Surface Temperature Reconstructions for the Last 2,000 Years (2006) / Chapter Skim
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Pages 5-24
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From page 5... ...
To study how climatic conditions varied prior to the time of the Industrial Revolution, paleoclimatologists rely on proxy evidence such as tree rings, corals, ocean and lake sediments, cave deposits, fossils, ice cores, borehole temperatures, glacier length records, and documentary evidence. For example, records of Alpine glacier length, some of which are derived from paintings and other documentary evidence, have been used to reconstruct the time series of surface temperature variations in south-central Europe for the last several centuries.
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From page 6... ...
· What is our current understanding of how the hemispheric mean or global mean surface temperature has varied over the last 2,000 years? · What conclusions can be drawn from large-scale surface temperature reconstructions?
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From page 7... ...
At high latitudes and/or at high elevations, tree ring growth is related to temperature, and thus trees from these sites are commonly used as a basis for surface temperature reconstructions. Cores extracted from the trees provide annually resolved time series of tree ring width and of wood properties, such as density and chemical composition, within each ring.
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From page 8... ...
Hence, large-scale surface temperature reconstructions based on borehole measurements typically extend back only over a few centuries, with coarse time resolution. Hundreds of holes have been drilled to depths of several hundred meters below the surface at sites throughout the Northern Hemisphere and at a smaller number of sites in the Southern Hemisphere.
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From page 9... ...
Borehole temperature measurements and glacier length records can be converted to temperature time series using physically based models with a few key variables. For all other proxies used for the reconstructions discussed in this report, statistical techniques are employed to define the relationship between the proxy measurements and the concurrent instrumental temperature record, and then this relationship is used to reconstruct past temperature variations from the remaining proxy data.
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From page 10... ...
Validation Reconstruction of Past Climate FIGURE O-1 Schematic diagram of the general methodology used to reconstruct past climates, including surface temperature reconstructions.
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From page 11... ...
Large-Scale Surface Temperature Reconstructions Several surface temperature reconstructions carried out since the mid-1990s involve the synthesis of data from many different locations, often from disparate sources such as tree rings, corals, and ice cores, to infer patterns of temperature variations over
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From page 12... ...
In the second approach, the individual proxy data are first composited and then this series is calibrated directly against the time series of largescale temperature variations. Both the number and the quality of the proxy records available for surface temperature reconstructions decrease dramatically moving backward in time.
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From page 13... ...
OVERVIEW 13 KEY : Number of records boreholes FIGURE O-2 Regional distribution of tree ring, borehole, ice core, and "other" records used to create the large-scale surface temperature reconstructions in Figure S-1 (and Figure O-5)
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From page 14... ...
What is our current understanding of how the hemispheric mean or global mean surface temperature has varied over the last 2,000 years? To understand the current state of the science surrounding large-scale surface temperature reconstructions, it is helpful to first review how these efforts have evolved over the last few decades.
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From page 15... ...
IPCC (2001) featured the multiproxy Northern Hemisphere surface temperature reconstruction reproduced in Figure O-4, which includes error bars.
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From page 16... ...
A more recent and complete description of what we know about the climate of the last two millennia can be gleaned from an inspection of Figure O-5, which was prepared by this committee to show the instrumental record compiled from traditional thermometer readings, several large-scale surface temperature reconstructions based on different kinds of proxy evidence, and results from a few paleoclimate model simulations. Figure O-5 is intended only to provide an illustration of the current state of the science, not a comprehensive review of all currently available large-scale surface temperature estimates.
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From page 17... ...
2006) Tree rings (Esper et al.
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From page 18... ...
Based on its deliberations, the plots shown in Figure O-5, and the evidence described in the chapters that follow and elsewhere, the committee draws the following conclusions: · The instrumentally measured warming of about 0.6°C during the 20th century is also reflected in borehole temperature measurements, the retreat of glaciers, and other observational evidence, and can be simulated with climate models. · Large-scale surface temperature reconstructions yield a generally consistent picture of temperature trends during the preceding millennium, including relatively warm conditions centered around A.D.
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From page 19... ...
only provides a few pieces of independent information to both calibrate and validate surface temperature reconstructions over large spatial scales and multidecade time periods. Instrumental records used for calibration and validation of proxy data have also been collected during a period when both global mean temperatures and human impacts on the environment have increased substantially.
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From page 20... ...
· Tree rings, the dominant data source in many large-scale surface temperature reconstructions, are derived from regional networks with extensive replication that reflect temperature variability at the regional scale. · Most surface temperature reconstructions incorporate proxy evidence from a variety of sources and wide geographic areas, and the resulting temperature estimates are often robust with respect to the removal of individual records.
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From page 21... ...
Surface temperature reconstructions have the potential to provide independent information about climate sensitivity and about the natural variability of the climate system that can be compared with estimates based on theoretical calculations and climate models, as well as other empirical data. However, large-scale surface temperature reconstructions for the last 2,000 years are not the primary evidence for the widely accepted views that global warming is occurring, that human activities are contributing, at least in part, to this warming, and that the Earth will continue to warm over the next century.
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From page 22... ...
· The stratosphere has cooled and the oceans have warmed in a manner that is consistent with the predicted spatial and temporal pattern of greenhouse warming. Surface temperature reconstructions for the last 2,000 years are consistent with other evidence of global climate change and can be considered as additional supporting evidence.
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From page 23... ...
Surface temperature reconstructions have the potential to further improve our knowledge of temperature variations over the last 2,000 years, particularly if additional proxy evidence can be identified and obtained. Additional proxy data that record decadal-to-centennial climate changes, especially for the period A.D.
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From page 24... ...
24 SURFACE TEMPERATURE RECONSTRUCTIONS FOR THE LAST 2,000 YEARS When analyzed in conjunction with historical and archeological evidence, paleoclimatic reconstructions can also tell us how past societies adapted to climate changes. This field of research is moving forward: Hypotheses are being tested, methods are being refined, and new ideas are being introduced.
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