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Papers on early 20th century warming

Posted by Ari Jokimäki on August 29, 2013

This is a list of papers on early 20th century warming. List contains both observational and theoretical studies. The list is not complete, and will most likely be updated in future in order to make it more thorough and more representative.

UPDATE (April 2, 2017): Johannessen et al. (2004) added.
UPDATE (April 15, 2015): Thompson et al. (2015) added.
UPDATE (April 8, 2014): Suo et al. (2013), Kelly et al. (1980), Petterssen (1949), Ahlmann (1948) added.

Early twentieth-century warming linked to tropical Pacific wind strength – Thompson et al. (2015)
“Of the rise in global atmospheric temperature over the past century, nearly 30% occurred between 1910 and 1940 when anthropogenic forcings were relatively weak. This early warming has been attributed to internal factors, such as natural climate variability in the Atlantic region, and external factors, such as solar variability and greenhouse gas emissions. However, the warming is too large to be explained by external factors alone and it precedes Atlantic warming by over a decade. For the late twentieth century, observations and climate model simulations suggest that Pacific trade winds can modulate global temperatures, but instrumental data are scarce in the early twentieth century. Here we present a westerly wind reconstruction (1894–1982) from seasonally resolved measurements of Mn/Ca ratios in a western Pacific coral that tracks interannual to multidecadal Pacific climate variability. We then reconstruct central Pacific temperatures using Sr/Ca ratios in a coral from Jarvis Island, and find that weak trade winds and warm temperatures coincide with rapid global warming from 1910 to 1940. In contrast, winds are stronger and temperatures cooler between 1940 and 1970, when global temperature rise slowed down. We suggest that variations in Pacific wind strength at decadal timescales significantly influence the rate of surface air temperature change.”
Diane M. Thompson, Julia E. Cole, Glen T. Shen, Alexander W. Tudhope & Gerald A. Meehl, Nature Geoscience 8, 117–121 (2015) doi:10.1038/ngeo2321.

External forcing of the early 20th century Arctic warming – Suo et al. (2013) “The observed Arctic warming during the early 20th century was comparable to present-day warming in terms of magnitude. The causes and mechanisms for the early 20th century Arctic warming are less clear and need to be better understood when considering projections of future climate change in the Arctic. The simulations using the Bergen Climate Model (BCM) can reproduce the surface air temperature (SAT) fluctuations in the Arctic during the 20th century reasonably well. The results presented here, based on the model simulations and observations, indicate that intensified solar radiation and a lull in volcanic activity during the 1920s–1950s can explain much of the early 20th century Arctic warming. The anthropogenic forcing could play a role in getting the timing of the peak warming correct. According to the model the local solar irradiation changes play a crucial role in driving the Arctic early 20th century warming. The SAT co-varied closely with local solar irradiation changes when natural external forcings are included in the model either alone or in combination with anthropogenic external forcings. The increased Barents Sea warm inflow and the anomalous atmosphere circulation patterns in the northern Europe and north Atlantic can also contribute to the warming. In summary, the early 20th century warming was largely externally forced.” Lingling Suo, Odd Helge Otterå, Mats Bentsen, Yongqi Gao, Ola M. Johannessen, Tellus A 2013, 65, 20578, http://dx.doi.org/10.3402/tellusa.v65i0.20578. [Full text]

Early 20th century warming in the Arctic: A review – Yamanouchi (2011) “From the 1920s to the 1940s, the Artic experienced significant warming that is comparable to the recent 30-year warming. The former warming was concentrated mostly in high latitudes, in contrast to the recent 30-year warming, which has occurred in all latitudes. Several explanations have been proposed; however, one of these proposed explanations, single external forcing, which could once explain the global average, failed to explain the early 20th century scenario. A second possible explanation was internal atmospheric variability with low frequency. Another candidate for the explanation was still forcing by black carbon deposited on snow and ice surfaces. The answer is most likely to be a combination of intrinsic internal natural climate variability and positive feedbacks that amplified the radiative and atmospheric forcing. We must continue our study by discovering historical data, analyzing ice cores, reanalyzing the Arctic system together with long-term reanalysis dating back to the 1880s, and also determine the contributions of each factor.” Takashi Yamanouchi, Polar Science, Volume 5, Issue 1, April 2011, Pages 53–71, http://dx.doi.org/10.1016/j.polar.2010.10.002.

Early 20th century Arctic warming in retrospect – Wood & Overland (2010) “The major early 20th century climatic fluctuation (∼1920–1940) has been the subject of scientific enquiry from the time it was detected in the 1920s. The papers of scientists who studied the event first-hand have faded into obscurity but their insights are relevant today. We review this event through a rediscovery of early research and new assessments of the instrumental record. Much of the inter-annual to decadal scale variability in surface air temperature (SAT) anomaly patterns and related ecosystem effects in the Arctic and elsewhere can be attributed to the superposition of leading modes of variability in the atmospheric circulation. Meridional circulation patterns were an important factor in the high latitudes of the North Atlantic during the early climatic fluctuation. Sea surface temperature (SST) anomalies that appeared during this period were congruent with low-frequency variability in the climate system but were themselves most likely the result of anomalous forcing by the atmosphere. The high-resolution data necessary to verify this hypothesis are lacking, but the consistency of multiple lines of evidence provides strong support. Our findings indicate that early climatic fluctuation is best interpreted as a large but random climate excursion imposed on top of the steadily rising global mean temperature associated with anthropogenic forcing.” Kevin R. Wood, James E. Overland, International Journal of Climatology, Volume 30, Issue 9, pages 1269–1279, July 2010, DOI: 10.1002/joc.1973.

Influence of volcanic activity and changes in solar irradiance on surface air temperatures in the early twentieth century – Shiogama et al. (2006) “Causes of the global surface air temperature warming in the early half of the 20th century are examined using a climate model and an optimal detection/attribution methodology. While the anthropogenic response seems to be underestimated in our model, our previous study detected the influence due to natural external forcing, including the combined effects of solar irradiance changes and the recovery from large volcanic activity. We further partition the responses between these two natural external factors, detecting both the solar and the volcanic signal in the observed early warming. A diagnosis of the sensitivity to solar forcing and a volcanic super-eruption simulation suggest that our model possesses larger climate sensitivities to solar forcing and longer relaxation times to volcanic forcing than HadCM3, enabling us to detect both the solar and volcanic forcing responses.” Hideo Shiogama, Tatsuya Nagashima, Tokuta Yokohata, Simon A. Crooks, Toru Nozawa, Geophysical Research Letters, Volume 33, Issue 9, May 2006, DOI: 10.1029/2005GL025622.

