating subjects. Here I offer the interview also to English reading world:

Ari: “Who is John Cook and how he became interested in climate science?”

John: “I started out studying physics at [University of Queensland, Australia]. After graduating, I spent my honours year focused on astrophysics (which is why I appreciate that the most popular skeptic argument is about the sun). However, after completing honours, I went into the workforce and while keeping a keen interest in science, didn’t pursue it professionally. However, a few years ago, a family member who was skeptical about global warming handed me a speech by Senator James Inhofe. After reading through it and researching the science, I was surprised at the weakness of the arguments against human caused global warming. So I started looking into the matter more deeply.

Being a compulsive data collector with an obsessive interest in plotting graphs and tabulating results, I began a database of skeptic arguments and how often each argument was used. I noticed there were a few other websites that systematically looked at skeptic arguments but often the answers referenced other websites rather than the peer review science. So I started researching what the peer review science said about each argument. And as I learnt the science, a pattern emerged. Each skeptic argument focused on a narrow piece of the puzzle while ignoring the broader picture. I set about trying to present the broader picture through the peer review science. Thus Skeptical Science was born.”

Ari: “Did you ever publish anything on astrophysics?”

John: “I haven’t published any peer reviewed papers. All I have published was a thesis on solar fraunhoffer line widths. The main achievement of that work was to point out the flaws in the PhD student’s thesis on the same subject – debunking even back then!”

Ari: “Being a newcomer in this issue, I never saw the Skeptical Science version 1. Was it similar to today with argument-meters and all? What was the initial response to your new website, instant success?”

John: “The initial ugly version was never meant to be seen by public eye – it was basic stuff. But somehow, someone saw it because I saw people refering to it on sites like Real Climate so at that point, I hurriedly asked my wife to come up with a web design (the current design) – at that point, the argument meters were all added. At that point, the site gradually grew – not an instant success. The biggest traffic jump was actually the climategate period.”

Ari: “With climate scientists like the people at RealClimate noticing and referring to your articles, how did/do you feel about that?”

John: “Once, I was reading a Real Climate post about the PMOD vs ACRIM debate and they mentioned that a good overview was found at Skeptical Science. That was somewhat cool firstly because I’d forgotten about that post and secondly because the Real Climate authors are actual climate scientists who have been working in their field professionally for decades. These are the guys working in the trenches, furthering understanding of our climate – I’m just a blogger trying to summarise their research in a way that us ordinary laymen will understand.

The more I read about climate science, the more I become aware of how much more there is to know. The Dunning-Kruger effect is rampant in the climate debate with many people with no scientific expertise thinking they know better than climate scientists who have been studying these matters for decades. I confess in the early days, I was just as susceptible to the Dunning-Kruger effect, thinking I had a clearer understanding of climate than I actually did. As time has progressed, I’ve realised how subtle, nuanced and complex climate is. Consequently, the website has evolved to put even more emphasis on the peer-reviewed science.”

Ari: “So, what are your thoughts on Skeptical Science today, especially considering what you originally thought it was going to be?”

John: “I didn’t really have much thoughts on what Skeptical Science might be at the beginning – it was more just the compulsion I have to sort and categorise things into neat little databases. At this point, I’m still not sure where Skeptical Science is heading – things are changing so quickly and the goal posts get moved from week to week. The basic principle is still the same though. Almost all skeptic arguments and tactics involve distracting people from the scientific realities of global warming. If it’s a science based argument, it involves focusing on narrow, cherry picked data. More commonly of late, it’s a focus on discrediting scientists and the IPCC. Either way, the best response is to point people back to what the peer review science is saying. So I continue to concentrate on that goal but the issue of how to communicate the science better to the general public is something I think about a lot.”

Ari: “Now there are people translating your articles to other languages. Being one of the translators I have seen how much work you have devoted just to make our work more pleasant, implementing new features to the translation system and repairing the glitches it has. I and I’m sure all the other translators thank you for the effort you have put in. There are now translators of several languages working on your articles. Your thoughts on all the translation projects?”

John: “The translation project is an exciting and unexpected development. It just kind of happened out of nowhere – initially, I was contacted by a Czech reader offering to translate some pages. We discussed where to put the translations and I offered to host them on my site. At that point, the idea came to me of multiple translations so I decided to create an admin system where any translator could login and add translations of any skeptical argument they chose. It was inspired somewhat by the Real Climate where a few of their posts have flags at the top of the page linking to translations. The system really took off when you and your team of Finnish translators really put the system through its paces and has grown steadily since then. As far as I’m concerned, the more people we can communicate the science to, the better.”

Ari: “We have seen from your articles that it’s not the sun and it’s not the clouds and it’s not the albedo and it’s not the cosmic rays and so on. What is it then?”

John: “What’s causing global warming? Initially I took the mindset of a process of elimination, ruling out sun, cosmic rays, clouds, etc. But gradually the light dawned that climate isn’t a simple mechanism ruled by one driving factor. You need to factor in all the different influences. Carbon dioxide, methane, internal variability, solar changes, etc. These all influence climate. And when you consider them all together, it becomes apparent that the forcing from carbon dioxide is currently the most dominant forcing. More disturbingly, it’s also the forcing that is changing faster than any other forcing – growing steadily. So many factors are influencing climate but the most dominant forcing is carbon dioxide.”

Observations of anthropogenic global warming

We have lot of observations of the Earth surface warming during the last few decades. Surface temperature measurements over land and ocean [1,2], satellite measurements [3,4], weather balloon measurements [5], ocean temperature measurements [6] and borehole measurements [7] all show clear increase in the average temperature of the Earth. Supporting these are the indicators of warming, such as changes in the behavior and ranges of species [8], melting glaciers [9], rising sea level [10], vanishing sea ice [11], etc.

Figure 1. Earth’s surface temperature presented with 11-year floating means. Data is from references [1] and [2].

Carbon dioxide has been shown to be able to intercept part of the thermal radiation in numerous laboratory experiments, starting with the results John Tyndall published in 1859 [12]. Already he was able to prove the simple fact that carbon dioxide intercepts some of the thermal radiation. Since then the carbon dioxide’s ability to intercept thermal radiation has been studied in various conditions, for example in different pressures, different carbon dioxide concentrations, different gas mixtures, different frequency bands, and so on [13]. All these laboratory experiments have one thing in common; they all have made our knowledge of carbon dioxide properties more precise. Today we are even more certain that carbon dioxide intercepts part of the thermal radiation. In addition to that we have a good understanding of how it does it and how large is the effect it causes [13].

