Papers on reconstructions of modern temperatures

This is a list of papers on proxy based reconstructions of modern temperatures with emphasis on global and hemispheric analyses. “Modern” here refers to last few thousands of years. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.

Globally resolved surface temperatures since the Last Glacial Maximum – Osman et al. (2021)
Abstract: Climate changes across the past 24,000 years provide key insights into Earth system responses to external forcing. Climate model simulations and proxy data have independently allowed for study of this crucial interval; however, they have at times yielded disparate conclusions. Here, we leverage both types of information using paleoclimate data assimilation to produce the first proxy-constrained, full-field reanalysis of surface temperature change spanning the Last Glacial Maximum to present at 200-year resolution. We demonstrate that temperature variability across the past 24 thousand years was linked to two primary climatic mechanisms: radiative forcing from ice sheets and greenhouse gases; and a superposition of changes in the ocean overturning circulation and seasonal insolation. In contrast with previous proxy-based reconstructions our results show that global mean temperature has slightly but steadily warmed, by ~0.5 °C, since the early Holocene (around 9 thousand years ago). When compared with recent temperature changes, our reanalysis indicates that both the rate and magnitude of modern warming are unusual relative to the changes of the past 24 thousand years.
Citation: Osman, M.B., Tierney, J.E., Zhu, J. et al. Globally resolved surface temperatures since the Last Glacial Maximum. Nature 599, 239–244 (2021). https://doi.org/10.1038/s41586-021-03984-4.

Seasonal origin of the thermal maxima at the Holocene and the last interglacial – Bova et al. (2021)
Abstract: Proxy reconstructions from marine sediment cores indicate peak temperatures in the first half of the last and current interglacial periods (the thermal maxima of the Holocene epoch, 10,000 to 6,000 years ago, and the last interglacial period, 128,000 to 123,000 years ago) that arguably exceed modern warmth. By contrast, climate models simulate monotonic warming throughout both periods. This substantial model–data discrepancy undermines confidence in both proxy reconstructions and climate models, and inhibits a mechanistic understanding of recent climate change. Here we show that previous global reconstructions of temperature in the Holocene and the last interglacial period reflect the evolution of seasonal, rather than annual, temperatures and we develop a method of transforming them to mean annual temperatures. We further demonstrate that global mean annual sea surface temperatures have been steadily increasing since the start of the Holocene (about 12,000 years ago), first in response to retreating ice sheets (12 to 6.5 thousand years ago), and then as a result of rising greenhouse gas concentrations (0.25 ± 0.21 degrees Celsius over the past 6,500 years or so). However, mean annual temperatures during the last interglacial period were stable and warmer than estimates of temperatures during the Holocene, and we attribute this to the near-constant greenhouse gas levels and the reduced extent of ice sheets. We therefore argue that the climate of the Holocene differed from that of the last interglacial period in two ways: first, larger remnant glacial ice sheets acted to cool the early Holocene, and second, rising greenhouse gas levels in the late Holocene warmed the planet. Furthermore, our reconstructions demonstrate that the modern global temperature has exceeded annual levels over the past 12,000 years and probably approaches the warmth of the last interglacial period (128,000 to 115,000 years ago).
Citation: Bova, S., Rosenthal, Y., Liu, Z. et al. Seasonal origin of the thermal maxima at the Holocene and the last interglacial. Nature 589, 548–553 (2021). https://doi.org/10.1038/s41586-020-03155-x.

Consistent multidecadal variability in global temperature reconstructions and simulations over the Common Era – PAGES 2k Consortium (2019)
Abstract: Multidecadal surface temperature changes may be forced by natural as well as anthropogenic factors, or arise unforced from the climate system. Distinguishing these factors is essential for estimating sensitivity to multiple climatic forcings and the amplitude of the unforced variability. Here we present 2,000-year-long global mean temperature reconstructions using seven different statistical methods that draw from a global collection of temperature-sensitive palaeoclimate records. Our reconstructions display synchronous multidecadal temperature fluctuations that are coherent with one another and with fully forced millennial model simulations from the Coupled Model Intercomparison Project Phase 5 across the Common Era. A substantial portion of pre-industrial (1300–1800 CE) variability at multidecadal timescales is attributed to volcanic aerosol forcing. Reconstructions and simulations qualitatively agree on the amplitude of the unforced global mean multidecadal temperature variability, thereby increasing confidence in future projections of climate change on these timescales. The largest warming trends at timescales of 20 years and longer occur during the second half of the twentieth century, highlighting the unusual character of the warming in recent decades.
Citation: PAGES 2k Consortium (2019). Consistent multidecadal variability in global temperature reconstructions and simulations over the Common Era. Nature Geoscience 12, 643–649. doi: 10.1038/s41561-019-0400-0.