Detecting natural influence on surface air temperature change in the early twentieth century – Nozawa et al. (2005) “We analyze surface air temperature datasets simulated by a coupled climate model forced with different external forcings, to diagnose the relative importance of these forcings to the observed warming in the early 20th century. The geographical distribution of linear temperature trends in the simulations forced only by natural contributions (volcanic eruptions and solar variability) shows better agreement with observed trends than that does the simulations forced only by well-mixed greenhouse gases. Using an optimal fingerprinting technique we robustly detect a significant natural contribution to the early 20th century warming. In addition, the amplitude of our simulated natural signal is consistent with the observations. Over the same period, however, we could not detect a greenhouse gas signal in the observed surface temperature in the presence of the external natural forcings. Hence our analysis suggests that external natural factors caused more warming in the early 20th century than anthropogenic factors.” Toru Nozawa, Tatsuya Nagashima, Hideo Shiogama, Simon A. Crooks, Geophysical Research Letters, Volume 32, Issue 20, October 2005, DOI: 10.1029/2005GL023540. [Full text]

Arctic climate change: observed and modelled temperature and sea-ice variability – Johannessen et al. (2004) “Changes apparent in the arctic climate system in recent years require evaluation in a century-scale perspective in order to assess the Arctic’s response to increasing anthropogenic greenhouse-gas forcing. Here, a new set of century- and multidecadal-scale observational data of surface air temperature (SAT) and sea ice is used in combination with ECHAM4 and HadCM3 coupled atmosphere–ice–ocean global model simulations in order to better determine and understand arctic climate variability. We show that two pronounced twentieth-century warming events, both amplified in the Arctic, were linked to sea-ice variability. SAT observations and model simulations indicate that the nature of the arctic warming in the last two decades is distinct from the early twentieth-century warm period. It is suggested strongly that the earlier warming was natural internal climate-system variability, whereas the recent SAT changes are a response to anthropogenic forcing. The area of arctic sea ice is furthermore observed to have decreased ∼8 × 105 km2 (7.4%) in the past quarter century, with record-low summer ice coverage in September 2002. A set of model predictions is used to quantify changes in the ice cover through the twenty-first century, with greater reductions expected in summer than winter. In summer, a predominantly sea-ice-free Arctic is predicted for the end of this century.” Johannessen, O. M., Bengtsson, L., Miles, M. W., Kuzmina, S. I., Semenov, V. A., Alekseev, G. V., Nagurnyi, A. P., Zakharov, V. F., Bobylev, L. P., Pettersson, L. H., Hasselmann, K. and Cattle, H. P. (2004), Arctic climate change: observed and modelled temperature and sea-ice variability. Tellus A, 56: 328–341. doi:10.1111/j.1600-0870.2004.00060.x. [Full text]

The Early Twentieth-Century Warming in the Arctic—A Possible Mechanism – Bengtsson et al. (2004) “The huge warming of the Arctic that started in the early 1920s and lasted for almost two decades is one of the most spectacular climate events of the twentieth century. During the peak period 1930–40, the annually averaged temperature anomaly for the area 60°–90°N amounted to some 1.7°C. Whether this event is an example of an internal climate mode or is externally forced, such as by enhanced solar effects, is presently under debate. This study suggests that natural variability is a likely cause, with reduced sea ice cover being crucial for the warming. A robust sea ice–air temperature relationship was demonstrated by a set of four simulations with the atmospheric ECHAM model forced with observed SST and sea ice concentrations. An analysis of the spatial characteristics of the observed early twentieth-century surface air temperature anomaly revealed that it was associated with similar sea ice variations. Further investigation of the variability of Arctic surface temperature and sea ice cover was performed by analyzing data from a coupled ocean–atmosphere model. By analyzing climate anomalies in the model that are similar to those that occurred in the early twentieth century, it was found that the simulated temperature increase in the Arctic was related to enhanced wind-driven oceanic inflow into the Barents Sea with an associated sea ice retreat. The magnitude of the inflow is linked to the strength of westerlies into the Barents Sea. This study proposes a mechanism sustaining the enhanced westerly winds by a cyclonic atmospheric circulation in the Barents Sea region created by a strong surface heat flux over the ice-free areas. Observational data suggest a similar series of events during the early twentieth-century Arctic warming, including increasing westerly winds between Spitsbergen and Norway, reduced sea ice, and enhanced cyclonic circulation over the Barents Sea. At the same time, the North Atlantic Oscillation was weakening.” Bengtsson, Lennart, Vladimir A. Semenov, Ola M. Johannessen, 2004: The Early Twentieth-Century Warming in the Arctic—A Possible Mechanism. J. Climate, 17, 4045–4057. doi: http://dx.doi.org/10.1175/1520-0442(2004)0172.0.CO;2. [Full text]

Solar and Greenhouse Gas Forcing and Climate Response in the Twentieth Century – Meehl et al. (2003) “Ensemble experiments with a global coupled climate model are performed for the twentieth century with time-evolving solar, greenhouse gas, sulfate aerosol (direct effect), and ozone (tropospheric and stratospheric) forcing. Observed global warming in the twentieth century occurred in two periods, one in the early twentieth century from about the early 1900s to the 1940s, and one later in the century from, roughly, the late 1960s to the end of the century. The model’s response requires the combination of solar and anthropogenic forcing to approximate the early twentieth-century warming, while the radiative forcing from increasing greenhouse gases is dominant for the response in the late twentieth century, confirming previous studies. Of particular interest here is the model’s amplification of solar forcing when this acts in combination with anthropogenic forcing. This difference is traced to the fact that solar forcing is more spatially heterogeneous (i.e., acting most strongly in areas where sunlight reaches the surface) while greenhouse gas forcing is more spatially uniform. Consequently, solar forcing is subject to coupled regional feedbacks involving the combination of temperature gradients, circulation regimes, and clouds. The magnitude of these feedbacks depends on the climate’s base state. Over relatively cloud-free oceanic regions in the subtropics, the enhanced solar forcing produces greater evaporation. More moisture then converges into the precipitation convergence zones, intensifying the regional monsoon and Hadley and Walker circulations, causing cloud reductions over the subtropical ocean regions, and, hence, more solar input. An additional response to solar forcing in northern summer is an enhancement of the meridional temperature gradients due to greater solar forcing over land regions that contribute to stronger West African and South Asian monsoons. Since the greenhouse gases are more spatially uniform, such regional circulation feedbacks are not as strong. These regional responses are most evident when the solar forcing occurs in concert with increased greenhouse gas forcing. The net effect of enhanced solar forcing in the early twentieth century is to produce larger solar-induced increases of tropical precipitation when calculated as a residual than for early century solar-only forcing, even though the size of the imposed solar forcing is the same. As a consequence, overall precipitation increases in the early twentieth century in the Asian monsoon regions are greater than late century increases, qualitatively consistent with observed trends in all-India rainfall. Similar effects occur in West Africa, the tropical Pacific, and the Southern Ocean tropical convergence zones.” Meehl, Gerald A., Warren M. Washington, T. M. L. Wigley, Julie M. Arblaster, Aiguo Dai, 2003: Solar and Greenhouse Gas Forcing and Climate Response in the Twentieth Century. J. Climate, 16, 426–444. doi: http://dx.doi.org/10.1175/1520-0442(2003)0162.0.CO;2. [Full text]

