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 A frozen mountaintop in Tibet may offer an important clue 
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about whether the Earth is warming perilously.
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 Researchers at Ohio State University and Lanzhou Institute of Glaciology and Geocryology in China have analyzed samples of glacial ice in Tibet 
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and say 
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temperatures there have been significantly higher on average over the past half-century than in any similar period in the past 10,000 years.
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 The ice samples are an important piece of evidence 
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supporting theories 
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that the Earth has warmed considerably in recent times, 
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largely because of pollutants in the air, 
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and will warm far more in the century ahead. 
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A substantial warming would melt some of the Earth's polar ice caps, 
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raising the level of the oceans 
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and causing widespread flooding of heavily populated coastal areas.
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 If you can use data 
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to reconstruct what happened in the past, 
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you have much more confidence in predictions for the future, 
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said Lonnie Thompson, a research scientist at Ohio State 
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who dug for and analyzed the ice samples.
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 To compare temperatures over the past 10,000 years,
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 researchers analyzed the changes in concentrations of two forms of oxygen.
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 These measurements can indicate temperature changes, 
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researchers said, 
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because the rates of evaporation of these oxygen atoms differ 
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as temperatures change.
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 Analysis of ice from the Dunde ice cap, a glacial plateau in Tibet 17,000 feet above sea level, show 
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that average temperatures were higher in 1937-87 than in any other 50-year period since before the last Ice Age,
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 Mr. Thompson said.
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 Some climate models project 
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that interior regions of Asia would be among the first 
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to heat up in a global warming 
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because they are far from oceans,
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 which moderate temperature changes.
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 But the ice-core samples aren't definitive proof 
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that the so-called greenhouse effect will lead to further substantial global heating, 
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Mr. Thompson acknowledged. 
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According to greenhouse theories,
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 increased carbon dioxide emissions,
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 largely caused by burning of fossil fuels,
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 will cause the Earth to warm up 
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because carbon dioxide prevents heat from escaping into space.
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 Skeptics say
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 that 
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if that's the case,
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 temperatures should have risen fairly uniformly over the past century, 
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reflecting the increase in carbon dioxide.
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 Instead, the Dunde ice-core record shows increasing temperatures from 1900 through the early 1950s, decreasing temperatures from the late 1950s through the mid-1970s, then higher temperatures again through last year.
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 Other temperature data show similar unexplained swings.
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 Climate varies drastically due to natural causes, 
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said Mr. Thompson.
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 But he said 
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ice samples from Peru, Greenland and Antarctica all show substantial signs of warming.
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