2006年VOA标准英语-Scientists Say Melting Permafrost Could Amplify（在线收听）

By Frank Ling
Washington
07 August 2006

New research shows that global warming might be worse than expected because of melting permafrost, permanently frozen soil, which can release the "greenhouse" gas, carbon dioxide. Computer models predict higher future temperatures when these gases are taken into consideration.

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Frozen tundra
  
Scientists say the Earth's rising atmospheric temperatures are largely caused by human-generated emissions of heat-trapping gases, mainly carbon dioxide. But recent findings suggest that the Earth is also a culprit. It, too, gives off carbon dioxide.

That is because vast amounts of carbon are trapped in permafrost. Over the ages, this frozen ground has accumulated layers of windblown dust, roots, and other organic, or carbon-containing, matter as glaciers advanced and retreated.

As long as the permafrost remains frozen, the carbon stays in the soil. But botanist Ted Schurr of the University of Florida says higher temperatures will melt the soil, releasing greenhouse gases, which would boost temperatures even more. Schurr traveled to Siberia to collect samples of permafrost up to three meters below the surface. He reports in the journal Science that when the permafrost melted in the laboratory, microbes digested the carbon and converted it to carbon dioxide.

"It's like food in your freezer," explained Schurr. "If It's really cold, those bacteria and fungi can't do their thing. Now if you warm this soil organic matter up from below there, you unfreeze it, then they can metabolize it and convert it to carbon dioxide. This gas then goes into the atmosphere and contributes to the carbon there."

Scientists know how much carbon dioxide people put into the air each year, but until now it was not clear how much greenhouse gas the Earth could give off. Schurr found that the deposits deep in the Siberian permafrost were much greater than previously thought and could potentially double current carbon dioxide concentrations if released.

"We describe a really large pool of about 500 billion tons of carbon," said Schurr. "In comparison, the atmosphere right now has about 730 billion tons. So we are talking about almost as much carbon stored in permafrost in Siberia as there is in the atmosphere now. "

In another study in the journal Geophysical Research Letters, Margaret Torn at the University of California at Berkeley shows the climate impact of this additional greenhouse factor. "We found a significant amount of warming coming back from these feedbacks that we're not yet estimating," said Torn. "The Intergovernmental Panel on Climate Change, or IPCC, currently estimates that we could have warming by as much as 5.8 degrees Celsius by the end of the 21st century. But if Earth responds as it has in the past, we would actually be committed to 7.7 degrees Celsius warming."

These latest studies are only steps toward a complete understanding of the atmosphere's complex nature. Scientists are still struggling to incorporate other greenhouse factors such as clouds, dust, and pollutants into their analysis. Torn says as more is known about them, they will be used to refine current climate models.