Estimation of natural and anthropogenic contributions to twentieth century temperature change – Tett et al. (2002) “Using a coupled atmosphere/ocean general circulation model, we have simulated the climatic response to natural and anthropogenic forcings from 1860 to 1997. The model, HadCM3, requires no flux adjustment and has an interactive sulphur cycle, a simple parameterization of the effect of aerosols on cloud albedo (first indirect effect), and a radiation scheme that allows explicit representation of well-mixed greenhouse gases. Simulations were carried out in which the model was forced with changes in natural forcings (solar irradiance and stratospheric aerosol due to explosive volcanic eruptions), well-mixed greenhouse gases alone, tropospheric anthropogenic forcings (tropospheric ozone, well-mixed greenhouse gases, and the direct and first indirect effects of sulphate aerosol), and anthropogenic forcings (tropospheric anthropogenic forcings and stratospheric ozone decline). Using an “optimal detection” methodology to examine temperature changes near the surface and throughout the free atmosphere, we find that we can detect the effects of changes in well-mixed greenhouse gases, other anthropogenic forcings (mainly the effects of sulphate aerosols on cloud albedo), and natural forcings. Thus these have all had a significant impact on temperature. We estimate the linear trend in global mean near-surface temperature from well-mixed greenhouse gases to be 0.9 ± 0.24 K/century, offset by cooling from other anthropogenic forcings of 0.4 ± 0.26 K/century, giving a total anthropogenic warming trend of 0.5 ± 0.15 K/century. Over the entire century, natural forcings give a linear trend close to zero. We found no evidence that simulated changes in near-surface temperature due to anthropogenic forcings were in error. However, the simulated tropospheric response, since the 1960s, is ∼50% too large. Our analysis suggests that the early twentieth century warming can best be explained by a combination of warming due to increases in greenhouse gases and natural forcing, some cooling due to other anthropogenic forcings, and a substantial, but not implausible, contribution from internal variability. In the second half of the century we find that the warming is largely caused by changes in greenhouse gases, with changes in sulphates and, perhaps, volcanic aerosol offsetting approximately one third of the warming. Warming in the troposphere, since the 1960s, is probably mainly due to anthropogenic forcings, with a negligible contribution from natural forcings.” Simon F. B. Tett, Gareth S. Jones, Peter A. Stott, David C. Hill, John F. B. Mitchell, Myles R. Allen, William J. Ingram, Tim C. Johns, Colin E. Johnson, Andy Jones, David L. Roberts, David M. H. Sexton, Margaret J. Woodage, Journal of Geophysical Research: Atmospheres (1984–2012), Volume 107, Issue D16, pages ACL 10-1–ACL 10-24, 27 August 2002, DOI: 10.1029/2000JD000028. [Full text]

Simulation of Early 20th Century Global Warming – Delworth & Knutson (2000) “The observed global warming of the past century occurred primarily in two distinct 20-year periods, from 1925 to 1944 and from 1978 to the present. Although the latter warming is often attributed to a human-induced increase of greenhouse gases, causes of the earlier warming are less clear because this period precedes the time of strongest increases in human-induced greenhouse gas (radiative) forcing. Results from a set of six integrations of a coupled ocean-atmosphere climate model suggest that the warming of the early 20th century could have resulted from a combination of human-induced radiative forcing and an unusually large realization of internal multidecadal variability of the coupled ocean-atmosphere system. This conclusion is dependent on the model’s climate sensitivity, internal variability, and the specification of the time-varying human-induced radiative forcing.” Thomas L. Delworth, Thomas R. Knutson, Science 24 March 2000: Vol. 287 no. 5461 pp. 2246-2250, DOI: 10.1126/science.287.5461.2246.

Changes in atmospheric circulation over northern hemisphere oceans associated with the rapid warming of the 1920s – Fu et al. (1999)
Global mean surface temperature has increased since the late 19th century. The warming occurred largely during two periods: 1920–1940, and since the mid-1970s. Although most recent studies have focused on the latter period, it is of interest to analyse the earlier period and compare its major features to the recent warming episode. The warming during 1920–1940 occurred most rapidly during the 1920s. It was strongest at high northern latitudes in winter, a pattern now believed to be characteristic of ‘greenhouse warming’. This warming of the Arctic was much discussed during the 1930s and 1940s, but the data available at that time were mostly derived from land areas. In this paper, we use the COADS marine data set and recent compilations of land surface temperature data sets to examine the behaviour of the surface fields over the ocean during this event. Considering the thermal and atmospheric fields at the surface, the strongest signal occurs in the North Atlantic Ocean during winter, being distinct but more gradual in the other oceans and seasons. The Northern Hemisphere continental record shows that both middle and high latitudes experienced rapid warming in the early 20th century warming interval (the 1920s and 1930s, hereafter referred to as ETCW). Temperature data for northern tropics, while displaying similar general characteristics, exhibit some differences with regard to timing and rates of change. There is a suggestion of weakening of the westerlies and the trade wind system in the 1930s, following an intensification of the westerlies across the North Atlantic during the previous two decades. This weakening may be related to a lessening of atmospheric baroclinicity in association with the fact that the amplitude of warming at high latitudes was much greater than that in low latitudes, reducing the mean meridional thermal gradient, and therefore the geostrophic pressure gradient. There is some indication that the North Atlantic and North Pacific high-pressure systems shifted northward. Coincident with this northward shift of the subtropical highs, typhoons in the Northwest Pacific and hurricanes in the North Atlantic became more numerous in this period of rising temperature, which we suggest is linked to a northward shift of the respective near-equatorial convergence zones. Concomitant to the weakening of the westerlies and trade wind systems, the Asian monsoon troughs deepened substantially, a situation generally favourable to the development of active monsoons. It is thought that the combination of these two features—enhanced continental monsoons and implied lowered vertical wind shear over the oceans—would tend to enhance the release of latent heat in the tropics, representing strengthened Hadley and Walker circulations, which may have been at least partly responsible for greater aridity in subtropical land areas of both hemispheres during this period. The latter is also consistent with an expansion and/or strengthening of the subtropical high-pressure belt into the continents.” Congbin Fu, Henry F. Diaz, Dongfeng Dong, Joseph O. Fletcher, International Journal of Climatology, Volume 19, Issue 6, pages 581–606, May 1999, DOI: 10.1002/(SICI)1097-0088(199905)19:63.0.CO;2-P.

Solar Forcing of Global Climate Change Since The Mid-17th Century – Reid et al. (1997) “Spacecraft measurements of the sun’s total irradiance since 1980 have revealed a long-term variation that is roughly in phase with the 11-year solar cycle. Its origin is uncertain, but may be related to the overall level of solar magnetic activity as well as to the concurrent activity on the visible disk. A low-pass Gaussian filtered time series of the annual sunspot number has been developed as a suitable proxy for solar magnetic activity that contains a long-term component related to the average level of activity as well as a short-term component related to the current phase of the 11-year cycle. This time series is also assumed to be a proxy for solar total irradiance, and the irradiance is reconstructed for the period since 1617 based on the estimate from climatic evidence that global temperatures during the Maunder Minimum of solar activity, which coincided with one of the coldest periods of the Little Ice Age, were about 1 °C colder than modern temperatures. This irradiance variation is used as the variable radiative forcing function in a one-dimensional ocean–climate model, leading to a reconstruction of global temperatures over the same period, and to a suggestion that solar forcing and anthropogenic greenhouse-gas forcing made roughly equal contributions to the rise in global temperature that took place between 1900 and 1955. The importance of solar variability as a factor in climate change over the last few decades may have been underestimated in recent studies.” George C. Reid, Climatic Change, October 1997, Volume 37, Issue 2, pp 391-405, DOI: 10.1023/A:1005307009726.

