Overview

In its Third Assessment Report the IPCC has concluded that there is new and stronger evidence that most of the warming (of the global surface temperature) observed over the last 50 years is attributable to human activities . The claim is based on an expressed belief that progress has been made in reducing uncertainty about the responses of the climate system to different external influences. The claimed major areas of progress relate to understanding how climate has actually varied over the recent past, and in the development of computer models that better quantify the relative contributions of natural and anthropogenic factors to climate change. The IPCC confidence that global warming is a consequence of anthropogenic causes is strongly linked to its conclusion that the warming of the past 100 years is very unlikely to be due to internal variability alone, as estimated by current models .

The reconstruction of global surface temperature over the past 1,000 years (see Figure 7), especially the limited variability of the preinstrument period, is the basis for the IPCC claim of better understanding of how climate has actually varied over the recent past. IPCC chooses to give weight to the reconstruction despite the fact that the envelope of uncertainty is as large as the range of variability being investigated. Ignoring the uncertainty envelope, the IPCC argues that because there is only limited global surface temperature variability until the 20th century then the warming of the 20th century must be unusual. In addition to the low skill...