Working Group I: "The Physical Science Basis"

"Unequivocal" Warming

Working Group I's section of the IPCC Fourth Assessment Report, issued on February 2, 2007, concluded that warming of the earth's climate system was "unequivocal." (WGI Summary for Policymakers/SfP, page 5)

This statement was based on evidence from "observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global mean sea level." (WGI SfP, page 5)

Emissions The "Very Likely" Cause

WGI said most of the warming since the middle of the 20th century is "very likely" the result of increasing concentrations of greenhouse gases emitted by human use of fossil fuels such as coal, oil, and natural gas. (WGI SfP, page 10)

This conclusion represented a greater level of certainty. In its Third Assessment Report in 2001, WGI said most of this warming was only "likely" to have resulted from greenhouse gases. (WGI SfP, page 10)

Future Changes Projected

Key projections of future changes included these points:

  • Warming and other climate changes would "very likely" be greater than those in the 20th century if greenhouse emissions continue at or above current rates." (WGI SfP, page 13)
  • "Best estimates" of average surface warming range from 3.2 to 7.2 degrees F, based on six scenarios representing different levels of emissions. (WGI SfP, page 13)
  • The authors declined to issue "best estimate" projections for sea level rise. Six projected ranges were given, based on the six emission scenarios, from 0.59 to 1.94 feet. The authors noted that melting in Greenland and Antarctica could be larger or smaller than the 1993-2003 observed rates they used in the projections. (WGI SfP, page 13, 14)
  • Greater frequency of weather events such as high temperature extremes, heat waves and heavy precipitation is "very likely." (WGI SfP, page 15)
  • It is "likely" that tropical cyclones (hurricanes and typhoons) in the future will be more intense. They would be marked by "larger peak wind speeds and more heavy precipitation." (WGI SfP, page 15)

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