The committee generally agrees with the assessment of human-caused climate change presented in the IPCC Working Group I (WGI) scientific report, but seeks here to articulate more clearly the level of confidence that can be ascribed to those assessments and the caveats that need to be attached to them. There also is a pressing need for a global observing system designed for monitoring climate. Reducing the wide range of uncertainty inherent in current model predictions of global climate change will require major advances in understanding and modeling of both (1) the factors that determine atmospheric concentrations of greenhouse gases and aerosols, and (2) the so-called “feedbacks” that determine the sensitivity of the climate system to a prescribed increase in greenhouse gases. Because there is considerable uncertainty in current understanding of how the climate system varies naturally and reacts to emissions of greenhouse gases and aerosols, current estimates of the magnitude of future warming should be regarded as tentative and subject to future adjustments (either upward or downward). Hence, national policy decisions made now and in the longer-term future will influence the extent of any damage suffered by vulnerable human populations and ecosystems later in this century.
This predicted temperature increase is sensitive to assumptions concerning future concentrations of greenhouse gases and aerosols. This estimate is also consistent with inferences about the sensitivityĢ of climate drawn from comparing the sizes of past temperature swings between ice ages and intervening warmer periods with the corresponding changes in the climate forcing. The predicted warming of 3☌ (5.4☏) by the end of the 21st century is consistent with the assumptions about how clouds and atmospheric relative humidity will react to global warming. The mid-range model estimate of human induced global warming by the Intergovernmental Panel on Climate Change (IPCC) is based on the premise that the growth rate of climate forcingġ agents such as carbon dioxide will accelerate. The impacts of these changes will be critically dependent on the magnitude of the warming and the rate with which it occurs. These include increases in rainfall rates and increased susceptibility of semi-arid regions to drought. Secondary effects are suggested by computer model simulations and basic physical reasoning.
Human-induced warming and associated sea level rises are expected to continue through the 21st century. The changes observed over the last several decades are likely mostly due to human activities, but we cannot rule out that some significant part of these changes is also a reflection of natural variability.
Greenhouse gases are accumulating in Earth's atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise.