Over the more than four-and-a-half billion years since the formation of the planet Earth, its climate has remained remarkably stable, and has apparently sustained life for about four billion of those years. Throughout that long period the oceans and the atmosphere have maintained an uneasy equilibrium; the sun has been a sufficiently steady source of heat so that the oceans have neither boiled their water away into space nor frozen down to the equator-fates that many other planets and satellites of the solar system have suffered.
Yet even in the recent past there have been dramatic shifts of climate. A mere 18,000 years ago most of Canada and northwestern Europe were covered by great ice sheets several kilometers thick in places (just as Greenland and the Antarctic are now). In fact, such ice ages and the kind of inter-glacial periods that we are in now have alternated approximately every 100,000 years for the last three million years or more.
If we turn the clock back more than 15 million years, however, we find an Earth with hardly any permanent ice at the poles and a generally much warmer climate. That was the condition that prevailed about 90 percent of the time for the past several hundred million years.
These historical facts illustrate the ever-changing character of our natural climate. Up to now the factors that have determined the climate have been the sun, gradual changes in the Earth's orbit around the sun, and the complex interactions within the planetary "climate system"-the atmosphere, the oceans, the land, the ice of the polar regions, and living things.
Within roughly the last 50 years a new factor has been added-the activities of man himself. A growing accumulation of evidence has persuaded most of the scientific community that human activity may be contributing to a substantial change in the Earth's climate on a global scale. In particular, large-scale consumption of fossil fuels (coal, petroleum and natural gas) is leading to an accumulation of carbon dioxide in the atmosphere, which
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