In addition to having measured the ability of carbon dioxide to intercept thermal radiation in laboratories, we have also measured the same properties directly from the atmosphere. For example, with satellites we are able to measure the spectrum (which is basically the intensity of the radiation at different frequencies) of the outgoing thermal radiation (also known as outgoing longwave radiation, OLR), and it shows same features as the spectra measured in laboratories [14]. From the spectrum we can identify the effects of different molecules (such as carbon dioxide or other chemicals mankind is emitting to the atmosphere) [15]. All the greenhouse gases are seen in the thermal radiation spectrum just because they have the ability to intercept the thermal radiation. Each gas molecule have their characteristic frequencies at which they intercept the thermal radiation. Therefore the spectrum has holes at each characteristic frequency of each molecule (if there is enough of those molecules in the atmosphere). From the spectrum it also can be determined how much there are the gases in question in the atmosphere. These direct measurements of the ability of carbon dioxide to intercept heat in laboratories and in atmosphere don’t reveal any properties that would disagree with the theory of the anthropogenic (man-made) global warming.

According to the theory, the sunlight first heats up the ground and ocean surface and the warmed up surface then emits heat as thermal radiation. The thermal radiation emitted by the surface then meets the greenhouse gases in the atmosphere. The greenhouse gases intercept part of the radiation and then emit it again to a random direction which causes about half of the radiation to return back towards the surface. When the amount of greenhouse gases increases in the atmosphere, they intercept more of the thermal radiation and more of the radiation will then return back to the surface. So there is more thermal radiation flying around in the lower parts of atmosphere and that means warming. Correspondingly, there is less thermal radiation in the upper atmosphere, especially in the stratosphere (at about 15-50 km height), and that means cooling [16]. The expected cooling of the stratosphere has also been observed in measurements [17].

So we know that carbon dioxide can intercept thermal radiation. In addition to that, we know that the carbon dioxide concentration of the atmosphere has been rising steadily for decades. We know that from the measurements of the atmospheric carbon dioxide concentration, which have been taken directly from the atmosphere with several different methods. We have measured the carbon dioxide concentration from air samples. Accurate measurements were started by Charles Keeling in 1950’s [18], but even before that there had been plenty of measurements[19] but with not enough accuracy to determine the long time changes in the carbon dioxide concentration of the atmosphere. These samples are still taken from many places all over the Earth [20]. Samples are taken for example from air planes or the from the surrounding air of the measurement stations. Many measurement stations now use automatic sampling. We have also measured the carbon dioxide concentration in many different ways from the atmosphere by utilizing spectral measurements [19]. We are able to measure the carbon dioxide concentration from the sunlight at the surface, from reflected sunlight by satellites [21] and from the outgoing thermal radiation of the Earth, that too by satellites [22]. It is noteworthy that when we measure the carbon dioxide concentration from the outgoing thermal radiation of the Earth, we are in fact measuring directly the amount of greenhouse effect caused by the carbon dioxide. Today it is beginning to be a routine to measure the carbon dioxide concentration from satellites in different places of the Earth with short time intervals, so we have knowledge of how carbon dioxide concentration varies in different regions [23].

Figure 2. Annual means of carbon dioxide concentration as measured from three different locations: South pole, Hawaii (Mauna Loa) and Alaska. The data is from reference [20].

So we know that carbon dioxide can intercept thermal radiation and that the atmospheric carbon dioxide concentration is increasing. In addition to those, we know that the increase in carbon dioxide concentration is mainly from the fossilized carbon burned by mankind. We know that because the carbon from the fossilized carbon is slightly different than the average carbon in the atmosphere. The difference is in the mass of the carbon atom in the carbon dioxide molecule. Atoms that are alike otherwise, but their masses are different, are called isotopes. In practice, the difference is caused by the amount of neutrons in the nucleus of the carbon atom. The natural isotopes of carbon are ¹²C, ¹³C, and ¹⁴C, where ¹²C is the lightest and ¹⁴C is heaviest. When trying to determine the source of the atmospheric carbon dioxide increase, the isotope ¹⁴C is the most relevant. Isotope ¹⁴C is also called as radiocarbon. Unlike isotopes ¹²C and ¹³C, the radiocarbon is not stable but decays by itself with a half-life of 5730 years [24]. New radiocarbon is being produced in the atmosphere when cosmic rays are reacting with nitrogen atoms, so there always is little radiocarbon in the atmosphere, even if it has limited lifetime. Because of that, living plants also use some radiocarbon during photosynthesis which means that when living plants are destroyed (by burning for example), they release also some radiocarbon among other isotopes. But fossil fuels (oil, coal) don’t have radiocarbon at all. Fossil fuels are from plants fossilized millions of years ago and because radiocarbon slowly decays by itself, there’s practically no radiocarbon left in fossil fuels. Therefore, when burning fossil fuels, no radiocarbon is released to the atmosphere. We are able to measure the carbon isotopes in the atmosphere and such measurements have shown that the radiocarbon content of the atmosphere has decreased while the carbon dioxide concentration has increased [25]. This can be explained only if the increase in the atmospheric carbon dioxide concentration is from fossil fuels (and it is called Suess effect).

We also have another isotopic method that gives a result that supports the radiocarbon method. Plants have a preference to lighter isotopes, so they have the isotope ¹²C the most. The most of the carbon from fossils is from ancient plants and isotopes ¹²C and ¹³C are stable isotopes (meaning that they don’t decay by themselves), so with those isotopes the carbon in fossils is almost the same as carbon in modern plants. Atmospheric measurements have shown that the carbon dioxide increase is from carbon dioxide that has lot of lightest carbon isotope (¹²C). It has been observed that in atmosphere the isotope ¹²C is getting more common while isotope ¹³C is getting more rare. This is exactly the expected result, if the carbon dioxide increase is from fossilized carbon or from modern plants. Furthermore, the isotope ¹³C has been getting rare at the same time as fossil fuel emissions have increased, so based on that it is more likely that the carbon dioxide increase is from fossil fuels [26]. When we also note the evidence cited above relating to the isotope ¹⁴C, it is practically certain that the increase is from fossil fuels. The increase in atmospheric carbon dioxide calls for so large amounts of carbon anyway, that it is difficult to get so much from anywhere else than from the wood mankind is burning (and from the forests that get destroyed for that) and from fossil fuels (oil, coal). For example volcanoes, which are thought to be significant sources for carbon dioxide, release much less carbon dioxide to the atmosphere than from the emissions of the mankind. The volcanic carbon dioxide emissions are only about 1 % of the emissions of the mankind [27].