Last phase of the Little Ice Age forced by volcanic eruptions – Brönnimann et al. (2019)
Abstract: During the first half of the nineteenth century, several large tropical volcanic eruptions occurred within less than three decades. The global climate effects of the 1815 Tambora eruption have been investigated, but those of an eruption in 1808 or 1809 whose source is unknown and the eruptions in the 1820s and 1830s have received less attention. Here we analyse the effect of the sequence of eruptions in observations, global three-dimensional climate field reconstructions and coupled climate model simulations. All the eruptions were followed by substantial drops of summer temperature over the Northern Hemisphere land areas. In addition to the direct radiative effect, which lasts 2–3 years, the simulated ocean–atmosphere heat exchange sustained cooling for several years after these eruptions, which affected the slow components of the climate system. Africa was hit by two decades of drought, global monsoons weakened and the tracks of low-pressure systems over the North Atlantic moved south. The low temperatures and increased precipitation in Europe triggered the last phase of the advance of Alpine glaciers. Only after the 1850s did the transition into the period of anthropogenic warming start. We conclude that the end of the Little Ice Age was marked by the recovery from a sequence of volcanic eruptions, which makes it difficult to define a single pre-industrial baseline.
Citation: Stefan Brönnimann, Jörg Franke, Samuel U. Nussbaumer, Heinz J. Zumbühl, Daniel Steiner, Mathias Trachsel, Gabriele C. Hegerl, Andrew Schurer, Matthias Worni, Abdul Malik, Julian Flückiger, Christoph C. Raible (2019). Last phase of the Little Ice Age forced by volcanic eruptions. Nature Geoscience 12, 650–656. doi: 10.1038/s41561-019-0402-y.

No evidence for globally coherent warm and cold periods over the preindustrial Common Era – Neukom et al. (2019)
Abstract: Earth’s climate history is often understood by breaking it down into constituent climatic epochs. Over the Common Era (the past 2,000 years) these epochs, such as the Little Ice Age, have been characterized as having occurred at the same time across extensive spatial scales. Although the rapid global warming seen in observations over the past 150 years does show nearly global coherence, the spatiotemporal coherence of climate epochs earlier in the Common Era has yet to be robustly tested. Here we use global palaeoclimate reconstructions for the past 2,000 years, and find no evidence for preindustrial globally coherent cold and warm epochs. In particular, we find that the coldest epoch of the last millennium—the putative Little Ice Age—is most likely to have experienced the coldest temperatures during the fifteenth century in the central and eastern Pacific Ocean, during the seventeenth century in northwestern Europe and southeastern North America, and during the mid-nineteenth century over most of the remaining regions. Furthermore, the spatial coherence that does exist over the preindustrial Common Era is consistent with the spatial coherence of stochastic climatic variability. This lack of spatiotemporal coherence indicates that preindustrial forcing was not sufficient to produce globally synchronous extreme temperatures at multidecadal and centennial timescales. By contrast, we find that the warmest period of the past two millennia occurred during the twentieth century for more than 98 per cent of the globe. This provides strong evidence that anthropogenic global warming is not only unparalleled in terms of absolute temperatures5, but also unprecedented in spatial consistency within the context of the past 2,000 years.
Citation: Raphael Neukom, Nathan Steiger, Juan José Gómez-Navarro, Jianghao Wang, Johannes P. Werner (2019). No evidence for globally coherent warm and cold periods over the preindustrial Common Era. Nature 571, 550–554. doi: 10.1038/s41586-019-1401-2.

Last millennium Northern Hemisphere summer temperatures from tree rings: Part II, spatially resolved reconstruction – Anchukaitis et al. (2017) [FULL TEXT] “Climate field reconstructions from networks of tree-ring proxy data can be used to characterize regional-scale climate changes, reveal spatial anomaly patterns associated with atmospheric circulation changes, radiative forcing, and large-scale modes of ocean-atmosphere variability, and provide spatiotemporal targets for climate model comparison and evaluation. Here we use a multiproxy network of tree-ring chronologies to reconstruct spatially resolved warm season (May–August) mean temperatures across the extratropical Northern Hemisphere (40-90°N) using Point-by-Point Regression (PPR). The resulting annual maps of temperature anomalies (750–1988 CE) reveal a consistent imprint of volcanism, with 96% of reconstructed grid points experiencing colder conditions following eruptions. Solar influences are detected at the bicentennial (de Vries) frequency, although at other time scales the influence of insolation variability is weak. Approximately 90% of reconstructed grid points show warmer temperatures during the Medieval Climate Anomaly when compared to the Little Ice Age, although the magnitude varies spatially across the hemisphere. Estimates of field reconstruction skill through time and over space can guide future temporal extension and spatial expansion of the proxy network.”

Last millennium northern hemisphere summer temperatures from tree rings: Part I: The long term context – Wilson et al. (2016) [FULL TEXT] “Large-scale millennial length Northern Hemisphere (NH) temperature reconstructions have been progressively improved over the last 20 years as new datasets have been developed. This paper, and its companion (Part II, Anchukaitis et al. in prep), details the latest tree-ring (TR) based NH land air temperature reconstruction from a temporal and spatial perspective. This work is the first product of a consortium called N-TREND (Northern Hemisphere Tree-Ring Network Development) which brings together dendroclimatologists to identify a collective strategy for improving large-scale summer temperature reconstructions. The new reconstruction, N-TREND2015, utilises 54 records, a significant expansion compared with previous TR studies, and yields an improved reconstruction with stronger statistical calibration metrics. N-TREND2015 is relatively insensitive to the compositing method and spatial weighting used and validation metrics indicate that the new record portrays reasonable coherence with large scale summer temperatures and is robust at all time-scales from 918 to 2004 where at least 3 TR records exist from each major continental mass. N-TREND2015 indicates a longer and warmer medieval period (∼900–1170) than portrayed by previous TR NH reconstructions and by the CMIP5 model ensemble, but with better overall agreement between records for the last 600 years. Future dendroclimatic projects should focus on developing new long records from data-sparse regions such as North America and eastern Eurasia as well as ensuring the measurement of parameters related to latewood density to complement ring-width records which can improve local based calibration substantially.”

The last millennium climate reanalysis project: Framework and first results – Hakim et al. (2016) “An “offline” approach to DA is used, where static ensemble samples are drawn from existing CMIP climate-model simulations to serve as the prior estimate of climate variables. We use linear, univariate forward models (“proxy system models (PSMs)”) that map climate variables to proxy measurements by fitting proxy data to 2 m air temperature from gridded instrumental temperature data; the linear PSMs are then used to predict proxy values from the prior estimate. Results for the LMR are compared against six gridded instrumental temperature data sets and 25% of the proxy records are withheld from assimilation for independent verification. Results show broad agreement with previous reconstructions of Northern Hemisphere mean 2 m air temperature, with millennial-scale cooling, a multicentennial warm period around 1000 C.E., and a cold period coincident with the Little Ice Age (circa 1450–1800 C.E.). Verification against gridded instrumental data sets during 1880–2000 C.E. reveals greatest skill in the tropics and lowest skill over Northern Hemisphere land areas. Verification against independent proxy records indicates substantial improvement relative to the model (prior) data without proxy assimilation. As an illustrative example, we present multivariate reconstructed fields for a singular event, the 1808/1809 “mystery” volcanic eruption, which reveal global cooling that is strongly enhanced locally due to the presence of the Pacific-North America wave pattern in the 500 hPa geopotential height field.” Hakim, G. J., J. Emile-Geay, E. J. Steig, D. Noone, D. M. Anderson, R. Tardif, N. Steiger, and W. A. Perkins (2016), The last millennium climate reanalysis project: Framework and first results, J. Geophys. Res. Atmos., 121, 6745–6764, doi:10.1002/2016JD024751. [Full text]

Inter-hemispheric temperature variability over the past millennium – Neukom et al. (2014) “The Earth’s climate system is driven by a complex interplay of internal chaotic dynamics and natural and anthropogenic external forcing. Recent instrumental data have shown a remarkable degree of asynchronicity between Northern Hemisphere and Southern Hemisphere temperature fluctuations, thereby questioning the relative importance of internal versus external drivers of past as well as future climate variability. However, large-scale temperature reconstructions for the past millennium have focused on the Northern Hemisphere, limiting empirical assessments of inter-hemispheric variability on multi-decadal to centennial timescales. Here, we introduce a new millennial ensemble reconstruction of annually resolved temperature variations for the Southern Hemisphere based on an unprecedented network of terrestrial and oceanic palaeoclimate proxy records. In conjunction with an independent Northern Hemisphere temperature reconstruction ensemble, this record reveals an extended cold period (1594–1677) in both hemispheres but no globally coherent warm phase during the pre-industrial (1000–1850) era. The current (post-1974) warm phase is the only period of the past millennium where both hemispheres are likely to have experienced contemporaneous warm extremes. Our analysis of inter-hemispheric temperature variability in an ensemble of climate model simulations for the past millennium suggests that models tend to overemphasize Northern Hemisphere–Southern Hemisphere synchronicity by underestimating the role of internal ocean–atmosphere dynamics, particularly in the ocean-dominated Southern Hemisphere. Our results imply that climate system predictability on decadal to century timescales may be lower than expected based on assessments of external climate forcing and Northern Hemisphere temperature variations alone.” Raphael Neukom, Joëlle Gergis, David J. Karoly, Heinz Wanner, Mark Curran, Julie Elbert, Fidel González-Rouco, Braddock K. Linsley, Andrew D. Moy, Ignacio Mundo, Christoph C. Raible, Eric J. Steig, Tas van Ommen, Tessa Vance, Ricardo Villalba, Jens Zinke & David Frank, Nature Climate Change, 4, 362–367 (2014), doi:10.1038/nclimate2174. [Full text]

