and Briffa (2006), together with other reconstructions, provide supporting evidence for the statement that the warming during the late 20th century is more spatially coherent than during previous warm episodes back to at least A.D. 900 (see also Bradley et al. 2003).

The basic conclusion of Mann et al. (1998, 1999) was that the late 20th century warmth in the Northern Hemisphere was unprecedented during at least the last 1,000 years. This conclusion has subsequently been supported by an array of evidence that includes the additional large-scale surface temperature reconstructions and documentation of the spatial coherence of recent warming described above (Cook et al. 2004, Moberg et al. 2005b, Rutherford et al. 2005, D’Arrigo et al. 2006, Osborn and Briffa 2006, Wahl and Ammann in press) and also the pronounced changes in a variety of local proxy indicators described in previous chapters (e.g., Thompson et al. in press).

Based on the analyses presented in the original papers by Mann et al. and this newer supporting evidence, the committee finds it plausible that the Northern Hemisphere was warmer during the last few decades of the 20th century than during any comparable period over the preceding millennium. The substantial uncertainties currently present in the quantitative assessment of large-scale surface temperature changes prior to about A.D. 1600 lower our confidence in this conclusion compared to the high level of confidence we place in the Little Ice Age cooling and 20th century warming. Even less confidence can be placed in the original conclusions by Mann et al. (1999) that “the 1990s are likely the warmest decade, and 1998 the warmest year, in at least a millennium” because the uncertainties inherent in temperature reconstructions for individual years and decades are larger than those for longer time periods, and because not all of the available proxies record temperature information on such short timescales. 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. The connections between proxy records and environmental variables are well justified in terms of physical, chemical, and biological processes.

  • Tree rings, the dominant data source in many reconstructions, are derived from regional networks with extensive replication, and they are a good indicator of environmental variables at the regional scale. Regional tree ring series are highly correlated with measures of temperature and drought. These connections have a convincing biophysical basis related to tree physiology and growing-season climate. Temperature dominates in some environments and precipitation in others, as is consistent with ecological expectations of limits to growth (Fritts 1976).

  • The same general temperature trends emerge from different reconstructions. Some reconstructions focus on temperature-dependent trees and use wood density measures (Briffa et al. 2002), others focus on temperature-dependent trees and use ring widths (D’Arrigo et al. 2006), and still others incorporate extensive data from precipitation-dependent trees and ice cores and use climate field correlations to derive



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