So we know that carbon dioxide can intercept thermal radiation and that the atmospheric carbon dioxide concentration is increasing because of mankind. In addition to those, we know that the outgoing thermal radiation from the Earth is decreasing at the spectral bands of greenhouse gases and that the atmosphere is emitting more thermal radiation back to the surface. Those too we know from direct measurements. We have measured the thermal radiation decreasing precisely in those parts of the spectrum which are known to be intercepted by carbon dioxide [28]. The amount of that decrease also agrees well of what is expected from the carbon dioxide concentration changes [29]. We have also measured the thermal radiation to increase on the surface of the Earth [30]. Also in this case the amount of increase fits well to the expected effects of greenhouse gas concentrations and we have even measured the change from the spectral bands of carbon dioxide [31]. So it seems that the outgoing thermal radiation from the Earth is decreasing at carbon dioxide spectral bands and that the decrease would show on the Earth’s surface as an increase in carbon dioxide spectral bands.

In addition to the things mentioned above, we also have lots of research results which indicate that carbon dioxide had significant role also in past climate changes. In the past mankind wasn’t pushing carbon dioxide to the sky, but back then the changes in sunlight arriving to the surface of the Earth first caused a little bit of warming, which then caused carbon dioxide concentration to increase in the atmosphere. This was mainly because of changes in the warming oceans. A warming ocean can cause atmospheric carbon dioxide concentration to increase by at least three ways; the warming affects the solubility of carbon dioxide to seawater, increases the ocean mixing (which flushes out more carbon dioxide from the depths of the ocean), and affects the biological activity in the ocean (and biological activity uses carbon dioxide). So increased atmospheric carbon dioxide concentration and the effects from it then strongly amplify the original warming [32]. In past ages the carbon dioxide has sometimes been very high, many times higher than today, and usually those times have been much warmer than today and for those times that weren’t warmer we have a good reason why they weren’t [32, 33].

Most of the information above is from direct measurements without having to use theories or climate models. So, just by using measurements we have been able to determine that carbon dioxide is causing the warming of the Earth (increased thermal radiation on the Earth’s surface causes the surface to warm), and during the last decades there hasn’t been other known factors that could have caused the observed warming. However, carbon dioxide by itself can only cause little warming to the Earth’s surface temperature, perhaps about one degrees of Celsius on average when carbon dioxide concentration is doubled. When carbon dioxide warms the Earth, the warming causes some things. The warming for example causes snow and ice to melt. When snow and ice melt from certain region revealing the bare ground or ocean surface, the ability to reflect light changes in that region. Snow and ice reflect sunlight much better than bare ground or the ocean surface. When less sunlight is reflected back to the space, there’s more sunlight that stays and heats up the ground and ocean. This is an example of the warming effect from the carbon dioxide causing a consequence that has the effect of causing more warming. The consequences like that are called feedbacks. If a consequence has an effect of causing more warming, it is called a positive feedback, and if a consequence restrains the warming, it is called a negative feedback.

In the studies of feedbacks it has turned out that there are lot of positive feedbacks but only little negative feedbacks. The most important feedbacks are the changes in water vapour and in cloudiness. The water vapour feedback has been determined in many measurements to be clearly positive [34]. When atmosphere warms, it can hold bigger water vapour concentration and because water vapour is a strong greenhouse gas, it causes much more warming. It is generally known that the biggest uncertainty, when predicting the amount of the future warming, has to do with the changes in cloudiness. Problem is mostly due to us not having enough stable and precise long time measurements, so that we would have been able to measure the changes in cloudiness due to warming with enough accuracy [35]. The situation is currently getting better, and latest research indicates that the cloud feedback is positive [36]. If this turns out to be true, it means that the total heating effect from carbon dioxide is large, because the changes in cloudiness has pretty much been the only factor that could have been a strong negative feedback. If clouds are a positive feedback too, as it seems in the light of latest research, there aren’t much things anymore that could restrain the strong warming in the future.

I thank Esko, Kaitsu, Jari, Matti and Timo for their good comments on my text to make it better.

References

1. NASA GISS (Goddard Institute for Space Studies) surface temperature analysis

2. HadCRUT3 (Hadley Centre ja Climate Research Unit, East Anglia) surface temperature analysis

3. RSS (Remote Sensing Systems)

4. UAH (University of Alabama in Huntsville) (link is directly to their data, they don’t seem to have a decent website)

5. HadAT: Upper-air temperatures from weather balloons

6. Pierce et al. (2006), [abstract, full text], “Anthropogenic Warming of the Oceans: Observations and Model Results”

7. Huang et al. (2000), [abstract, full text], “Temperature trends over the past five centuries reconstructed from borehole temperatures”

8. Parmesan (2006), [abstract, full text], “Ecological and Evolutionary Responses to Recent Climate Change”

9. Zemp et al. (2009), [abstract, full text], “Six decades of glacier mass-balance observations: a review of the worldwide monitoring network”

10. Church & White (2006), [abstract, full text], “A 20th century acceleration in global sea-level rise”

11. Comiso et al. (2008), [abstract, full text], “Accelerated decline in the Arctic sea ice cover”

12. Wikipedia: Tyndall’s Setup For Measuring Radiant Heat Absorption By Gases

13. AGW Observer: Papers on laboratory measurements of CO2 absorption properties

14. Niro et al. (2005), [abstract, full text], “Spectra calculations in central and wing regions of CO₂ IR bands between 10 and 20 μm. III: atmospheric emission spectra”

15. Clerbaux et al. (2003), [abstract, full text], “Trace gas measurements from infrared satellite for chemistry and climate applications”

16. Uherek (2006), [full text], “Stratospheric cooling”

17. AGW Observer: Papers on temperature trends in stratosphere

18. Keeling (1960), [full text], “The concentration and isotopic abundances of carbon dioxide in the atmosphere”

19. AGW Observer: Papers on atmospheric carbon dioxide concentration measurements

20. CDIAC Trends: Atmospheric Carbon Dioxide and Carbon Isotope Records

21. Buchwitz et al. (2007), [abstract, full text], “First direct observation of the atmospheric CO₂ year-to-year increase from space”

22. Chédin et al. (2003), [abstract, full text], “First global measurement of midtropospheric CO2 from NOAA polar satellites: Tropical zone”

23. Buchwitz (2008), [full text], “Visualization of the global distribution of greenhouse gases using satellite measurements”

