Climate in the Twentyfirst Century

Climate forecasts are inherently imprecise largely because of two different sorts of uncertainty: incomplete knowledge about how the system works— understandable for a system governed by processes the spatial scales of which range from the molecular to the global and uncertainty about how important climate factors will evolve in the future. A variety of factors affect temperature near the surface of the earth, including variability in solar output, volcanic activity, and dust and other aerosols, in addition to concentrations of greenhouse gases.

However, this uncertainty does not stop one from making some broad statements about (1) the likelihood of the sources of observed global warming and (2) the likely effects of continued warming. In the first case, attempts by climate modelers to reproduce the observed global near-surface temperature

SEA LEVEL RISE

Thermal expansion only

4XCO2

4XCO2

Year

Year source: GFDL climate model

record using only natural variability in climate models have proved inadequate. The Third Assessment Report (2001) of the Intergovernmental Panel on Climate Change (IPCC) attributes some 80 percent of recent rises in global temperature to human activities, with other important contributions coming from volcanic and solar sources. Over the coming century, likely effects of continued warming include higher daily maximum and minimum temperatures, more hot days over most land areas, fewer frosts in winter, fewer cold days over most land areas, a reduced daily range of temperatures, more extreme precipitation events (all very likely), increased risk of drought, increases in cyclone peak wind, and precipitation intensity (likely). Other effects, such as the disintegration of Antarctic ice sheets, carry potentially enormous implications, but are considered very unlikely.

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