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WIREs Clim Change
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Reconstructing Earth's surface temperature over the past 2000 years: the science behind the headlines

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The last quarter century spans the publication of the first assessment report of the Intergovernmental Panel on Climate Change in 1990 and the latest report published in 2013–2014. The five assessment reports appearing over that interval reveal a marked increase in the number of paleoclimate studies addressing the climate of the last 2000 years (the Common Era). An important focus of this work has been on reconstruction of hemispheric and global temperatures. Several early studies in this area generated considerable scientific and public interest, and were followed by high‐profile and sometimes vitriolic debates about the magnitude of temperature changes over all or part of the Common Era and their comparison to 20th‐ and 21st‐century global temperature increases due to increasing levels of atmospheric greenhouse gases. Behind the more public debates, however, several consistent themes of scientific inquiry have developed to better characterize climate variability and change over the Common Era. These include attempts to collect more climate proxy archives and understand the signals they contain, improve the statistical methods used to estimate past temperature variability from proxies and their associated uncertainties, and to compare reconstructed temperature variability and change with climate model simulations. All of these efforts are driving a new age of research on the climate of the Common Era that is developing more cohesive and collaborative investigations into the dynamics of climate on time scales of decades to centuries, and an understanding of the implications for modeled climate projections of the future. WIREs Clim Change 2016, 7:746–771. doi: 10.1002/wcc.418

Collection of diagrams plotting temperature reconstructions for the Common Era in each of the five IPCC assessment reports. Vertical scaling and horizontal alignment are exactly comparable for panels (c)–(e), while only approximate for panels (a) and (b). As noted in most of the panel descriptions below, the baseline or reference periods for the anomalies in each panel are different: in panel (a) the baseline is not specified, but is approximately the mean near the beginning of the 20th century; in panel (b) the baseline is not specified, but is approximately the mean of the four decades ranging from 1930 to 1960; in panels (c) and (d) the baseline is the mean between 1961 and 1990; in panel (e) the baseline is the mean between 1881 and 1980, approximately 0.2°C below the baseline for panels (c) and (d). For reference, the following panel captions are reproduced as presented in each of the IPCC assessment reports, although some minor editing has been included for clarity and referencing style. (a) Figure 7.1 from Climate Change 1990: The IPCC Scientific Assessment: Schematic diagram of global temperature variations over the last thousand years. The dashed line nominally represents mean conditions near the beginning of the 20th century. (b) Figure 10 (TS) from Climate Change 1995—The Science of Climate Change: Decadal summer temperature index for the Northern Hemisphere, up to 1970–1979. The record is based on the average of 16 proxy summer temperature records from North America, Europe, and East Asia. The smooth line was created using an approximately 50‐year Gaussian filter. Recent instrumental data for Northern Hemisphere summer temperature anomalies (over land and ocean) are also plotted (thick line). The instrumental record is probably biased high in the mid‐19th century because of exposures differing from current techniques. (c) Figure 2.20 from Climate Change 2001: The Scientific Basis: Comparison of warm‐season and annual mean multiproxy‐based and warm season tree‐ring‐based millennial Northern Hemisphere temperature reconstructions. The recent instrumental annual mean Northern Hemisphere temperature record to 1999 is shown for comparison. Also shown is an extra‐tropical sampling of the Ref temperature pattern reconstructions more directly comparable in its latitudinal sampling of Ref . The self‐consistently estimated two standard error limits (shaded region) for the smoothed Ref series are shown. The horizontal zero line denotes the 1961–1990 reference period mean temperature. All series were smoothed with a 40‐year Hamming‐weights lowpass filter, with boundary constraints imposed by padding the series with its mean values during the first and last 25 years. (d) Figure 6.10 (b) from Climate Change 2007: The Physical Science Basis: Records of NH temperature variation during the last 1.3 kyr. Reconstructions using multiple climate proxy records, including three records shown in AR3, and the instrumental temperature record in black. All series have been smoothed with a Gaussian‐weighted filter to remove fluctuations on time scales less than 30 years; smoothed values are obtained up to both ends of each record by extending the records with the mean of the adjacent existing values. All temperatures represent anomalies (°C) from the 1961 to 1990 mean. (e) Figure 5.7 (a) from Climate Change 2013: The Physical Science Basis: Reconstructed Northern Hemisphere annual temperatures during the last 2000 years. Individual reconstructions, grouped by color according to their spatial representation (red: land‐only all latitudes; orange: land‐only extratropical latitudes; light blue: land and sea extra‐tropical latitudes; dark blue: land and sea all latitudes) and instrumental temperatures shown in black (Hadley Centre/ Climatic Research Unit (CRU) gridded surface temperature‐4 data set (HadCRUT4) land and sea, and CRU Gridded Dataset of Global Historical Near‐Surface Air TEMperature Anomalies Over Land version 4 (CRUTEM4) land‐only). All series represent anomalies (°C) from the 1881 to 1980 mean (horizontal dashed line) and have been smoothed with a filter that reduces variations on time scales less than about 50 years.
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