Reconstructing past temperatures from natural proxies and estimated climate forcings using short- and long-memory models – Barboza et al. (2014) “We produce new reconstructions of Northern Hemisphere annually averaged temperature anomalies back to 1000 AD, and explore the effects of including external climate forcings within the reconstruction and of accounting for short-memory and long-memory features. Our reconstructions are based on two linear models, with the first linking the latent temperature series to three main external forcings (solar irradiance, greenhouse gas concentration and volcanism), and the second linking the observed temperature proxy data (tree rings, sediment record, ice cores, etc.) to the unobserved temperature series. Uncertainty is captured with additive noise, and a rigorous statistical investigation of the correlation structure in the regression errors is conducted through systematic comparisons between reconstructions that assume no memory, short-memory autoregressive models, and long-memory fractional Gaussian noise models. We use Bayesian estimation to fit the model parameters and to perform separate reconstructions of land-only and combined land-and-marine temperature anomalies. For model formulations that include forcings, both exploratory and Bayesian data analysis provide evidence against models with no memory. Model assessments indicate that models with no memory underestimate uncertainty. However, no single line of evidence is sufficient to favor short-memory models over long-memory ones, or to favor the opposite choice. When forcings are not included, the long-memory models appear to be necessary. While including external climate forcings substantially improves the reconstruction, accurate reconstructions that exclude these forcings are vital for testing the fidelity of climate models used for future projections. Finally, we use posterior samples of model parameters to arrive at an estimate of the transient climate response to greenhouse gas forcings of 2.5°C (95% credible interval of [2.16, 2.92]°C), which is on the high end of, but consistent with, the expert-assessment-based uncertainties given in the recent Fifth Assessment Report of the IPCC.” Luis Barboza, Bo Li, Martin P. Tingley, and Frederi G. Viens, Ann. Appl. Stat. Volume 8, Number 4 (2014), 1966-2001. [Full text]

A Reconstruction of Regional and Global Temperature for the Past 11,300 Years – Marcott et al. (2013) “Surface temperature reconstructions of the past 1500 years suggest that recent warming is unprecedented in that time. Here we provide a broader perspective by reconstructing regional and global temperature anomalies for the past 11,300 years from 73 globally distributed records. Early Holocene (10,000 to 5000 years ago) warmth is followed by ~0.7°C cooling through the middle to late Holocene (<5000 years ago), culminating in the coolest temperatures of the Holocene during the Little Ice Age, about 200 years ago. This cooling is largely associated with ~2°C change in the North Atlantic. Current global temperatures of the past decade have not yet exceeded peak interglacial values but are warmer than during ~75% of the Holocene temperature history. Intergovernmental Panel on Climate Change model projections for 2100 exceed the full distribution of Holocene temperature under all plausible greenhouse gas emission scenarios.” Shaun A. Marcott, Jeremy D. Shakun, Peter U. Clark, Alan C. Mix, Science 8 March 2013: Vol. 339 no. 6124 pp. 1198-1201, DOI: 10.1126/science.1228026. [Full text]

Northern Hemisphere temperature patterns in the last 12 centuries – Ljungqvist et al. (2012) “We analyse the spatio-temporal patterns of temperature variability over Northern Hemisphere land areas, on centennial time-scales, for the last 12 centuries using an unprecedentedly large network of temperature-sensitive proxy records. Geographically widespread positive temperature anomalies are observed from the 9th to 11th centuries, similar in extent and magnitude to the 20th century mean. A dominance of widespread negative anomalies is observed from the 16th to 18th centuries. Though we find the amplitude and spatial extent of the 20th century warming is within the range of natural variability over the last 12 centuries, we also find that the rate of warming from the 19th to the 20th century is unprecedented in the context of the last 1200 yr. The positive Northern Hemisphere temperature change from the 19th to the 20th century is clearly the largest between any two consecutive centuries in the past 12 centuries. These results remain robust even after removing a significant number of proxies in various tests of robustness showing that the choice of proxies has no particular influence on the overall conclusions of this study.” Ljungqvist, F. C., Krusic, P. J., Brattström, G., and Sundqvist, H. S.: Northern Hemisphere temperature patterns in the last 12 centuries, Clim. Past, 8, 227-249, doi:10.5194/cp-8-227-2012, 2012. [Full text]

A statistical analysis of multiple temperature proxies: Are reconstructions of surface temperatures over the last 1000 years reliable? – McShane & Wyner (2011) “Predicting historic temperatures based on tree rings, ice cores, and other natural proxies is a difficult endeavor. The relationship between proxies and temperature is weak and the number of proxies is far larger than the number of target data points. Furthermore, the data contain complex spatial and temporal dependence structures which are not easily captured with simple models. In this paper, we assess the reliability of such reconstructions and their statistical significance against various null models. We find that the proxies do not predict temperature significantly better than random series generated independently of temperature. Furthermore, various model specifications that perform similarly at predicting temperature produce extremely different historical backcasts. Finally, the proxies seem unable to forecast the high levels of and sharp run-up in temperature in the 1990s either in-sample or from contiguous holdout blocks, thus casting doubt on their ability to predict such phenomena if in fact they occurred several hundred years ago. We propose our own reconstruction of Northern Hemisphere average annual land temperature over the last millennium, assess its reliability, and compare it to those from the climate science literature. Our model provides a similar reconstruction but has much wider standard errors, reflecting the weak signal and large uncertainty encountered in this setting.” [Full text], [Discussion papers on this]