24. Wikipedia: Suess effect

25. Levin & Hessheimer (2000), [abstract, full text], “Radiocarbon – a unique tracer of global carbon cycle dynamics”

26. RealClimate, Steig (2004), “How do we know that recent CO₂ increases are due to human activities?”

27. Hards (2005), [full text], “Volcanic Contributions to Global Carbon Cycle”

28. AGW Observer: Papers on changes in OLR due to GHG’s

29. Griggs & Harries (2004), [abstract, full text], “Comparison of spectrally resolved outgoing longwave data between 1970 and present”

30. AGW Observer: Papers on changes in DLR

31. Evans & Puckrin (2006), [abstract, full text], “Measurements of the Radiative Surface Forcing of Climate”

32. AGW Observer: Papers on GHG role in historical climate changes

33. Royer (2008), [abstract, full text], “Linkages between CO₂, climate, and evolution in deep time”

34. AGW Observer: Papers on water vapor feedback observations

35. Loeb et al. (2007), [abstract], “Variability in global top-of-atmosphere shortwave radiation between 2000 and 2005”. Relevant quote: “As a minimum, radiation budget instruments should be stable enough to detect a change in net cloud forcing corresponding to a 25% cloud feedback. A 25% cloud feedback would reduce or amplify the influence of the anthropogenic radiative forcing by the same amount. Estimates of anthropogenic total radiative forcing in the next few decades are 0.6 Wm⁻² per decade [IPCC, 2001, Figure 9.13]. A 25% cloud feedback would change cloud net radiative forcing by 25% of the anthropogenic radiative forcing, or 0.15 Wm⁻² per decade. The global average shortwave (SW) or solar reflected cloud radiative forcing by clouds is ~50 Wm⁻², so that the observation requirements for global broadband radiation budget to directly observe such a cloud feedback is approximately 0.15/50 = 0.3% per decade in SW broadband calibration stability [Ohring et al., 2005]. Achieving this stability per decade in calibration is extremely difficult and has only recently been demonstrated for the first time by the ERBS and CERES broadband radiation budget instruments [Wong et al., 2006; Loeb et al., 2007].”

36. Clement et al. (2009), [abstract, full text], “Observational and Model Evidence for Positive Low-Level Cloud Feedback”

Yet another new climate science blog

With a few friends we have started a new blog called “Ilmastotieto” (Finnish word for “climate information”). Here’s the link. As you can guess from the title, it’s in Finnish. So from now on I’ll be writing there as well. I don’t think it will make my posting here much less as I probably will be publishing the articles I write there in Finnish here in English. There’s one already that I will publish here as soon as I have translated it to English. It will probably make the paperlist postings here little slower, though.

Papers on black carbon

This is a list of papers on the climatic effects of black carbon. This subject was suggested by Kaj Luukko in this thread. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.

Evaluation of black carbon estimations in global aerosol models – Koch et al. (2009) “We evaluate black carbon (BC) model predictions from the AeroCom model intercomparison project by considering the diversity among year 2000 model simulations and comparing model predictions with available measurements. … In regions other than Asia, most models are biased high compared to surface concentration measurements. However compared with (column) AAOD or BC burden retreivals, the models are generally biased low.” [Full text]

Global and regional climate changes due to black carbon – Ramanathan & Carmichael (2008) A review paper. “Anthropogenic sources of black carbon, although distributed globally, are most concentrated in the tropics where solar irradiance is highest. … Because of the combination of high absorption, a regional distribution roughly aligned with solar irradiance, and the capacity to form widespread atmospheric brown clouds in a mixture with other aerosols, emissions of black carbon are the second strongest contribution to current global warming, after carbon dioxide emissions.” [Full text]

Present-day climate forcing and response from black carbon in snow – Flanner et al. (2007) “We apply our Snow, Ice, and Aerosol Radiative (SNICAR) model, coupled to a general circulation model with prognostic carbon aerosol transport, to improve understanding of climate forcing and response from black carbon (BC) in snow. … Applying biomass burning BC emission inventories for a strong (1998) and weak (2001) boreal fire year, we estimate global annual mean BC/snow surface radiative forcing from all sources (fossil fuel, biofuel, and biomass burning) of +0.054 (0.007–0.13) and +0.049 (0.007–0.12) W m⁻², respectively.” [Full text]

20th-Century Industrial Black Carbon Emissions Altered Arctic Climate Forcing – McConnell et al. (2007) “Black carbon (BC) from biomass and fossil fuel combustion alters chemical and physical properties of the atmosphere and snow albedo, yet little is known about its emission or deposition histories. Measurements of BC, vanillic acid, and non–sea-salt sulfur in ice cores indicate that sources and concentrations of BC in Greenland precipitation varied greatly since 1788 as a result of boreal forest fires and industrial activities. Beginning about 1850, industrial emissions resulted in a sevenfold increase in ice-core BC concentrations, with most change occurring in winter. BC concentrations after about 1951 were lower but increasing. At its maximum from 1906 to 1910, estimated surface climate forcing in early summer from BC in Arctic snow was about 3 watts per square meter, which is eight times the typical preindustrial forcing value.” [Supporting information]

Aerosol organic carbon to black carbon ratios: Analysis of published data and implications for climate forcing – Novakov et al. (2005) “Measurements of organic carbon (OC) and black carbon (BC) concentrations over a variety of locations worldwide have been analyzed to infer the spatial distributions of the ratios of OC to BC. Since these ratios determine the relative amounts of scattering and absorption, they are often used to estimate the radiative forcing due to aerosols. … The OC to BC ratios, ranging from 1.3 to 2.4, appear relatively constant and are generally unaffected by seasonality, sources, or technology changes, at the locations considered here. The ratios compare well with emission inventories over Europe and China but are a factor of 2 lower in other regions. The reduced estimate for OC/BC in aerosols strengthens the argument that reduction of soot emissions maybe a useful approach to slow global warming.” [Full text]

Climate response of direct radiative forcing of anthropogenic black carbon – Chung & Seinfeld (2005) “The equilibrium climate effect of direct radiative forcing of anthropogenic black carbon (BC) is examined by 100-year simulations in the Goddard Institute for Space Studies General Circulation Model II-prime coupled to a mixed-layer ocean model. … The climate sensitivity of BC direct radiative forcing is calculated to be 0.6 K W⁻¹ m², which is about 70% of that of CO₂, independent of the assumption of BC mixing state. The largest surface temperature response occurs over the northern high latitudes during winter and early spring.” [Full text]

Climate sensitivity to black carbon aerosol from fossil fuel combustion – Roberts & Jones (2004) “However, it is unclear how the climate sensitivity to black carbon aerosol forcing compares with the sensitivity to greenhouse gas forcing. Here we investigate this question using the HadSM4 configuration of the Hadley Centre climate model, extended by the addition of interactive black carbon and sulphate aerosol schemes. The results confirm earlier suggestions that the climate sensitivities are not necessarily similar and indicate that the black carbon sensitivity may be weaker.”