Changes in Global Surface Temperature From 1880 to 1977 Derived From Historical Records of Sea Surface Temperature – Paltridge & Woodruff (1981) “A preliminary analysis based primarily on historical records of sea surface temperature (SST) gives estimates of the change since 1880 of global, hemispheric and zonal average surface temperatures. The global change with time is roughly similar in shape and magnitude to that derived by Mitchell from land station data alone, but lags the Mitchell curve by 10-20 years. That is, the present data show a minimumof temperature somewhere between 1900 and 1925 and a maximum somewhere between 1945 and 1970. Comparing the means of these 25-year periods, the rise from minimum to maximum was (roughly) 0.6 K for the Northern Hemisphere and 0.9 K for the Southern Hemisphere. Comparing the means of the 50 years before 1930 and the 48 years from 1930 to 1977, the rise was 0.3 K for the Northern Hemisphere and 0.6 K for the Southern Hemisphere. The figures do not take into account the polar regions which, on linear extrapolation from lower latitudes, may have risen in temperature by twice the hemispheric averages. The temperature of the tropical zone (l0°N-10°S) has not changed over the years, so that the meridionaltemperature gradient has decreased in both hemispheres. The detail of the various conclusions may be revised later in the light of further analysis of the errors associated with the SST data sets. This furtheranalysis is underway at the Environmental Research Laboratories of NOAA.” Paltridge, G., S. Woodruff, 1981: Changes in Global Surface Temperature From 1880 to 1977 Derived From Historical Records of Sea Surface Temperature. Mon. Wea. Rev., 109, 2427–2434. doi: http://dx.doi.org/10.1175/1520-0493(1981)1092.0.CO;2. [Full text]

Variations in Surface Air Temperatures: Part 2. Arctic Regions, 1881–1980 – Kelly et al. (1980) “We describe annual and seasonal changes in air temperatures over high latitudes of the Northern Hemisphere during the period 1881–1980. Trends (that is, fluctuations on time scales greater than 20 years) in the average temperature of the Arctic are compared with those of the Northern Hemisphere. Seasonal and regional departures from the long-term trends in the average temperature of the Arctic are identified. Spatial patterns of variation in the Arctic temperature field are determined by principal component analysis and the major characteristics of the time series of the dominant patterns are summarized. Trends in Arctic temperatures have been broadly similar to those for the Northern Hemisphere during the study period. The Arctic variations were, however, greater in magnitude and more rapid. The spatial pattern of change associated with the trend in Arctic temperatures is clearly identified by principal component analysis. It shows that the trends have, in general, been Arctic-wide, but that certain regions are particularly sensitive to long-term variations, most notably northwest Greenland and around the Kara Sea. There is some evidence that the pattern of Arctic cooling that occurred after 1940 was more complex than the warming that affected the whole Arctic during the 1920’s and 1930’s. Warming of the Arctic has occurred during the 1970’s, but is not yet of sufficient duration to be considered long term, except, perhaps, in spring. The average temperature of the Arctic during the 1970’s was equal to that of the 1960’s, indicating a cessation of the long-term cooling trend but not, as yet, a shift to long-term warming. Short-term variations in temperature appear to be most pronounced close to major regions of sea-ice production and decay.” Kelly, P. M., P. D. Jones, C. B. Sear, B. S. G. Cherry, R. K. Tavakol, 1982: Variations in Surface Air Temperatures: Part 2. Arctic Regions, 1881–1980. Mon. Wea. Rev., 110, 71–83. doi: http://dx.doi.org/10.1175/1520-0493(1982)1102.0.CO;2. [Full text]

Temperature fluctuations and trends over the earth – Callendar (1961) “The annual temperature deviations at over 400 meteorological stations are combined on a regional basis to give the integrated fluctuations over large areas and zones. These are shown in graphical form, and it is concluded that a solar or atmospheric dust hypothesis is necessary to explain the world-wide fluctuations of a few years duration. An important change in the relationships of the zonal fluctuations has occurred since 1920. The overall temperature trends found from the data are considered in relation to the homogeneity of recording, and also to the evidence of glacial recession in different zones. It is concluded that the rising trend, shown by the instruments during recent decades, is significant from the Arctic to about 45°S lat., but quite small in most regions below 35°N. and not yet apparent in some. It is thought that the regional and zonal distribution of recent climatic trends is incompatible with the hypothesis of increased solar heating as the cause. On the other hand, the major features of this distribution are not incompatible with the hypothesis of increased carbon dioxide radiation, if the rate of atmospheric mixing between the hemispheres is a matter of decades rather than years.” G. S. Callendar, Quarterly Journal of the Royal Meteorological Society, Volume 87, Issue 371, pages 1–12, January 1961, DOI: 10.1002/qj.49708737102.

Changes in the General Circulation Associated with the Recent Climatic Variation – Petterssen (1949) No abstract. S. Petterssen, Geografiska Annaler, Vol. 31, Glaciers and Climate: Geophysical and Geomorphological Essays (1949), pp. 212-221.

The Present Climatic Fluctuation – Ahlmann (1948) No abstract. Hans W:Son Ahlmann, The Geographical Journal, Vol. 112, No. 4/6 (Oct. – Dec., 1948), pp. 165-193.

The artificial production of carbon dioxide and its influence on temperature – Callendar (1938) “By fuel combustion man has added about 150,000 million tons of carbon dioxide to the air during the past half century. The author estimates from the best available data that approximately three quarters of this has remained in the atmosphere. The radiation absorption coefficients of carbon dioxide and water vapour are used to show the effect of carbon dioxide on “sky radiation.” From this the increase in mean temperature, due to the artificial production of carbon dioxide, is estimated to be at the rate of 0.003°C. per year at the present time. The temperature observations at 200 meteorological stations are used to show that world temperatures have actually increased at an average rate of 0.005°C. per year during the past half century.” G. S. Callendar, Quarterly Journal of the Royal Meteorological Society, Volume 64, Issue 275, pages 223–240, April 1938, DOI: 10.1002/qj.49706427503. [Full text]

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Global warming has not stopped

Posted by Ari Jokimäki on August 19, 2013

The apparent lack of warming in Earth’s surface temperature measurements since 1998 is not yet significant from climatic perspective. Surface temperature also seems to be changing according to IPCC projections. Climate model simulations show similar warming breaks, and have done so even before current break started, even if they include the effect of carbon dioxide. Models also can re-create the current break and the cause for the break seems to be known: warming has gone to the oceans instead of warming the surface. The ocean warming has been observed. Also the continuing warming effect of greenhouse gases has been observed. Global warming as a whole seems to continue despite the apparent break in surface measurements.

Several decades of long-term warming is evident in recent surface temperature measurements. However, since 1998 surface temperature records don’t show clear warming. This is not very clear because the time period is not very long and the possible trends might not be statistically significant. Skeptical Science trend calculator shows warming trends since 1998 but they are not statistically significant. Santer and others (2011) estimated that it takes 17 years of satellite measurements, before effect of mankind to lower atmosphere can be detected. In some cases, 15 years has been mentioned for the limit of statistical significance, so the situation seems to be quite borderline. For example, from 1983 and 1998, which was time of rapid warming, SkS trend calculator still shows a trend that is not statistically significant. From 1982 to 1998 trend is significant. (Even if statistically enough time would pass without warming, it still wouldn’t mean that increases in greenhouse gases wouldn’t have a warming effect. This we will see below in more detail.)

