1 A frozen mountaintop in Tibet may offer an important clue about whether the Earth is warming perilously.
2 Researchers at Ohio State University and Lanzhou Institute of Glaciology and Geocryology in China have analyzed samples of glacial ice in Tibet and say temperatures there have been significantly higher on average over the past half-century than in any similar period in the past 10,000 years.
3 The ice samples are an important piece of evidence supporting theories that the Earth has warmed considerably in recent times, largely because of pollutants in the air, and will warm far more in the century ahead.
4 A substantial warming would melt some of the Earth's polar ice caps, raising the level of the oceans and causing widespread flooding of heavily populated coastal areas.
5 'If you can use data to reconstruct what happened in the past, you have much more confidence in predictions for the future,' said Lonnie Thompson, a research scientist at Ohio State who dug for and analyzed the ice samples.
6 To compare temperatures over the past 10,000 years, researchers analyzed the changes in concentrations of two forms of oxygen.
7 These measurements can indicate temperature changes, researchers said, because the rates of evaporation of these oxygen atoms differ as temperatures change.
8 Analysis of ice from the Dunde ice cap, a glacial plateau in Tibet 17,000 feet above sea level, show that average temperatures were higher in 1937-87 than in any other 50-year period since before the last Ice Age, Mr. Thompson said.
9 Some climate models project that interior regions of Asia would be among the first to heat up in a global warming because they are far from oceans, which moderate temperature changes.
10 But the ice-core samples aren't definitive proof that the so-called greenhouse effect will lead to further substantial global heating, Mr. Thompson acknowledged.
11 According to greenhouse theories, increased carbon dioxide emissions, largely caused by burning of fossil fuels, will cause the Earth to warm up because carbon dioxide prevents heat from escaping into space.
12 Skeptics say that if that's the case, temperatures should have risen fairly uniformly over the past century, reflecting the increase in carbon dioxide.
13 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.
14 Other temperature data show similar unexplained swings.
15 'Climate varies drastically due to natural causes,' said Mr. Thompson.
16 But he said ice samples from Peru, Greenland and Antarctica all show substantial signs of warming.

A substantial warming would melt some of the Earth ' s polar ice caps , raising the level of the oceans and causing widespread flooding of heavily populated coastal areas .
substantial warming
How much warmer would the Earth need to be to melt 20% of the polar ice caps?