Surface Temperature Reconstructions for the Last 2,000 Years (2006) / Chapter Skim
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11 Large-Scale Multiproxy Reconstruction Techniques
11 Large-Scale Multiproxy Reconstruction Techniques
Pages 110-119
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From page 110... ...
In the first approach, proxies are calibrated against time series of the dominant patterns of spatial variability in the instrumental temperature record and the results are combined to obtain a time series of large-scale mean surface temperature. In the second approach, the individual proxy data are first composited, and this composite series is then calibrated directly against the time series of large-scale temperature variations.
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From page 111... ...
, the coarse spatial sampling limits our confidence in hemispheric mean or global mean temperature estimates prior to A.D. 1600 and makes it very difficult to generate meaningful quantitative estimates of global temperature variations prior to about A.D.
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From page 112... ...
Criticisms and Advances of Reconstruction Techniques There have been criticisms of the techniques used to create large-scale surface temperature reconstructions and, in particular, of the work done by Mann et al. (e.g., Zorita and von Storch 2005; McIntyre and McKitrick 2003, 2005a,b; von Storch et al.
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From page 113... ...
Methods for evaluation of uncertainties are discussed in Chapter 9. Several recent research efforts have explored how the selection of proxies affects surface temperature reconstructions.
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From page 114... ...
2 0 2 E Asia (regional) -2 0 -2 800 1000 1200 1400 1600 1800 2000 Year 1.0 0.5 records threshold of 0.0 -0.5 Fraction exceeding -1.0 800 1000 1200 1400 1600 1800 2000 FIGURE 11-2 Upper panel: Fourteen temperature-related proxy records selected on the basis of their correlation with the instrumental record, as described by Osborn and Briffa (2006)
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From page 115... ...
However, the methods in use are evolving and are expected to improve. STRENGTHS AND LIMITATIONS OF LARGE-SCALE SURFACE TEMPERATURE RECONSTRUCTIONS The committee identified the following key strengths of large-scale surface temperature reconstructions: · Large-scale surface temperature reconstructions are based on proxy records that are meaningful recorders of environmental variables.
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From page 116... ...
1600 to the present derived from largescale surface temperature reconstructions are consistent with other sources of temperature information for the period, including borehole temperatures and glacier length records. · Prior to about 1600, information is sparser and the pattern of change is not necessarily synchronous, but periods of medieval warmth are seen in a number of diverse records, including historical information from Europe and Asia; cave deposits; marine and lake sediments; and ice cores from Greenland, Ellesmere Island, Tibet, and the equatorial Andes.
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From page 117... ...
OVERALL FINDINGS AND CONCLUSIONS Based on its deliberations and the materials presented in Chapters 111 and elsewhere, the committee draws the following overall conclusions regarding large-scale surface temperature reconstructions for the last 2,000 years: · 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.
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From page 118... ...
The existence of a Little Ice Age from roughly 1500 to 1850 is supported by a wide variety of evidence including ice cores, tree rings, borehole temperatures, glacier length records, and historical documents. Evidence for regional warmth during medieval times can be found in a diverse but more limited set of records including ice cores, tree rings, marine sediments, and historical sources from Europe and Asia, but the exact timing and duration of warm periods may have varied from region to region, and the magnitude and geographic extent of the warmth are uncertain.
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From page 119... ...
Each individual proxy provides a record of environmental change, but the process of combining these signals into a spatially averaged temperature signal requires careful statistical evaluation. It might be possible to circumvent some of the limitations associated with these reconstructions by employing a number of complementary strategies in analyzing the proxy data, including using them to constrain climate models, and by attempting to calibrate the proxy data against climatic variables in different ways.
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