A noodle, hockey stick, and spaghetti plate: a perspective on high-resolution paleoclimatology – Frank et al. (2010) “The high-resolution reconstruction of hemispheric-scale temperature variation over the past-millennium benchmarks recent warming against more naturally driven climate episodes, such as the Little Ice Age and the Medieval Warm Period, thereby allowing assessment of the relative efficacies of natural and anthropogenic forcing factors. Icons of past temperature variability, as featured in the Intergovernmental Panel on Climate Change (IPCC) reports over nearly two decades, have changed from a schematic sketch in 1990, to a seemingly well-solved story in 2001, to more explicit recognition of significant uncertainties in 2007. In this article, we detail the beginning of the movement to reconstruct large-scale temperatures, highlight major steps forward, and present our views on what remains to be accomplished. Despite significant efforts and progress, the spatial representation of reconstructions is limited, and the interannual and centennial variation are poorly quantified. Research priorities to reduce reconstruction uncertainties and improve future projections, include (1) increasing the role of expert assessment in selecting and incorporating the highest quality proxy data in reconstructions (2) employing reconstruction ensemble methodology, and (3) further improvements of forcing series. We suggest that much of the sensitivity in the reconstructions, a topic that has dominated scientific debates, can be traced back to the input data. It is perhaps advisable to use fewer, but expert-assessed proxy records to reduce errors in future reconstruction efforts.” David Frank, Jan Esper, Eduardo Zorita, Rob Wilson, Wiley Interdisciplinary Reviews: Climate Change, Volume 1, Issue 4, pages 507–516, July/August 2010. [Full text]

Recent Warming Reverses Long-Term Arctic Cooling – Kaufman et al. (2009) “The temperature history of the first millennium C.E. is sparsely documented, especially in the Arctic. We present a synthesis of decadally resolved proxy temperature records from poleward of 60°N covering the past 2000 years, which indicates that a pervasive cooling in progress 2000 years ago continued through the Middle Ages and into the Little Ice Age. A 2000-year transient climate simulation with the Community Climate System Model shows the same temperature sensitivity to changes in insolation as does our proxy reconstruction, supporting the inference that this long-term trend was caused by the steady orbitally driven reduction in summer insolation. The cooling trend was reversed during the 20th century, with four of the five warmest decades of our 2000-year-long reconstruction occurring between 1950 and 2000.” Darrell S. Kaufman, David P. Schneider, Nicholas P. McKay, Caspar M. Ammann, Raymond S. Bradley, Keith R. Briffa, Gifford H. Miller, Bette L. Otto-Bliesner, Jonathan T. Overpeck, Bo M. Vinther and Arctic Lakes 2k Project Members, Science 4 September 2009: Vol. 325 no. 5945 pp. 1236-1239, DOI: 10.1126/science.1173983. [Full text]

Analysis of the Moberg et al. (2005) hemispheric temperature reconstruction – Moberg et al. (2008) “The Moberg et al. (Nature 433(7026):613–617, 2005. doi:10.1038/nature03265; M05) reconstruction of northern hemisphere temperature variations from proxy data has been criticised; the M05 method may artificially inflate low-frequency variance relative to reality. We test this assertion by undertaking several pseudoproxy experiments in three climate model simulations—one control run and two forced simulations that include several time-varying radiative forcings. The pseudoproxy series are designed to have the same variance spectra as the real M05 proxies, primarily to mimic the low-resolution character of several series. A simple composite-plus-scale (CPS) method is also analysed. In the CPS case all input data behave like annually resolved proxies. The spectral domain performance of both M05 and CPS is found to be dependent on the noise type and noise level in pseudoproxies, on the variance spectrum of the climate model simulation, and on the degree of data smoothing. CPS performs better than M05 in most investigated cases with the control run, but leads to deflated low-frequency variance in some cases. With M05, low-frequency variance tend to be inflated for the control run but not for one of the forced runs and only very slightly with the other forced simulation. Hence, the M05 approach does not routinely inflate low-frequency variance. In our experiment, the M05 approach performs better in the spectral domain than CPS when applied to forced climate model simulations. The results underscore the importance of evaluating the variance spectrum of climate reconstructions.” Anders Moberg, Rezwan Mohammad and Thorsten Mauritsen, Climate Dynamics, Volume 31, Numbers 7-8, 957-971, DOI: 10.1007/s00382-008-0392-8.