Climate is usually considered as average weather over longer period of time. Standard length for the climatic time period is 30 years. Let us see what this means for the surface temperature. Following figure shows Earth’s surface temperature as a running 30 year mean (this means that the value of each point in the graph is the average of the surrounding 30 years of temperature values, for example, the running 30 year mean value for year 1990 is the average of temperatures of 30 years between 1975 and 2004):

30yrTemp

As we can see from the graph, there are no signs of global warming stopping or even slowing down in this kind of inspection. From climatic perspective global warming still continues. Those with sharp eyes notice that the time shown in X axis stops in the graph before 2000. This is because in 30 year running mean the year 1998 is currently last one that can be shown. However, the temperature evolution after 1998 is included and is affecting the graph starting from 1983. The fact that graph stops at 1998 means that we have to wait 15 years before we know how climate has evolved since 1998.

Although global surface temperature evolution since 1998 is not yet climatically important, there has been quite a lot of research on the issue. One question appearing in public has been that does the temperature follow earlier projections. Rahmstorf and others (2012) have analyzed how IPCC projections match the surface temperature measurements. Here is a graph of their results:

1998ipcc
The evolution of Earth’s surface temperature and IPCC projections. Surface temperature without corrections is shown in pink (one year running mean averaged from all surface temperature analyses) and corrected surface temperature is shown in red (corrections are explained in text). Blue area and blue lines are IPCC third assessment report projections. Green area and green lines are IPCC fourth assessment report (AR4) projections.

As we can see from the graph, surface temperature changes shown in pink sometimes go outside the projections. This is because some factors are not included in IPCC projections. Such factors are solar activity changes and eruptions of volcanoes. Additionally, the variation of El Niño/La Niña is random and therefore it doesn’t change in simulations at the same time it does in real life. IPCC projections are combined results from many simulations, so the El Niño/La Niña variations of different simulations tend to cancel out when simulation results are combined. This means that the projections don’t actually include El Niño/La Niña variation either. It should be noted that even if surface temperature shown in pink doesn’t stay within limits of projections, it does stay within limits of all individual simulations (not shown in the Figure above).

The effects of the Sun, volcanoes, and El Niño/La Niña variation have been corrected for in the temperature evolution shown in red. This graph stays quite well within limits of projections especially during last few years. It sometimes goes outside the projections in the beginning of the time period. There still might be some factors which would need to be corrected for. On the average it does seem to follow the projections even in the beginning of the time period (at least it doesn’t deviate permanently to one direction).

Already happened temperature evolution can be recreated with models also so that internal variability of climate system and changes in solar activity and in volcanoes are included (afterwards we have knowledge for example when a volcanic eruption has happened). Lean & Rind (2009) have done this and the following Figure shows their results:

1998lean
a) The observed surface temperature (black) and simulation of surface temperature from a simple model (orange). b) The factors affecting surface temperature. Graphs are from Lean & Rind (2009).

The result of the simple model shown in the graph is so close to observed surface temperature evolution that we have good reason to suspect this study might be able to answer us why surface temperature has not apparently increased since 1998. Lower part of the Figure shows the factors affecting surface temperature, and from there we can see that ENSO (that is, the variation of El Niño and La Niña) seems to have varied in quite a similar manner than surface temperature in the period in question. Also the cause of the longer-term temperature rise seems to be clear: the effect of mankind is the only one of the factors, which shows long-term increase.

Also the future projections of climate models show periods where surface temperature doesn’t increase, even if models have the effect of greenhouse gases included. Here are some examples of such simulations:

1998malli
Projections of climate models show periods of slowed/paused warming similar to that in the observations since 1998. On the left: simulations with three different emission scenarios from IPCC AR4. Upper right: simulation example from Easterling & Wehner (2009). Lower right: simulation example from Meehl and others (2011).

All shown simulation examples show periods where long-term warming trend is paused even for decades, and warming continues after that. The Figure also shows simulation examples from model runs for IPCC AR4 projections. These show similar pauses. AR4 discusses the expected temperature evolution rather carelessly. From the texts of the report one might get an impression that surface temperature should rise certain amount in each decade. They of course mean that on average certain rise in temperature is expected per decade, even if it doesn’t occur during each decade. Some people have used the carelessly worded texts in IPCC AR4 to distort the issue, even when the same report shows the simulations presented above, where the truth in the matter can be seen.

Simulation examples shown above are all quite recent. Model simulations have shown similar features also earlier. Here we see an example from IPCC second assessment report (SAR), which was published in 1995, before current warming pause apparently started:

1998ipcc2
Model simulations from IPCC second assessment report. Year 0 means year 1990.

One interesting detail in the SAR model simulations is that one of them shows strong spike around 1995. This is comparable to the 1998 peak in observed temperatures. Even if the post-1995 evolution is difficult to see in the graph, we can be sure that the simulation in question shows quite long pause in the warming after 1995. In principle, we could say that the simulation in question predicted the warming pause, albeit being off by few years. It’s not genuine prediction of course, but just a coincidence. Nevertheless, it’s a quite curious detail.

So, the simulations of climate models show clearly that while the increase of greenhouse gases in the atmosphere increases Earth’s surface temperature in the long run, other factors cause pauses to the warming every now and then. Similarly, those other factors also speed up the warming in other times. This has been explicitly stated in Easterling & Wehner (2009): “We show that the climate over the 21st century can and likely will produce periods of a decade or two where the globally averaged surface air temperature shows no trend or even slight cooling in the presence of longer-term warming”.

At the moment it seems that the factor causing the pause has been ENSO, which in practice means that the warming effect of greenhouse gases has gone deeper to the oceans instead of warming the surface. This has been the subject of some recent studies.

Already shown above were the results of Lean & Rind (2009) and Meehl and others (2011). Similar results have also been reported by Kaufmann and others (2011), Hunt (2011), Guemas and others (2013), and Watanabe and others (2013). According to all these studies, the primary cause for the apparent pause in the surface warming is that the warming has gone to the oceans. Solar activity has also been said to have played some role on the issue. Also, Solomon and others (2010) have suggested that changes in water vapor content in the atmosphere might have speeded up the warming during the 1990’s and slowed the warming during 2000’s.

The warming going into oceans has also been observed. Following Figure shows the ocean heat content in the top layer (0-700 m) of the oceans (from Lyman and others, 2010):

1998meri

The graph shows that after 1998 the heat content in the oceans has increased substantially.

But when can we expect surface warming to continue? Surface warming continues when the sum of all factors affecting Earth’s surface temperature has a warming effect. It can take decades and decades, as long as there are other factors that have large enough cooling effect to mask the warming effect from greenhouse gases. However, our current knowledge suggests that there are no such factors that could have large enough cooling effect in order to make this pause much longer.

There are reasons to think that warming might continue soon. Earth’s surface temperature has been very high recently, close to record temperatures, while solar activity has been very low and La Niña has been the prevailing state of ENSO. Without the effect of greenhouse gases these factors would have cooled Earth’s surface substantially. We haven’t seen such cooling. When La Niña changes to El Niño, it is expected that warming will continue.

So it seems that the warming effect of greenhouse gases seems to be still there. Fortunately, we don’t need to guess this, as we also have observations of the effect of greenhouse gases, as we see next.

A group of researchers have studied spectral measurements of outgoing long-wave radiation taken from satellites (Chapman and others, 2013). They found out that the warming effect of carbon dioxide has continued to increase during the 2000’s. They calculated from the spectral measurements that between 2002 and 2012 the amount of outgoing long-wave radiation decreased in the characteristic absorption frequencies of greenhouse gases. This was the case at least for carbon dioxide, ozone, and methane. Largest warming effect was from carbon dioxide. Observed decreases in the outgoing long-wave radiation matched the expectation from the increased greenhouse gas concentrations during the study period. Here are their results in a graph:

1998olr

It should be noted that the study of Chapman and others was presented in a conference in April 2013, and apparently official research paper has not been published yet. Information presented here is from the conference paper.