A modeling study on the climate impacts of black carbon aerosols – Wang (2004) “A three-dimensional interactive aerosol-climate model has been developed and used to study the climatic impact of black carbon (BC) aerosols. When compared with the model’s natural variability, significant global-scale changes caused by BC aerosols occurred in surface latent and sensible heat flux, surface net long-wave radiative flux, planetary boundary layer height, convective precipitation (all negative), and low-cloud coverage (positive), all closely related to the hydrological cycle. … The result of this study suggests that with a constant annual emission of 14 TgC, BC aerosols do not cause a significant change in global-mean surface temperature. The calculated surface temperature change is determined by a subtle balance among changes in surface energy budget as well as in the hydrological cycle, all caused by BC forcing and often compensate each other. The result of this study shows that the influences of BC aerosols on climate and environment are more significant in regional scale than in global scale.” [Full text]

Global atmospheric black carbon inferred from AERONET – Sato et al. (2003) “The spectral range of AERONET allows discrimination between constituents that absorb most strongly in the UV region, such as soil dust and organic carbon, and the more ubiquitously absorbing black carbon (BC). … We find that the amount of BC in current climatologies must be increased by a factor of 2–4 to yield best agreement with AERONET, in the approximation in which BC is externally mixed with other aerosols. The inferred climate forcing by BC, regardless of whether it is internally or externally mixed, is ≈1 W/m², most of which is probably anthropogenic.” [Full text]

Large historical changes of fossil-fuel black carbon aerosols – Novakov et al. (2003) “We estimate historical trends of fossil-fuel BC emissions in six regions that represent about two-thirds of present day emissions and extrapolate these to global emissions from 1875 onward. Qualitative features in these trends show rapid increase in the latter part of the 1800s, the leveling off in the first half of the 1900s, and the re-acceleration in the past 50 years as China and India developed. We find that historical changes of fuel utilization have caused large temporal change in aerosol absorption, and thus substantial change of aerosol single scatter albedo in some regions, which suggests that BC may have contributed to global temperature changes in the past century.” [Full text]

Climate Effects of Black Carbon Aerosols in China and India – Menon et al. (2002) “In recent decades, there has been a tendency toward increased summer floods in south China, increased drought in north China, and moderate cooling in China and India while most of the world has been warming. We used a global climate model to investigate possible aerosol contributions to these trends. We found precipitation and temperature changes in the model that were comparable to those observed if the aerosols included a large proportion of absorbing black carbon (“soot”), similar to observed amounts. Absorbing aerosols heat the air, alter regional atmospheric stability and vertical motions, and affect the large-scale circulation and hydrologic cycle with significant regional climate effects.” [Full text]

Strong radiative heating due to the mixing state of black carbon in atmospheric aerosols – Jacobson (2001) “Here I simulate the evolution of the chemical composition of aerosols, finding that the mixing state and direct forcing of the black-carbon component approach those of an internal mixture, largely due to coagulation and growth of aerosol particles. This finding implies a higher positive forcing from black carbon than previously thought, suggesting that the warming effect from black carbon may nearly balance the net cooling effect of other anthropogenic aerosol constituents. The magnitude of the direct radiative forcing from black carbon itself exceeds that due to CH₄, suggesting that black carbon may be the second most important component of global warming after CO₂ in terms of direct forcing.”

A global black carbon aerosol model – Cooke & Wilson (1996) “A global inventory has been constructed for emissions of black carbon from fossil fuel combustion and biomass burning. … The modeled values of black carbon mass concentration compare within a factor of 2 in continental regions and some remote regions but are higher than measured values in other remote marine regions and in the upper troposphere.” [Full text]

Effect of black carbon on the optical properties and climate forcing of sulfate aerosols – Chýlek et al. (1995) “We study the optical properties of anthropogenic sulfate aerosols containing black carbon using a recently developed exact solution of the scattering problem for a spherical particle (sulfate aerosol) containing an eccentrically located spherical inclusion (black carbon). … The black carbon within the sulfate aerosol reduces the expected sulfate direct cooling effect by about 0.034 W/m² for each 1% of the black carbon to sulfate mass mixing ratio. Thus the presence of black carbon within sulfate in the background aerosol does not significantly change the previous estimates of the global aerosol direct cooling effect. However, in regions where the black carbon in sulfate concentrations are of the order of 5% or more, the local and regional effects are significant.”

Papers on CO2 records from ice cores

This is a list of papers on the past CO₂ records measured from the ice cores. Specifically this list contains papers that present new or improved carbon dioxide records from ice cores. The list is based to the Ice Core Data Sets at NOAA. The subject for this list was suggested by Magnus W here. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.

UPDATE, March 14, 2011: Siegenthaler & Oeschger (1987) added.
UPDATE, October 13, 2010: Neftel et al. (1982) added.

High-resolution carbon dioxide concentration record 650,000–800,000 years before present – Lüthi et al. (2008) “So far, the Antarctic Vostok and EPICA Dome C ice cores have provided a composite record of atmospheric carbon dioxide levels over the past 650,000 years. Here we present results of the lowest 200 m of the Dome C ice core, extending the record of atmospheric carbon dioxide concentration by two complete glacial cycles to 800,000 yr before present.” [Data]

Atmospheric CO₂ and Climate on Millennial Time Scales During the Last Glacial Period – Ahn & Brook (2008) “We compared CO₂ variations on millennial time scales between 20,000 and 90,000 years ago with an Antarctic temperature proxy and records of abrupt climate change in the Northern Hemisphere. CO₂ concentration and Antarctic temperature were positively correlated over millennial-scale climate cycles, implying a strong connection to Southern Ocean processes.” [Full text] [Data]

Northern Hemisphere forcing of climatic cycles in Antarctica over the past 360,000 years – Kawamura et al. (2007) “Here we present a new chronology of Antarctic climate change over the past 360,000 years that is based on the ratio of oxygen to nitrogen molecules in air trapped in the Dome Fuji and Vostok ice cores. … Our results indicate that orbital-scale Antarctic climate change lags Northern Hemisphere insolation by a few millennia, and that the increases in Antarctic temperature and atmospheric carbon dioxide concentration during the last four terminations occurred within the rising phase of Northern Hemisphere summer insolation.” [Supplement] [Data]
Data description: “CO2 record from the Dome Fuji ice core (Antarctica) covering the last 260 kyr with relatively low resolution.”