On the reliability of millennial reconstructions of variations in surface air temperature in the Northern Hemisphere – Datsenko & Sonechkin (2008) “The reliability of the recently published reconstructions of the surface air temperature variability in the Northern Hemisphere over the past 2000 yr is discussed. For this purpose, the power spectra of the two best known reconstructions (Mann et al.[10–12] and Moberg et al. [13]) are calculated and compared to the spectra of the 150-yr temperature series based on instrumental observations and simulated 1000-yr series. It is found that the Mann et al. reconstruction drastically underestimates low-frequency temperature variations, whereas the Moberg et al. reconstruction reproduces them much better, although with a certain underestimation rather than overestimation, as Mann et al. have recently argued.”

Proxy-based reconstructions of hemispheric and global surface temperature variations over the past two millennia – Mann et al. (2008) “Following the suggestions of a recent National Research Council report [NRC (National Research Council) (2006) Surface Temperature Reconstructions for the Last 2,000 Years (Natl Acad Press, Washington, DC).], we reconstruct surface temperature at hemispheric and global scale for much of the last 2,000 years using a greatly expanded set of proxy data for decadal-to-centennial climate changes, recently updated instrumental data, and complementary methods that have been thoroughly tested and validated with model simulation experiments. Our results extend previous conclusions that recent Northern Hemisphere surface temperature increases are likely anomalous in a long-term context. Recent warmth appears anomalous for at least the past 1,300 years whether or not tree-ring data are used.” [Full text]

Millennial temperature reconstruction intercomparison and evaluation – Juckes et al. (2007) “Here recent work is reviewed and some new calculations performed with an aim to clarifying the consequences of the different approaches used. A range of proxy data collections introduced by different authors is used to estimate Northern Hemispheric annual mean temperatures with two reconstruction algorithms: (1) inverse regression and, (2) compositing followed by variance matching (CVM). … A reconstruction using 13 proxy records extending back to AD 1000 shows a maximum pre-industrial temperature of 0.25 K (relative to the 1866 to 1970 mean). The standard error on this estimate, based on the residual in the calibration period, is 0.14 K. Instrumental temperatures for two recent years (1998 and 2005) have exceeded the pre-industrial estimated maximum by more than 4 standard deviations of the calibration period residual.” [Link to PDF]

On the long-term context for late twentieth century warming – D’Arrigo et al. (2006) “Previous tree-ring–based Northern Hemisphere temperature reconstructions portray a varying amplitude range between the “Medieval Warm Period” (MWP), “Little Ice Age” (LIA) and present. We describe a new reconstruction, developed using largely different methodologies and additional new data compared to previous efforts. Unlike earlier studies, we quantify differences between more traditional (STD) and Regional Curve Standardization (RCS) methodologies, concluding that RCS is superior for retention of low-frequency trends. Continental North American versus Eurasian RCS series developed prior to merging to the hemispheric scale cohere surprisingly well, suggesting common forcing, although there are notable deviations (e.g., fifteenth to sixteenth century). Results indicate clear MWP (warm), LIA (cool), and recent (warm) episodes. Direct interpretation of the RCS reconstruction suggests that MWP temperatures were nearly 0.7°C cooler than in the late twentieth century, with an amplitude difference of 1.14°C from the coldest (1600–1609) to warmest (1937–1946) decades. However, we advise caution with this analysis. Although we conclude, as found elsewhere, that recent warming has been substantial relative to natural fluctuations of the past millennium, we also note that owing to the spatially heterogeneous nature of the MWP, and its different timing within different regions, present palaeoclimatic methodologies will likely “flatten out” estimates for this period relative to twentieth century warming, which expresses a more homogenous global “fingerprint.” Therefore we stress that presently available paleoclimatic reconstructions are inadequate for making specific inferences, at hemispheric scales, about MWP warmth relative to the present anthropogenic period and that such comparisons can only still be made at the local/regional scale.” D’Arrigo, R., R. Wilson, and G. Jacoby (2006), J. Geophys. Res., 111, D03103, doi:10.1029/2005JD006352. [Full text]

Climate sensitivity constrained by temperature reconstructions over the past seven centuries – Hegerl et al. (2006) “A number of observational studies, however, find a substantial probability of significantly higher sensitivities, yielding upper limits on climate sensitivity of 7.7 K to above 9 K (refs 3–8). Here we demonstrate that such observational estimates of climate sensitivity can be tightened if reconstructions of Northern Hemisphere temperature over the past several centuries are considered. … After accounting for the uncertainty in reconstructions and estimates of past external forcing, we find an independent estimate of climate sensitivity that is very similar to those from instrumental data. If the latter are combined with the result from all proxy reconstructions, then the 5–95 per cent range shrinks to 1.5–6.2 K, thus substantially reducing the probability of very high climate sensitivity.” [Link to PDF]