References:

IPCC second assessment report (over 50 MB PDF file, the graph shown here is on the PDF page 314, page 300 of the report).

IPCC AR4 simulations: Figure 10.5 with caption.

D. Chapman, P. Nguyen, M. Halem, A decade of measured greenhouse forcings from AIRS, Proc. SPIE 8743, Algorithms and Technologies for Multispectral, Hyperspectral, and Ultraspectral Imagery XIX, 874313 (May 18, 2013); doi:10.1117/12.2017019. [abstract]

John A. Church, Neil J. White, Leonard F. Konikow, Catia M. Domingues, J. Graham Cogley, Eric Rignot, Jonathan M. Gregory, Michiel R. van den Broeke, Andrew J. Monaghan, Isabella Velicogna, Revisiting the Earth’s sea-level and energy budgets from 1961 to 2008, Geophysical Research Letters, Volume 38, Issue 18, 28 September 2011, DOI: 10.1029/2011GL048794. [abstract, full text]

David R. Easterling, Michael F. Wehner, 2009, Is the climate warming or cooling? Geophysical Research Letters, Volume 36, Issue 8, April 2009, DOI: 10.1029/2009GL037810. [abstract, full text]

Virginie Guemas, Francisco J. Doblas-Reyes, Isabel Andreu-Burillo & Muhammad Asif, Retrospective prediction of the global warming slowdown in the past decade, Nature Climate Change, 3, 649–653 (2013) doi:10.1038/nclimate1863. [abstract]

B. G. Hunt, The role of natural climatic variation in perturbing the observed global mean temperature trend, Climate Dynamics, February 2011, Volume 36, Issue 3-4, pp 509-521, DOI: 10.1007/s00382-010-0799-x. [abstract]

Robert K. Kaufmann, Heikki Kauppi, Michael L. Mann, and James H. Stock, Reconciling anthropogenic climate change with observed temperature 1998–2008, PNAS July 19, 2011 vol. 108 no. 29 11790-11793, doi: 10.1073/pnas.1102467108. [abstract, full text]

John M. Lyman, Simon A. Good, Viktor V. Gouretski, Masayoshi Ishii, Gregory C. Johnson, Matthew D. Palmer, Doug M. Smith, & Josh K. Willis, Robust warming of the global upper ocean, Nature 465, 334–337 (20 May 2010) doi:10.1038/nature09043. [abstract, full text]

Gerald A. Meehl, Julie M. Arblaster, John T. Fasullo, Aixue Hu & Kevin E. Trenberth, 2011, Model-based evidence of deep-ocean heat uptake during surface-temperature hiatus periods, Nature Climate Change, 1, 360–364 (2011) doi:10.1038/nclimate1229. [abstract, full text]

Stefan Rahmstorf et al 2012, Comparing climate projections to observations up to 2011, Environ. Res. Lett. 7 044035 doi:10.1088/1748-9326/7/4/044035. [abstract, full text]

B. D. Santer, C. Mears, C. Doutriaux, P. Caldwell, P. J. Gleckler, T. M. L. Wigley, S. Solomon, N. P. Gillett, D. Ivanova, T. R. Karl, J. R. Lanzante, G. A. Meehl, P. A. Stott, K. E. Taylor, P. W. Thorne, M. F. Wehner, F. J. Wentz, 2011, Separating signal and noise in atmospheric temperature changes: The importance of timescale, Journal of Geophysical Research: Atmospheres (1984–2012), Volume 116, Issue D22, November 2011, DOI: 10.1029/2011JD016263. [abstract, full text]

Susan Solomon, Karen H. Rosenlof, Robert W. Portmann, John S. Daniel, Sean M. Davis, Todd J. Sanford, Gian-Kasper Plattner, Contributions of Stratospheric Water Vapor to Decadal Changes in the Rate of Global Warming, Science 5 March 2010: Vol. 327 no. 5970 pp. 1219-1223, DOI: 10.1126/science.1182488. [abstract, full text]

Masahiro Watanabe, Youichi Kamae, Masakazu Yoshimori, Akira Oka, Makiko Sato, Masayoshi Ishii, Takashi Mochizuki, Masahide Kimoto, Strengthening of ocean heat uptake efficiency associated with the recent climate hiatus, Geophysical Research Letters, Volume 40, Issue 12, pages 3175–3179, 28 June 2013, DOI: 10.1002/grl.50541. [abstract]

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Papers on global surface temperature since 1998

Posted by Ari Jokimäki on August 2, 2013

This is a list of papers on global surface temperature since 1998. The list is not complete, and will most likely be updated in future in order to make it more thorough and more representative.

UPDATE (February 22, 2014): England et al. (2014) added.
UPDATE (November 14, 2013): Otto et al. (2013) and Cowtan & Way (2013) added.
UPDATE (October 3, 2013): Kosaka & Xie (2013) and Fyfe et al. (2013) added.

See also the list on global surface temperature for additional relevant papers.

Recent intensification of wind-driven circulation in the Pacific and the ongoing warming hiatus – England et al. (2014) “Despite ongoing increases in atmospheric greenhouse gases, the Earth’s global average surface air temperature has remained more or less steady since 2001. A variety of mechanisms have been proposed to account for this slowdown in surface warming. A key component of the global hiatus that has been identified is cool eastern Pacific sea surface temperature, but it is unclear how the ocean has remained relatively cool there in spite of ongoing increases in radiative forcing. Here we show that a pronounced strengthening in Pacific trade winds over the past two decades—unprecedented in observations/reanalysis data and not captured by climate models—is sufficient to account for the cooling of the tropical Pacific and a substantial slowdown in surface warming through increased subsurface ocean heat uptake. The extra uptake has come about through increased subduction in the Pacific shallow overturning cells, enhancing heat convergence in the equatorial thermocline. At the same time, the accelerated trade winds have increased equatorial upwelling in the central and eastern Pacific, lowering sea surface temperature there, which drives further cooling in other regions. The net effect of these anomalous winds is a cooling in the 2012 global average surface air temperature of 0.1–0.2 °C, which can account for much of the hiatus in surface warming observed since 2001. This hiatus could persist for much of the present decade if the trade wind trends continue, however rapid warming is expected to resume once the anomalous wind trends abate.” Matthew H. England, Shayne McGregor, Paul Spence, Gerald A. Meehl, Axel Timmermann, Wenju Cai, Alex Sen Gupta, Michael J. McPhaden, Ariaan Purich & Agus Santoso, Nature Climate Change (2014), doi:10.1038/nclimate2106. [Full text]

Coverage bias in the HadCRUT4 temperature series and its impact on recent temperature trends – Cowtan & Way (2013) “Incomplete global coverage is a potential source of bias in global temperature reconstructions if the unsampled regions are not uniformly distributed over the planet’s surface. The widely used HadCRUT4 dataset covers on average about 84% of the globe over recent decades, with the unsampled regions being concentrated at the poles and over Africa. Three existing reconstructions with near-global coverage are examined, each suggesting that HadCRUT4 is subject to bias due to its treatment of unobserved regions. Two alternative approaches for reconstructing global temperatures are explored, one based on an optimal interpolation algorithm and the other a hybrid method incorporating additional information from the satellite temperature record. The methods are validated on the basis of their skill at reconstructing omitted sets of observations. Both methods provide superior results than excluding the unsampled regions, with the hybrid method showing particular skill around the regions where no observations are available. Temperature trends are compared for the hybrid global temperature reconstruction and the raw HadCRUT4 data. The widely quoted trend since 1997 in the hybrid global reconstruction is two and a half times greater than the corresponding trend in the coverage-biased HadCRUT4 data. Coverage bias causes a cool bias in recent temperatures relative to the late 1990s which increases from around 1998 to the present. Trends starting in 1997 or 1998 are particularly biased with respect to the global trend. The issue is exacerbated by the strong El Niño event of 1997-1998, which also tends to suppress trends starting during those years.” Kevin Cowtan, Robert G. Way, Quarterly Journal of the Royal Meteorological Society, DOI: 10.1002/qj.2297.