Stable Carbon Cycle–Climate Relationship During the Late Pleistocene – Siegenthaler et al. (2005) “A record of atmospheric carbon dioxide (CO₂) concentrations measured on the EPICA (European Project for Ice Coring in Antarctica) Dome Concordia ice core extends the Vostok CO₂ record back to 650,000 years before the present (yr B.P.). Before 430,000 yr B.P., partial pressure of atmospheric CO₂ lies within the range of 260 and 180 parts per million by volume. This range is almost 30% smaller than that of the last four glacial cycles; however, the apparent sensitivity between deuterium and CO₂ remains stable throughout the six glacial cycles, suggesting that the relationship between CO₂ and Antarctic climate remained rather constant over this interval.” [Full text] [Data]

Supporting evidence from the EPICA Dronning Maud Land ice core for atmospheric CO₂ changes during the past millennium – Siegenthaler et al. (2005) “Here we present a new detailed CO₂ record from the Dronning Maud Land (DML) ice core, drilled in the framework of the European Project for Ice Coring in Antarctica (EPICA) and some new measurements on a previously drilled ice core from the South Pole. The DML CO₂ record shows an increase from about 278 to 282 parts per million by volume (ppmv) between ad 1000 and ad 1200 and a fairly continuous decrease to a mean value of about 277 ppmv around ad 1700.” [Full text] [Data]

Evidence for substantial accumulation rate variability in Antarctica during the Holocene, through synchronization of CO₂ in the Taylor Dome, Dome C and DML ice cores – Monnin et al. (2004) “High resolution records of atmospheric CO₂ concentration during the Holocene are obtained from the Dome Concordia and Dronning Maud Land (Antarctica) ice cores. These records confirm that the CO₂ concentration varied between 260 and 280 ppmv in the Holocene as measured in the Taylor Dome ice core. However, there are differences in the CO₂ records most likely caused by mismatches in timescales. Matching the Taylor Dome timescale to the Dome C timescale by synchronization of CO₂ indicates that the accumulation rate at Taylor Dome increased through the Holocene by a factor two and bears little resemblance to the stable isotope record used as a proxy for temperature. This result shows that different locations experienced substantially different accumulation changes, and casts doubt on the often-used assumption that accumulation rate scales with the saturation vapor pressure as a function of temperature, at least for coastal locations.” [Full text] [Data]

High-resolution Holocene N₂O ice core record and its relationship with CH₄ and CO₂ – Flückiger et al. (2002) “Here we fill this gap with a high-resolution N2O record measured along the European Project for Ice Coring in Antarctica (EPICA) Dome C Antarctic ice core. On the same ice we obtained high-resolution methane and carbon dioxide records. This provides the unique opportunity to compare variations of the three most important greenhouse gases (after water vapor) without any uncertainty in their relative timing. The CO₂ and CH₄ records are in good agreement with previous measurements on other ice cores.” [Full text] [Data]

Atmospheric CO₂ Concentrations over the Last Glacial Termination – Monnin et al. (2001) “A record of atmospheric carbon dioxide (CO₂) concentration during the transition from the Last Glacial Maximum to the Holocene, obtained from the Dome Concordia, Antarctica, ice core, reveals that an increase of 76 parts per million by volume occurred over a period of 6000 years in four clearly distinguishable intervals. The close correlation between CO₂ concentration and Antarctic temperature indicates that the Southern Ocean played an important role in causing the CO₂ increase. However, the similarity of changes in CO₂ concentration and variations of atmospheric methane concentration suggests that processes in the tropics and in the Northern Hemisphere, where the main sources for methane are located, also had substantial effects on atmospheric CO₂ concentrations.” [Data]

Atmospheric CO₂ concentration from 60 to 20 kyr BP from the Taylor Dome Ice Core, Antarctica – Indermühle et al. (2000) “A high‐resolution record of the atmospheric CO₂ concentration from 60 to 20 thousand years before present (kyr BP) based on measurements on the ice core of Taylor Dome, Antarctica is presented. This record shows four distinct peaks of 20 parts per million by volume (ppmv) on a millennial time scale. Good correlation of the CO₂ record with temperature reconstructions based on stable isotope measurements on the Vostok ice core (Antarctica) is found.” [Full text] [Data]

Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica – Petit et al. (1999) “Atmospheric concentrations of carbon dioxide and methane correlate well with Antarctic air-temperature throughout the record. Present-day atmospheric burdens of these two important greenhouse gases seem to have been unprecedented during the past 420,000 years.” [Full text] [Data]
Data description: “They include the deuterium content of the ice (δD_ice, a proxy of local temperature change), the dust content (desert aerosols), the concentration of sodium (marine aerosol), and from the entrapped air the greenhouse gases CO₂ and CH₄, and the δ¹⁸O of O₂ (hereafter δ¹⁸O_atm) which reflects changes in global ice volume and in the hydrological cycle.”

Ice Core Records of Atmospheric CO₂ Around the Last Three Glacial Terminations – Fischer et al. (1999) “High-resolution records from Antarctic ice cores show that carbon dioxide concentrations increased by 80 to 100 parts per million by volume 600 ± 400 years after the warming of the last three deglaciations. Despite strongly decreasing temperatures, high carbon dioxide concentrations can be sustained for thousands of years during glaciations; the size of this phase lag is probably connected to the duration of the preceding warm period, which controls the change in land ice coverage and the buildup of the terrestrial biosphere.” [Full text] [Data]

Dual modes of the carbon cycle since the Last Glacial Maximum – Smith et al. (1999) “We have measured the CO₂ concentration and δ¹³CO₂ of air trapped in ice from Taylor Dome, Antarctica, across the last glacial termination in order to develop a time series for the carbonisotope composition of atmospheric CO₂ and to constrain the mechanisms of carbon cycling between the main sources and sinks of atmospheric CO₂. The atmospheric CO₂ concentration data (Fig. 1) form a record comparable to that from Byrd and Vostok.” [Full text] [Data]

Holocene carbon-cycle dynamics based on CO₂ trapped in ice at Taylor Dome, Antarctica – Indermühle et al. (1999) “A high-resolution ice-core record of atmospheric CO₂ concentration over the Holocene epoch shows that the global carbon cycle has not been in steady state during the past 11,000 years. Analysis of the CO₂ concentration and carbon stable-isotope records, using a one-dimensional carbon-cycle model,uggests that changes in terrestrial biomass and sea surface temperature were largely responsible for the observed millennial-scale changes of atmospheric CO₂ concentrations.” [Full text] [Data]

