9.3.3 What Can be Learned from the Past 1,000 Years?

External forcing relative to the present is generally small for the last millennium when compared to that for the mid-Holocene and LGM. Nonetheless, there is evidence that climatic responses to forcing, together with natural internal variability of the climate system, produced several well-defined climatic events, such as the cool conditions during the 17th century or relatively warm periods early in the millennium.

9.3.3.1 Evidence of External Influence on the Climate Over the Past 1,000 Years

A substantial number of proxy reconstructions of annual or decadal NH mean surface temperature are now available (see Figure 6.11, and the reviews by Jones et al., 2001 and Jones and Mann, 2004). Several new reconstructions have been published, some of which suggest larger variations over the last millennium than assessed in the TAR, but uncertainty remains in the magnitude of inter-decadal to inter-centennial variability. This uncertainty arises because different studies rely on different proxy data or use different reconstruction methods (Section 6.6.1). Nonetheless, NH mean temperatures in the second half of the 20th century were likely warmer than in any other 50-year period in the last 1.3 kyr (Chapter 6), and very likely warmer than any such period in the last 500 years. Temperatures subsequently decreased, and then rose rapidly during the most recent 100 years. This long-term tendency is punctuated by substantial shorter-term variability (Figure 6.10). For example, cooler conditions with temperatures 0.5°C to 1°C below the 20th-century mean value are found in the 17th and early 18th centuries.

A number of simulations of the last millennium (Figure 6.13) have been performed using a range of models, including some simulations with AOGCMs (e.g., Crowley, 2000; Goosse and Renssen, 2001; Bertrand et al., 2002; Bauer et al., 2003; Gerber et al., 2003; see also Gonzalez-Rouco et al., 2003; Jones and Mann, 2004; Zorita et al., 2004; Weber, 2005; Tett et al., 2007). These simulations use different reconstructions of external forcing, particularly solar, volcanic and greenhouse gas forcing, and often include land use changes (e.g., Bertrand et al., 2002; Stendel et al., 2006; Tett et al., 2007). While the use of different models and forcing reconstructions leads to differences, the simulated evolution of the NH annual mean surface temperature displays some common characteristics between models that are consistent with the broad features of the data (Figures 6.13 and 9.4). For example, all simulations show relatively cold conditions during the period around 1675 to 1715 in response to natural forcing, which is in qualitative agreement with the proxy reconstructions. In all simulations shown in Figure 6.13, the late 20th century is warmer than any other multi-decadal period during the last millennium. In addition, there is significant correlation between simulated and reconstructed variability (e.g., Yoshimori et al., 2005). By comparing simulated and observed atmospheric CO₂ concentration during the last 1 kyr, Gerber et al. (2003) suggest that the amplitude of the temperature evolution simulated by simple climate models and EMICs is consistent with the observed evolution of CO₂. Since reconstructions of external forcing are virtually independent from the reconstructions of past temperatures, this broad consistency increases confidence in the broad features of the reconstructions and the understanding of the role of external forcing in recent climate variability. The simulations also show that it is not possible to reproduce the large 20th-century warming without anthropogenic forcing regardless of which solar or volcanic forcing reconstruction is used (Crowley, 2000; Bertrand et al., 2002; Bauer et al., 2003; Hegerl et al., 2003, 2007), stressing the impact of human activity on the recent warming.

While there is broad qualitative agreement between simulated and reconstructed temperatures, it is difficult to fully assess model-simulated variability because of uncertainty in the magnitude of historical variations in the reconstructions and differences in the sensitivity to external forcing (Table 8.2). The role of internal variability has been found to be smaller than that of the forced variability for hemispheric temperature means at decadal or longer time scales (Crowley, 2000; Hegerl et al., 2003; Goosse et al., 2004; Weber et al., 2004; Hegerl et al., 2007; Tett et al., 2007), and thus internal variability is a relatively small contributor to differences between different simulations of NH mean temperature. Other sources of uncertainty in simulations include model ocean initial conditions, which, for example, explain the warm conditions found in the Zorita et al. (2004) simulation during the first part of the millennium (Goosse et al., 2005b; Osborn et al., 2006) (Errata).