Energy budget constraints on climate response – Otto et al. (2013) “The rate of global mean warming has been lower over the past decade than previously. It has been argued that this observation might require a downwards revision of estimates of equilibrium climate sensitivity, that is, the long-term (equilibrium) temperature response to a doubling of…” Alexander Otto, Friederike E. L. Otto, Olivier Boucher, John Church, Gabi Hegerl, Piers M. Forster, Nathan P. Gillett, Jonathan Gregory, Gregory C. Johnson, Reto Knutti, Nicholas Lewis, Ulrike Lohmann, Jochem Marotzke, Gunnar Myhre, Drew Shindell, Bjorn Stevens & Myles R. Allen, Nature Geoscience 6, 415–416 (2013), doi:10.1038/ngeo1836.

Recent global-warming hiatus tied to equatorial Pacific surface cooling – Kosaka & Xie (2013) “Despite the continued increase in atmospheric greenhouse gas concentrations, the annual-mean global temperature has not risen in the twenty-first century, challenging the prevailing view that anthropogenic forcing causes climate warming. Various mechanisms have been proposed for this hiatus in global warming, but their relative importance has not been quantified, hampering observational estimates of climate sensitivity. Here we show that accounting for recent cooling in the eastern equatorial Pacific reconciles climate simulations and observations. We present a novel method of uncovering mechanisms for global temperature change by prescribing, in addition to radiative forcing, the observed history of sea surface temperature over the central to eastern tropical Pacific in a climate model. Although the surface temperature prescription is limited to only 8.2% of the global surface, our model reproduces the annual-mean global temperature remarkably well with correlation coefficient r = 0.97 for 1970–2012 (which includes the current hiatus and a period of accelerated global warming). Moreover, our simulation captures major seasonal and regional characteristics of the hiatus, including the intensified Walker circulation, the winter cooling in northwestern North America and the prolonged drought in the southern USA. Our results show that the current hiatus is part of natural climate variability, tied specifically to a La-Niña-like decadal cooling. Although similar decadal hiatus events may occur in the future, the multi-decadal warming trend is very likely to continue with greenhouse gas increase.” Yu Kosaka, Shang-Ping Xie, Nature 501,403–407 (19 September 2013) doi:10.1038/nature12534.

Overestimated global warming over the past 20 years – Fyfe et al. (2013) “Recent observed global warming is significantly less than that simulated by climate models. This difference might be explained by some combination of errors in external forcing, model response and internal climate variability.” John C. Fyfe, Nathan P. Gillett, Francis W. Zwiers, Nature Climate Change 3,767–769(2013)doi:10.1038/nclimate1972. [Full text]

Strengthening of ocean heat uptake efficiency associated with the recent climate hiatus – Watanabe et al. (2013) “The rate of increase of global-mean surface air temperature (SATg) has apparently slowed during the last decade. We investigated the extent to which state-of-the-art general circulation models (GCMs) can capture this hiatus period by using multimodel ensembles of historical climate simulations. While the SATg linear trend for the last decade is not captured by their ensemble means regardless of differences in model generation and external forcing, it is barely represented by an 11-member ensemble of a GCM, suggesting an internal origin of the hiatus associated with active heat uptake by the oceans. Besides, we found opposite changes in ocean heat uptake efficiency (κ), weakening in models and strengthening in nature, which explain why the models tend to overestimate the SATg trend. The weakening of κ commonly found in GCMs seems to be an inevitable response of the climate system to global warming, suggesting the recovery from hiatus in coming decades.” Masahiro Watanabe, Youichi Kamae, Masakazu Yoshimori, Akira Oka, Makiko Sato, Masayoshi Ishii, Takashi Mochizuki, Masahide Kimoto, Geophysical Research Letters, Volume 40, Issue 12, pages 3175–3179, 28 June 2013, DOI: 10.1002/grl.50541.

Retrospective prediction of the global warming slowdown in the past decade – Guemas et al. (2013) “Despite a sustained production of anthropogenic greenhouse gases, the Earth’s mean near-surface temperature paused its rise during the 2000–2010 period. To explain such a pause, an increase in ocean heat uptake below the superficial ocean layer has been proposed to overcompensate for the Earth’s heat storage. Contributions have also been suggested from the deep prolonged solar minimum, the stratospheric water vapour, the stratospheric and tropospheric aerosols. However, a robust attribution of this warming slowdown has not been achievable up to now. Here we show successful retrospective predictions of this warming slowdown up to 5 years ahead, the analysis of which allows us to attribute the onset of this slowdown to an increase in ocean heat uptake. Sensitivity experiments accounting only for the external radiative forcings do not reproduce the slowdown. The top-of-atmosphere net energy input remained in the [0.5–1] W m−2 interval during the past decade, which is successfully captured by our predictions. Most of this excess energy was absorbed in the top 700 m of the ocean at the onset of the warming pause, 65% of it in the tropical Pacific and Atlantic oceans. Our results hence point at the key role of the ocean heat uptake in the recent warming slowdown. The ability to predict retrospectively this slowdown not only strengthens our confidence in the robustness of our climate models, but also enhances the socio-economic relevance of operational decadal climate predictions.” Virginie Guemas, Francisco J. Doblas-Reyes, Isabel Andreu-Burillo & Muhammad Asif, Nature Climate Change, 3, 649–653 (2013) doi:10.1038/nclimate1863.