Natural and anthropogenic changes in atmospheric CO₂ over the last 1000 years from air in Antarctic ice and firn – Etheridge et al. (1996) “A record of atmospheric CO2 mixing ratios from 1006 A.D. to 1978 A.D. has been produced by analysing the air enclosed in three ice cores from Law Dome, Antarctica. The enclosed air has unparalleled age resolution and extends into recent decades, because of the high rate of snow accumulation at the ice core sites. The CO2 data overlap with the record from direct atmospheric measurements for up to 20 years.” [Data]

CO₂ measurements from polar ice cores: more data from different sites – Staffelbach et al. (1991) “In this paper, we will discuss possible small deviations of the CO₂ concentration in air bubbles from that of the atmosphere at the time of enclosure. To do this, new results from Crête (Central Greenland) ice cores, covering the period since the beginning of industrialisation are presented, showing a good agreement with the data from Antarctic ice cores. In addition, the record of the atmospheric CO₂ concentration during the transition from the last glaciation to the Holocene and the fast variations in the concentration of atmospheric CO₂ during parts of the last glaciation, as suggested by Greenland ice core data, will be discussed.”

CO₂ record in the Byrd ice core 50,000–5,000 years bp – Neftel et al. (1988) “To achieve this, we have studied a great number of samples from the deep ice core from Byrd station, Westantarctica, drilled in 1968. These measurements allow us to reconstruct the atmospheric CO₂ concentration in the time period 50,000–15,000 yr bp in great detail.” [Full text] [Data]

Vostok ice core provides 160,000-year record of atmospheric CO₂ – Barnola et al. (1987) “Direct evidence of past atmospheric CO₂ changes has been extended to the past 160,000 years from the Vostok ice core. These changes are most notably an inherent phenomenon of change between glacial and interglacial periods. Besides this major 100,000-year cycle, the CO₂ record seems to exhibit a cyclic change with a period of some 21,000 years.” [Data]

Biospheric CO2 emissions during the past 200 years reconstructed by deconvolution of ice core data – Siegenthaler & Oeschger (1987) “Measurements on air trapped in old polar ice have revealed that the pre-industrial atmosphere contained 280 ppm of CO2 and that δ13C of atmospheric CO2 decreased by about 1.1 %, until 1980. These measurements show that considerable amounts of non-fossil CO2 must have already been emitted into the atmosphere in the 19th century. Quantitative estimates of the emission rates were performed by deconvolving the CO2 and δ13C records, using models of the global carbon cycle (box-diffusion and outcrop-diffusion ocean, four-box biosphere). Depending on the structure of the ocean submodel, deconvolution of the CO2 record yields a cumulative non-fossil production of about 90 to 150 Gt C until 1980, of which more than 50% were released prior to 1900. According to the model results, the net non-fossil production rate was roughly constant in the 19th and the first part of the 20th century. In the past 30 years, smaller values are obtained (0-0.9 Gt C yr−1) which are at the lower limit or below current ecological estimates for deforestation and land use (1.6 ± 0.8 Gt C yr−1). The difference might possibly be due to other sinks, e.g., stimulation of plant productivity by the enhanced CO2 concentration. Calculated 13C and 14C time histories agree well with the observed changes. While the change of the atmospheric CO2 concentration reflects more the cumulative carbon release, the isotope concentrations are more sensitive to short-term changes of the emission rate. The reason is that the oceanic uptake capacity is smaller for excess CO2 by the buffer factor of ˜ 10 than for an isotopic perturbation.” U. Siegenthaler, H. Oeschger, Tellus B, Volume 39B, Issue 1-2, pages 140–154, February-April 1987, DOI: 10.1111/j.1600-0889.1987.tb00278.x.

Evidence from polar ice cores for the increase in atmospheric CO₂ in the past two centuries – Neftel et al. (1985) “An ice core from Siple Station (West Antartica) that allows determination of the enclosed gas concentration with very good time resolution has recently become available. We report here measurements of this core which now allow us to trace the development of the atmospheric CO₂ from a period overlapping the Mauna Loa record back over the past two centuries.”

Ice core sample measurements give atmospheric CO₂ content during the past 40,000 yr – Neftel et al. (1982) “Recent measurements on ice samples from Camp Century (Greenland, 77°10’N, 61°08’W), Byrd Station (Antarctica, 80°01’S, 110°31’W) and Dome C (74°40’S, 125°10’E) suggest that during the late part of the last glaciation the atmospheric CO₂ concentration was significantly lower than during the Holocene. Further investigation of this natural increase of the atmospheric CO₂ concentration in the past should aid our understanding of the climatic implications of the man-made CO₂ increase since the beginning of industrialization3. Here we report new and precise measurements of the CO₂ concentration of the air occluded in bubbles of ice samples from Camp Century and Byrd Station, using a new dry extraction technique. The extracted gases were analysed with an IR-laser spectrometer (IRLS). Samples from 22 different depths were analysed from each core. The samples are distributed over a depth interval corresponding approximately to the past 40,000 yr. In addition results for ice samples from selected depth horizons from a colder region (North Central, Greenland 74°37’N, 39°36’W) and from a warmer region (Dye-3, Greenland 65°11’N, 43°50’W) are given. Based on these results we estimate the trend of the atmospheric CO₂ concentration during the past 40,000 yr.” [Full text]

Papers on Amazon and global warming

This is a list of papers on the effect of global warming to the Amazon rainforest. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.

UPDATE (February 3, 2010): 2 x Phillips et al. (2009) added, thanks to Skeptical Science for pointing them out (see the comment section below).

