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Recent studies comparing reconstructions of surface temperature and natural (solar and volcanic) radiative forcing (e.g., Lean et al., 1995; Crowley and Kim, 1996, 1999; Overpeck et al., 1997; Mann et al., 1998; Damon and Peristykh, 1999; Free and Robock, 1999; Waple et al., 2001) suggest that a combination of solar and volcanic influences have affected large-scale temperature in past centuries. The primary features of the Northern Hemisphere mean annual temperature histories of Mann et al. (1999a) and Crowley and Lowery (2000) from AD 1000 to 1900 have been largely reproduced based on experiments using an Energy Balance Model forced by estimates of these natural radiative forcings (Crowley, 2000; Mann, 2000) making the argument that the “Little Ice Age” and “Medieval Warm Period”, at the hemispheric mean scale, are consistent with estimates of naturally-forced climate variability. Several studies indicate that the combined effect of these influences has contributed a small component to the warming of the 20th century. Most of these studies isolate greenhouse radiative forcing as being dominant during late 20th century warming (see Crowley, 2000). This argues against a close empirical relationship between certain sun-climate parameters and large-scale temperature that has been claimed for the 20th century (Hoyt and Schatten, 1997). The reader is referred to Chapter 6 for a detailed discussion of these radiative forcings, and to Chapter 12 for comparisons of observed and model simulations of recent climate change.
Since the SAR there have been considerable advances in our knowledge of temperature change over the last millennium. It is likely that temperatures were relatively warm in the Northern Hemisphere as a whole during the earlier centuries of the millennium, but it is much less likely that a globally-synchronous, well defined interval of “Medieval warmth” existed, comparable to the near global warmth of the late 20th century. Marked warmth seems to have been confined to Europe and regions neighbouring the North Atlantic. Relatively colder hemispheric or global-scale conditions did appear to set in after about AD 1400 and persist through the 19th century, but peak coldness is observed during substantially different epochs in different regions. By contrast, the warming of the 20th century has had a much more convincing global signature (see Figure 2.9). This is consistent with the palaeoclimate evidence that the rate and magnitude of global or hemispheric surface 20th century warming is likely to have been the largest of the millennium, with the 1990s and 1998 likely to have been the warmest decade and year, respectively, in the Northern Hemisphere. Independent estimates of hemispheric and global ground temperature trends over the past five centuries from sub-surface information contained in borehole data confirm the conclusion that late 20th century warmth is anomalous in a long-term context. Decreasing temporal resolution back in time of these estimates and potential complications in inferring surface air temperature trends from sub-surface ground temperature measurements precludes, however, a meaningful direct comparison of the borehole estimates with high-resolution temperature estimates based on other proxy climate data. Because less data are available, less is known about annual averages prior to 1,000 years before the present and for conditions prevailing in most of the Southern Hemisphere prior to 1861.
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