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Earth's Ancient Past Broadens A Debate on Global Warming
By WILLIAM J. BROAD

楊文瀚 報告

In recent years, scientists have made sizable gains in what was once considered an impossible art--reconstructing the history of earth's atmosphere back into the dim past. They can now peer across more than a half billion years.

The scientists have learned about the changing makeup of the vanished gases by teasing subtle clues from fossilized soils, plants and sea creatures. They have also gained insights from computer models that predict how phenomena like eroding rocks and erupting volcanoes have altered the plant's evolving air. 「It's getting a lot more attention,」 Michael C. MacCracken, chief scientist of the Climate Institute, a research group in Washington, said of the growing field.

For the first time, the intergovernmental Panel on Climate Change, a United Nations group that analyzes global warming, plans to include a chapter on the reconstructions in its latest report, due early next year.

At issue is whether the findings back or undermine the prevailing view on global warming. One side foresees a looming crisis of planetary heating; the other, temperature increases that would be more nuisance than catastrophe.

Perhaps surprisingly, both hail from the same world: scientists who study the big picture of Earth's past, including geologists and paleoclimatologists.

Most public discussions of global warming concentrate on evidence from the last few hundred or, at most, few thousand years. And some climate scientists remain unconvinced that data from deep past are solid enough to be relevant to the debates.

But the experts who peer back millions of years, though they may debate what their work means, do agree on the relevance of their findings. They also agree that the eon known as the Phanerozoic, a lengthy span from the present to 550 million years ago, the dawn of complex life, typically bore concentrations of carbon dioxide that were up to 18 times the levels present in the short reign of Homo sapiens.

The carbon dioxide, the scientists agree, came from volcanoes and other natural sources, as on Mars and Venus. The levels have generally dropped over the ages, as the carbon became a building block of many rock formations and all living things.

Moreover, the opponents tend to agree on why the early Earth's high carbon dioxide levels failed to roast the planet. First, the Sun was dimmer in its youth. Second, as the gas concentrations increase, its heat trapping capacity slows and reaches a plateau.

Where the specialists clash is on what the evidence means for the idea that industrial civilization and the burning of fossil fuels are the main culprits in climate change. The two sides agree that carbon dioxide can block solar energy that would otherwise radiate back into space, an effect known as greenhouse warming. But they differ on its strength.

Some argue that CO2 fluctuations over the Phanerozoic follow climate trends fairly well, supporting a casual relationship between high gas levels and high temperatures. 「The geologic record over the past 550 million years indicates a good correlation,」 said Robert A. Berner, a Yale University geologist and pioneer of paleoclimate analysis. 「There are other factors at work here. But in general, global warming is due to CO2. It was in the past and is now.

Other experts say that is an oversimplification of a complex picture of natural variation. The fluctuations in the gas levels, they say, include changes in sea currents, Sun cycles and cosmic rays that bombard the planet.

「In my view, the uncertainties are too great to draw any conclusions right now,」 Michael Oppenheimer, a professor of geosciences and international affairs at Princeton, said. 「It could be that when the dust settles, some insight will emerge that will be germane to the current problem--how do we keep the climate from spinning out of control.」