Drought Sensitivity of the Amazon Rainforest – Phillips et al. (2009) “We used records from multiple long-term monitoring plots across Amazonia to assess forest responses to the intense 2005 drought, a possible analog of future events. … Relative to pre-2005 conditions, forest subjected to a 100-millimeter increase in water deficit lost 5.3 megagrams of aboveground biomass of carbon per hectare. The drought had a total biomass carbon impact of 1.2 to 1.6 petagrams (1.2 x 10¹⁵ to 1.6 x 10¹⁵ grams). Amazon forests therefore appear vulnerable to increasing moisture stress, with the potential for large carbon losses to exert feedback on climate change.” [Full text] [Conference abstract]

Changes in Amazonian Forest Biomass, Dynamics, and Composition, 1980–2002 – Phillips et al. (2009) “Long-term, on-the-ground monitoring of forest plots distributed across Amazonia provides a powerful means to quantify stocks and fluxes of biomass and biodiversity. Here we examine the evidence for concerted changes in the structure, dynamics, and functional composition of old-growth Amazonian forests over recent decades. … The most likely driver(s) of changes are recent increases in the supply of resources such as atmospheric carbon dioxide, which would increase net primary productivity, increasing tree growth and recruitment, and, in turn, mortality. In the future the growth response of remaining undisturbed Amazonian forests is likely to saturate, and there is a risk of these ecosystems transitioning from sink to source driven by higher respiration (temperature), higher mortality (drought), or compositional change (functional shifts toward lighterwooded plants).” [Full text]

Exploring the likelihood and mechanism of a climate-change-induced dieback of the Amazon rainforest – Malhi et al. (2009) “We examine the evidence for the possibility that 21st-century climate change may cause a large-scale “dieback” or degradation of Amazonian rainforest. … We then examine climate simulations by 19 global climate models (GCMs) in this context and find that most tend to underestimate current rainfall. … Our analysis suggests that dry-season water stress is likely to increase in E. Amazonia over the 21st century, but the region tends toward a climate more appropriate to seasonal forest than to savanna.” [Full text]

Interactions among Amazon land use, forests and climate: prospects for a near-term forest tipping point – Nepstad et al. (2008) “Rising worldwide demands for biofuel and meat are creating powerful new incentives for agro-industrial expansion into Amazon forest regions. Forest fires, drought and logging increase susceptibility to further burning while deforestation and smoke can inhibit rainfall, exacerbating fire risk. If sea surface temperature anomalies (such as El Niño episodes) and associated Amazon droughts of the last decade continue into the future, approximately 55% of the forests of the Amazon will be cleared, logged, damaged by drought or burned over the next 20 years, emitting 15–26 Pg of carbon to the atmosphere.” [Full text]

Towards quantifying uncertainty in predictions of Amazon ‘dieback’ – Huntingford et al. (2008) “We analyse how the modelled vegetation cover in Amazonia responds to (i) uncertainty in the parameters specified in the atmosphere component of HadCM3 and their associated influence on predicted surface climate. … The potential for human-induced climate change to trigger the loss of Amazon rainforest appears robust within the context of the uncertainties explored in this paper.” [Full text]

Climate Change, Deforestation, and the Fate of the Amazon – Malhi et al. (2008) “The forest biome of Amazonia is one of Earth’s greatest biological treasures and a major component of the Earth system. This century, it faces the dual threats of deforestation and stress from climate change. Here, we summarize some of the latest findings and thinking on these threats, explore the consequences for the forest ecosystem and its human residents, and outline options for the future of Amazonia.” [Full text]

New views on an old forest: assessing the longevity, resilience and future of the Amazon rainforest – Maslin et al. (2005) “The aim of this paper is to investigate the longevity and diversity of the Amazonian rainforest and to assess its likely future. Palaeoclimate and palaeoecological records suggest that the Amazon rainforest originated in the late Cretaceous and has been a permanent feature of South America for at least the last 55 million years. The Amazon rainforest has survived the high temperatures of the Early Eocene climate optimum, the gradual Cenozoic cooling, and the drier and lower carbon dioxide levels of the Quaternary glacial periods. Two new theories for the great diversity of the Amazon rainforest are discussed – the canopy density hypothesis and the precessional-forced seasonality hypothesis. We suggest the Amazon rainforest should not be viewed as a geologically ephemeral feature of South America, but rather as a constant feature of the global Cenozoic biosphere. The forest is now, however, entering a set of climatic conditions with no past analogue. The predicted future hotter and more arid tropical climates may have a disastrous effect on the Amazon rainforest.” [Full text]

The role of ecosystem-atmosphere interactions in simulated Amazonian precipitation decrease and forest dieback under global climate warming – Betts et al. (2004) “A suite of simulations with the HadCM3LC coupled climate-carbon cycle model is used to examine the various forcings and feedbacks involved in the simulated precipitation decrease and forest dieback. Rising atmospheric CO₂ is found to contribute 20% to the precipitation reduction through the physiological forcing of stomatal closure, with 80% of the reduction being seen when stomatal closure was excluded and only radiative forcing by CO₂ was included. … The precipitation reduction is enhanced by 20% by the biogeophysical feedback, and 5% by the carbon cycle feedback from the forest dieback.” [Full text]

Contrasting simulated past and future responses of the Amazonian forest to atmospheric change – Cowling et al. (2004) “We contrasted HadCM3LC simulations of Amazonian forest at the last glacial maximum (LGM; 21 kyr ago) and a Younger Dryas–like period (13–12 kyr ago) with predicted responses of future warming to provide estimates of the climatic limits under which the Amazon forest remains relatively stable. Our simulations indicate that despite lower atmospheric CO₂ concentrations and increased aridity during the LGM, Amazonia remains mostly forested, and that the cooling climate of the Younger Dryas–like period in fact causes a trend toward increased above–ground carbon balance relative to today. … Although elevated atmospheric CO₂ contributes to a positive enhancement of plant carbon and water balance, decreased stomatal conductance and increased plant and soil respiration cause a positive feedback that amplifies localized drying and climate warming. We speculate that the Amazonian forest is currently near its critical resiliency threshold, and that even minor climate warming may be sufficient to promote deleterious feedbacks on forest integrity.” [Full text]

Using a GCM analogue model to investigate the potential for Amazonian forest dieback – Huntingford et al. (2004) “A combined GCM analogue model and GCM land surface representation is used to investigate the influences of climatology and land surface parameterisation on modelled Amazonian vegetation change. … The timing of forest dieback is found to be sensitive to the initial pre-industrial climate, as well as uncertainties in the representation of land-atmosphere CO₂ exchange. … Further advances are required in both GCM rainfall simulation and land-surface process representation before a clearer picture will emerge on the timing of possible Amazonian forest dieback.”

A crisis in the making: responses of Amazonian forests to land use and climate change – Laurance (1998) “At least three global-change phenomena are having major impacts on Amazonian forests: (1) accelerating deforestation and logging; (2) rapidly changing patterns of forest loss; and (3) interactions between human land-use and climatic variability. Additional alterations caused by climatic change, rising concentrations of atmospheric carbon dioxide, mining, overhunting and other large-scale phenomena could also have important effects on the Amazon ecosystem.”