Testing the Fidelity of Methods Used in Proxy-Based Reconstructions of Past Climate – Mann et al. (2005) “Two widely used statistical approaches to reconstructing past climate histories from climate “proxy” data such as tree rings, corals, and ice cores are investigated using synthetic “pseudoproxy” data derived from a simulation of forced climate changes over the past 1200 yr. These experiments suggest that both statistical approaches should yield reliable reconstructions of the true climate history within estimated uncertainties, given estimates of the signal and noise attributes of actual proxy data networks.” Mann, Michael E., Scott Rutherford, Eugene Wahl, Caspar Ammann, 2005, J. Climate, 18, 4097–4107, doi: http://dx.doi.org/10.1175/JCLI3564.1. [Full text]

Extracting a Climate Signal from 169 Glacier Records – Oerlemans et al. (2005) “I constructed a temperature history for different parts of the world from 169 glacier length records. Using a first-order theory of glacier dynamics, I related changes in glacier length to changes in temperature. … Moderate global warming started in the middle of the 19th century. The reconstructed warming in the first half of the 20th century is 0.5 kelvin.” [Link to PDF]

Proxy-Based Northern Hemisphere Surface Temperature Reconstructions: Sensitivity to Method, Predictor Network, Target Season, and Target Domain – Rutherford et al. (2005) “Results are presented from a set of experiments designed to investigate factors that may influence proxy-based reconstructions of large-scale temperature patterns in past centuries. The factors investigated include 1) the method used to assimilate proxy data into a climate reconstruction, 2) the proxy data network used, 3) the target season, and 4) the spatial domain of the reconstruction. Estimates of hemispheric-mean temperature are formed through spatial averaging of reconstructed temperature patterns that are based on either the local calibration of proxy and instrumental data or a more elaborate multivariate climate field reconstruction approach. The experiments compare results based on the global multiproxy dataset used by Mann and coworkers, with results obtained using the extratropical Northern Hemisphere (NH) maximum latewood tree-ring density set used by Briffa and coworkers. Mean temperature reconstructions are compared for the full NH (Tropics and extratropics, land and ocean) and extratropical continents only, withvarying target seasons (cold-season half year, warm-season half year, and annual mean). The comparisons demonstrate dependence of reconstructions on seasonal, spatial, and methodological considerations, emphasizing the primary importance of the target region and seasonal window of the reconstruction. The comparisons support the generally robust nature of several previously published estimates of NH mean temperature changes in past centuries and suggest that further improvements in reconstructive skill are most likely to arise from an emphasis on the quality, rather than quantity, of available proxy data.” Rutherford, S., M. E. Mann, T. J. Osborn, K. R. Briffa, P D. Jones, R. S. Bradley, M. K. Hughes, 2005, J. Climate, 18, 2308–2329, doi: http://dx.doi.org/10.1175/JCLI3351.1. [Full text]

Hockey sticks, principal components, and spurious significance – McIntyre & McKitrick (2005) “The “hockey stick” shaped temperature reconstruction of Mann et al. (1998, 1999) has been widely applied. However it has not been previously noted in print that, prior to their principal components (PCs) analysis on tree ring networks, they carried out an unusual data transformation which strongly affects the resulting PCs. Their method, when tested on persistent red noise, nearly always produces a hockey stick shaped first principal component (PC1) and overstates the first eigenvalue. In the controversial 15th century period, the MBH98 method effectively selects only one species (bristlecone pine) into the critical North American PC1, making it implausible to describe it as the “dominant pattern of variance”. Through Monte Carlo analysis, we show that MBH98 benchmarks for significance of the Reduction of Error (RE) statistic are substantially under-stated and, using a range of cross-validation statistics, we show that the MBH98 15th century reconstruction lacks statistical significance.” McIntyre, S., and R. McKitrick (2005), Geophys. Res. Lett., 32, L03710, doi:10.1029/2004GL021750. [Full text]

Proxy-Based Northern Hemisphere Surface Temperature Reconstructions: Sensitivity to Method, Predictor Network, Target Season, and Target Domain – Rutherford et al. (2005) “Results are presented from a set of experiments designed to investigate factors that may influence proxy-based reconstructions of large-scale temperature patterns in past centuries. … The comparisons demonstrate dependence of reconstructions on seasonal, spatial, and methodological considerations, emphasizing the primary importance of the target region and seasonal window of the reconstruction.” [Link to PDF]

Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data – Moberg et al. (2005) “Here we reconstruct Northern Hemisphere temperatures for the past 2,000 years by combining low-resolution proxies with tree-ring data, using a wavelet transform technique11 to achieve timescale-dependent processing of the data. … According to our reconstruction, high temperatures—similar to those observed in the twentieth century before 1990—occurred around ad 1000 to 1100, and minimum temperatures that are about 0.7 K below the average of 1961–90 occurred around ad 1600.” [Link to PDF] [Their own 2008-paper that criticises this work]

