forcing due to the greenhouse gases injected into the atmosphere by human activities is about 2.5 watts per square meter (IPCC 2001).
Based on a climate sensitivity of 3°C for a carbon dioxide doubling, as estimated in the preceding section, a greenhouse forcing of 2.5 watts per square meter is sufficient to produce a warming of around 2°C. The observed warming during the 20th century of around 0.6°C is less than the estimated response to the greenhouse forcing for two reasons:
it is partially offset by increases in the concentration of sulfate and other aerosols, which tend to produce cooling at the Earth’s surface (e.g., Santer et al. 1995), and
part of the warming has not been realized yet because the oceans and polar ice sheets have not had sufficient time to equilibrate with the forcing (e.g., Hansen et al. 2005).
The observed 0.6°C warming during the 20th century is much larger than the internal variability in climate models. Model simulations that include both externally forced and internal variability, including plausible prescriptions of time-varying sulfate aerosols, yield time series of global mean temperature that resemble the observations (Stott et al. 2000, Ammann et al. 2003). To the extent that the warmth of the most recent one or two decades stands out above the natural variability in mean surface temperature over the last 2,000 years, the surface temperature record serves as supporting evidence that human activities are largely responsible for the recent warming. However, the attribution of the recent global warming to human activities does not rest solely or even principally upon paleoclimate evidence.
The next chapter of this report provides a brief description of the instrumental record and some considerations that apply to estimating large-scale surface temperature variations on the basis of observations at a limited number of sites. Most of what we know about how the temperature of the Earth has varied on the timescale of the last 2,000 years is based on proxy records, including documentary records, archeological evidence, and a variety of natural sources including tree rings, corals, ice cores, ocean and lake sediments, borehole temperatures, and glacier length records. The sources and characteristics of the various proxy datasets are discussed in Chapters 3–8. The statistical procedures and assumptions that are used in reconstructions of surface temperatures from proxy data are discussed in Chapter 9. Paleoclimate models and an expanded discussion of variations in climate forcing over the last two millennia are presented in Chapter 10. Finally, Chapter 11 describes the synthesis of evidence derived from a variety of different proxies to produce large-scale surface temperature reconstructions. These techniques have been a subject of controversy in a number of recent papers in the refereed literature, so Chapter 11 also assesses their strengths, limitations, and prospects for improvement.