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The American Geophysical Union (AGU) stated that human activities, especially the burning of fossil fuels, such as coal and petroleum, have significantly increased atmospheric concentrations of greenhouse gases (gases such as carbon dioxide that trap solar radiation). The continued build-up of these gases in the atmosphere is expected to cause major disruptions to the global climate system, including rising average surface temperatures, rising sea levels caused by the melting of polar ice caps, and increasing rates of evaporation and precipitationthat several warming periods during the last several thousand years have been unrelated to the burning of fossil fuels. It also acknowledged that the approximately 0.5° C (1° F) increase in the earth's average temperature during the last 150 years was within the range of natural climate variation.
The AGU found that climate studies of the distant past showed that changes in atmospheric carbon dioxide levels are associated with large climate variations. “There is no known geologic precedent for the transfer of carbon from the earth's crust to atmospheric carbon dioxide, in quantities comparable to the burning of fossil fuels, without simultaneous changes in ... the climate system".