Borehole climate reconstructions: Spatial structure and hemispheric averages – Pollack & Smerdon (2004) “Ground surface temperature (GST) reconstructions determined from temperature profiles measured in terrestrial boreholes, when averaged over the Northern Hemisphere, estimate a surface warming of ∼1 K during the interval AD 1500–2000. Other traditional proxy-based estimates suggest less warming during the same interval. … 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. … Reconstructions assembled after excluding low-occupancy grid elements show a five-century GST change in the range of 1.02–1.06 K.” [Link to PDF]

Climate over past millennia – Jones & Mann (2004) “We review evidence for climate change over the past several millennia from instrumental and high-resolution climate “proxy” data sources and climate modeling studies. … We devote particular attention to proxy-based reconstructions of temperature patterns in past centuries, which place recent large-scale warming in an appropriate longer-term context. Our assessment affirms the conclusion that late 20th century warmth is unprecedented at hemispheric and, likely, global scales.” [Link to PDF]

Global surface temperatures over the past two millennia – Mann & Jones (2003) “We present reconstructions of Northern and Southern Hemisphere mean surface temperature over the past two millennia based on high-resolution ‘proxy’ temperature data which retain millennial-scale variability. These reconstructions indicate that late 20th century warmth is unprecedented for at least roughly the past two millennia for the Northern Hemisphere.” [Link to PDF]

Low-frequency temperature variations from a northern tree ring density network – Briffa et al. (2001) “We describe new reconstructions of northern extratropical summer temperatures for nine subcontinental-scale regions and a composite series representing quasi “Northern Hemisphere” temperature change over the last 600 years. … The 20th century is clearly shown by all of the palaeoseries composites to be the warmest during this period.” [Link to PDF]

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 record6, but suggest that the cumulative change over the past five centuries amounts to about 1 K, exceeding recent estimates from conventional climate proxies.” [Link to PDF]

Annual climate variability in the Holocene: interpreting the message of ancient trees – Briffa (2000) “As for assessing the significance of 20th century global warming, the evidence from dendroclimatology in general, supports the notion that the last 100 years have been unusually warm, at least within a context of the last two millennia. However, this evidence should not be considered equivocal. The activities of humans may well be impacting on the ‘natural’ growth of trees in different ways, making the task of isolating a clear climate message subtly difficult.” [Link to PDF]

Northern hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations – Mann et al. (1999) “Building on recent studies, we attempt hemispheric temperature reconstructions with proxy data networks for the past millennium. We focus not just on the reconstructions, but the uncertainties therein, and important caveats. Though expanded uncertainties prevent decisive conclusions for the period prior to AD 1400, our results suggest that the latter 20th century is anomalous in the context of at least the past millennium. The 1990s was the warmest decade, and 1998 the warmest year, at moderately high levels of confidence. The 20th century warming counters a millennial‐scale cooling trend which is consistent with long‐term astronomical forcing.” [Link to PDF]

Global-scale temperature patterns and climate forcing over the past six centuries – Mann et al. (1998) “Spatially resolved global reconstructions of annual surface temperature patterns over the past six centuries are based on the multivariate calibration of widely distributed high-resolution proxy climate indicators. 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. Northern Hemisphere mean annual temperatures for three of the past eight years are warmer than any other year since (at least) ad 1400.” [Link to PDF]

High-resolution palaeoclimatic records for the last millennium: interpretation, integration and comparison with General Circulation Model control-run temperatures – Jones et al. (1998) “We have averaged 17 temperature reconstructions (representing various seasons of the year), all extending back at least to the mid-seventeenth century, to form two annually resolved hemispheric series (NH10 and SH7). … The coldest year of the millennium over the NH is ad 1601, the coldest decade 1691–1700 and the seventeenth is the coldest century.” [Link to PDF]

Influence of volcanic eruptions on Northern Hemisphere summer temperature over the past 600 years – Briffa et al. (1998) “Here we use this well dated, high-resolution composite time-series to suggest that large explosive volcanic eruptions produced different extents of Northern Hemisphere cooling during the past 600 years. The large effect of some recent eruptions is apparent, such as in 1816, 1884 and 1912, but the relative effects of other known, and perhaps some previously unknown, pre-nineteenth-century eruptions are also evaluated. The most severe short-term Northern Hemisphere cooling event of the past 600 years occurred in 1601, suggesting that either the effect on climate of the eruption of Huaynaputina, Peru, in 1600 has previously been greatly underestimated, or another, as yet unidentified, eruption occurred at the same time.”

‘Little Ice Age’ summer temperature variations: their nature and relevance to recent global warming trends – Bradley & Jones (1993) “Using historical, tree-ring and ice core data, we examine climatic variations during the period commonly called the ‘Little Ice Age’. The coldest conditions of the last 560 years were between AD 1570 and 1730, and in the nineteenth century. Unusually warm conditions have prevailed since the 1920s, probably related to a relative absence of major explosive volcanic eruptions and higher levels of greenhouse gases.”

Closely related

Papers on the MWP as Global Event