Separating signal and noise in atmospheric temperature changes: The importance of timescale – Santer et al. (2011) “We compare global-scale changes in satellite estimates of the temperature of the lower troposphere (TLT) with model simulations of forced and unforced TLT changes. While previous work has focused on a single period of record, we select analysis timescales ranging from 10 to 32 years, and then compare all possible observed TLT trends on each timescale with corresponding multi-model distributions of forced and unforced trends. We use observed estimates of the signal component of TLT changes and model estimates of climate noise to calculate timescale-dependent signal-to-noise ratios (S/N). These ratios are small (less than 1) on the 10-year timescale, increasing to more than 3.9 for 32-year trends. This large change in S/N is primarily due to a decrease in the amplitude of internally generated variability with increasing trend length. Because of the pronounced effect of interannual noise on decadal trends, a multi-model ensemble of anthropogenically-forced simulations displays many 10-year periods with little warming. A single decade of observational TLT data is therefore inadequate for identifying a slowly evolving anthropogenic warming signal. Our results show that temperature records of at least 17 years in length are required for identifying human effects on global-mean tropospheric temperature.” B. D. Santer, C. Mears, C. Doutriaux, P. Caldwell, P. J. Gleckler, T. M. L. Wigley, S. Solomon, N. P. Gillett, D. Ivanova, T. R. Karl, J. R. Lanzante, G. A. Meehl, P. A. Stott, K. E. Taylor, P. W. Thorne, M. F. Wehner, F. J. Wentz, Journal of Geophysical Research: Atmospheres (1984–2012), Volume 116, Issue D22, November 2011, DOI: 10.1029/2011JD016263. [Full text]

Reconciling anthropogenic climate change with observed temperature 1998–2008 – Kaufmann et al. (2011) “Given the widely noted increase in the warming effects of rising greenhouse gas concentrations, it has been unclear why global surface temperatures did not rise between 1998 and 2008. We find that this hiatus in warming coincides with a period of little increase in the sum of anthropogenic and natural forcings. Declining solar insolation as part of a normal eleven-year cycle, and a cyclical change from an El Nino to a La Nina dominate our measure of anthropogenic effects because rapid growth in short-lived sulfur emissions partially offsets rising greenhouse gas concentrations. As such, we find that recent global temperature records are consistent with the existing understanding of the relationship among global surface temperature, internal variability, and radiative forcing, which includes anthropogenic factors with well known warming and cooling effects.” Robert K. Kaufmann, Heikki Kauppi, Michael L. Mann, and James H. Stock, PNAS July 19, 2011 vol. 108 no. 29 11790-11793, doi: 10.1073/pnas.1102467108. [Full text]

The role of natural climatic variation in perturbing the observed global mean temperature trend – Hunt (2011) “Controversy continues to prevail concerning the reality of anthropogenically-induced climatic warming. One of the principal issues is the cause of the hiatus in the current global warming trend. There appears to be a widely held view that climatic change warming should exhibit an inexorable upwards trend, a view that implies there is no longer any input by climatic variability in the existing climatic system. The relative roles of climatic change and climatic variability are examined here using the same coupled global climatic model. For the former, the model is run using a specified CO2 growth scenario, while the latter consisted of a multi-millennial simulation where any climatic variability was attributable solely to internal processes within the climatic system. It is shown that internal climatic variability can produce global mean surface temperature anomalies of ±0.25 K and sustained positive and negative anomalies sufficient to account for the anomalous warming of the 1940s as well as the present hiatus in the observed global warming. The characteristics of the internally-induced negative temperature anomalies are such that if this internal natural variability is the cause of the observed hiatus, then a resumption of the observed global warming trend is to be expected within the next few years.” B. G. Hunt, Climate Dynamics, February 2011, Volume 36, Issue 3-4, pp 509-521, DOI: 10.1007/s00382-010-0799-x.

Model-based evidence of deep-ocean heat uptake during surface-temperature hiatus periods – Meehl et al. (2011) “There have been decades, such as 2000–2009, when the observed globally averaged surface-temperature time series shows little increase or even a slightly negative trend (a hiatus period). However, the observed energy imbalance at the top-of-atmosphere for this recent decade indicates that a net energy flux into the climate system of about 1 W m−2 (refs 2, 3) should be producing warming somewhere in the system. Here we analyse twenty-first-century climate-model simulations that maintain a consistent radiative imbalance at the top-of-atmosphere of about 1 W m−2 as observed for the past decade. Eight decades with a slightly negative global mean surface-temperature trend show that the ocean above 300 m takes up significantly less heat whereas the ocean below 300 m takes up significantly more, compared with non-hiatus decades. The model provides a plausible depiction of processes in the climate system causing the hiatus periods, and indicates that a hiatus period is a relatively common climate phenomenon and may be linked to La Niña-like conditions.” Gerald A. Meehl, Julie M. Arblaster, John T. Fasullo, Aixue Hu & Kevin E. Trenberth, Nature Climate Change, 1, 360–364 (2011) doi:10.1038/nclimate1229. [Full text]

Contributions of Stratospheric Water Vapor to Decadal Changes in the Rate of Global Warming – Solomon et al. (2010) “Stratospheric water vapor concentrations decreased by about 10% after the year 2000. Here we show that this acted to slow the rate of increase in global surface temperature over 2000–2009 by about 25% compared to that which would have occurred due only to carbon dioxide and other greenhouse gases. More limited data suggest that stratospheric water vapor probably increased between 1980 and 2000, which would have enhanced the decadal rate of surface warming during the 1990s by about 30% as compared to estimates neglecting this change. These findings show that stratospheric water vapor is an important driver of decadal global surface climate change.” Susan Solomon, Karen H. Rosenlof, Robert W. Portmann, John S. Daniel, Sean M. Davis, Todd J. Sanford, Gian-Kasper Plattner, Science 5 March 2010: Vol. 327 no. 5970 pp. 1219-1223, DOI: 10.1126/science.1182488. [Full text]

An imperative for climate change planning: tracking Earth’s global energy – Trenberth (2009) “Planned adaptation to climate change requires information about what is happening and why. While a long-term trend is for global warming, short-term periods of cooling can occur and have physical causes associated with natural variability. However, such natural variability means that energy is rearranged or changed within the climate system, and should be traceable. An assessment is given of our ability to track changes in reservoirs and flows of energy within the climate system. Arguments are given that developing the ability to do this is important, as it affects interpretations of global and especially regional climate change, and prospects for the future.” Kevin E Trenberth, Current Opinion in Environmental Sustainability, Volume 1, Issue 1, October 2009, Pages 19–27, http://dx.doi.org/10.1016/j.cosust.2009.06.001. [Full text]

How will Earth’s surface temperature change in future decades? – Lean & Rind (2009) “Reliable forecasts of climate change in the immediate future are difficult, especially on regional scales, where natural climate variations may amplify or mitigate anthropogenic warming in ways that numerical models capture poorly. By decomposing recent observed surface temperatures into components associated with ENSO, volcanic and solar activity, and anthropogenic influences, we anticipate global and regional changes in the next two decades. From 2009 to 2014, projected rises in anthropogenic influences and solar irradiance will increase global surface temperature 0.15 ± 0.03°C, at a rate 50% greater than predicted by IPCC. But as a result of declining solar activity in the subsequent five years, average temperature in 2019 is only 0.03 ± 0.01°C warmer than in 2014. This lack of overall warming is analogous to the period from 2002 to 2008 when decreasing solar irradiance also countered much of the anthropogenic warming. We further illustrate how a major volcanic eruption and a super ENSO would modify our global and regional temperature projections.” Judith L. Lean, David H. Rind, Geophysical Research Letters, Volume 36, Issue 15, 16 August 2009, DOI: 10.1029/2009GL038932. [Full text]

Is the climate warming or cooling? – Easterling & Wehner (2009) “Numerous websites, blogs and articles in the media have claimed that the climate is no longer warming, and is now cooling. Here we show that periods of no trend or even cooling of the globally averaged surface air temperature are found in the last 34 years of the observed record, and in climate model simulations of the 20th and 21st century forced with increasing greenhouse gases. We show that the climate over the 21st century can and likely will produce periods of a decade or two where the globally averaged surface air temperature shows no trend or even slight cooling in the presence of longer-term warming.” David R. Easterling, Michael F. Wehner, Geophysical Research Letters, Volume 36, Issue 8, April 2009, DOI: 10.1029/2009GL037810. [Full text]

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