constrained (Ramaswamy et al. 2001, NRC 2005). Results using a Monte Carlo approach, climate model simulations, and observations of atmospheric and oceanic warming suggest a total radiative forcing from preindustrial times to the present of 1.4–2.4 W·m–2, which is consistent with the observed warming (Knutti et al. 2002).

Simulations with energy balance and intermediate complexity models indicate that a combination of solar and volcanic forcings can explain periods of relative warmth and cold between A.D. 1000 and 1900, but anthropogenic forcings, particularly increases in greenhouse gases, are needed to reproduce the late 20th century warming (Crowley 2000, Bertrand et al. 2002, Bauer et al. 2003). Coupled atmosphere–ocean models have been used to simulate the relative roles of natural versus human-induced climate forcings in explaining the 20th century changes in global surface temperature constructed from instrumental records (Stott et al. 2000, Ammann et al. 2003) (Figure 10-6). Although the different model simulations use different specifications of the various natural and anthropogenic forcings and different parameterizations, the simulations are in agreement that anthropogenic forcing is the largest contributor to late 20th century warmth.

Current climate models have been tuned to optimize their ability to simulate the present climate, and they exhibit a range of climate sensitivities associated with different treatments of processes such as those associated with clouds and snow and ice (Webb et al. 2006, Winton 2006). Some models have been compared against data for past time periods that encompass major changes in forcings and climate responses, for example, the Last Interglaciation (Kaspar et al. 2005, Otto-Bliesner et al. 2006a) and the Last Glacial Maximum (Masson-Delmotte et al. 2006, Otto-Bliesner et al. 2006b). That these models’ simulated climates for those epochs are consistent with proxy evidence lends credibility to their use for attribution of 20th century climate change and projections of future climate change.

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