Side Lights – evidence of human responsibility for global warming
* Some of the strongest evidence that humans are at least partially responsible for global warming comes from a 1998 analysis of a 2-mile-deep ice core extracted from the giant ice sheet covering Antarctica. The ice core contains a record of snowfall, atmospheric chemicals, dust, and trapped bubbles of air going back more than 420,000 years. “The study is probably the most convincing evidence to date that humans are making some really large changes to Earth’s climate system,” said Jonathan Overpeck, a U.S. expert on ancient climates. “What [the ice-core analysis] says is that we’re going well beyond the bounds of natural variation.”
* Analysis of the core shows that, during ancient warm periods, the levels of greenhouse gases, such as carbon dioxide, rose. But they rose, say scientists working on the project, by amounts smaller than their present increase. The scientists blame the recent increase on human industrial activity over the last several centuriesx.
* Fossil evidence abounds of vast climate changes in Earth’s history. The fossil (shown at right) is of a fish that once swam millions of years ago in an inland sea that covered Kansas. At that same time, Antarctica had a semitropical climate and the Sahara was a vast grassland.
* During the Ice Ages, large sheets of ice covered much of North America and Europe. The last Ice Age ended only 10,000 years ago. During its height, glaciers extended as far south as what is today northern Iowa and southern Illinois.
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