北极寒季时排放的甲烷超过预期

The amount of methane¹ gas escaping from the ground during the long cold period in the Arctic each year and entering Earth's atmosphere is likely much higher than estimated by current climate change models, concludes a major new study led by San Diego State University. A team comprising ecologists Walter Oechel (SDSU and Open University) and Donatella Zona (SDSU and the University of Sheffield) and scientists from NASA's Jet Propulsion Laboratory (JPL), Harvard University, the National Oceanic and Atmospheric² Administration, and the University of Montana, found that far more methane is escaping from Arctic tundra³ during the cold months -- when the soil surface is frozen (generally from September through May) -- as well as from upland tundra, than prevailing⁴ assumptions and climate modelers previously⁵ believed. In fact, they found that at least half of the annual methane emissions⁷ occur in the cold months, and that drier, upland tundra can be a larger emitter of methane than wet tundra. The finding challenges critical assumptions in current global climate models. The results are published today in the Proceedings⁸ of the National Academy of Sciences.

 

Methane is a potent⁹ greenhouse gas that intensifies¹⁰ atmospheric warming and is approximately 25 times more potent per molecule¹¹ than carbon dioxide over a 100-year period. Methane trapped in the Arctic tundra comes primarily from microbial decomposition¹² of organic matter in soil that thaws¹³ seasonally¹⁴. This methane naturally seeps¹⁵ out of the soil over the course of the year, but scientists worry that climate change could lead to the release of even larger emissions from organic matter that is currently stabilized¹⁶ in a deep, frozen soil layer called permafrost.

 

Over the past several decades, scientists have used specialized¹⁷ instruments to accurately¹⁸ measure methane emissions in the Arctic and incorporated those results into global climate models. However, almost all of these measurements have been obtained during the Arctic's short summer. The region's long, brutal¹⁹ cold period, which accounts for between 70 and 80 percent of the year, has been largely 'overlooked and ignored,' according to Oechel. Most researchers, he said, figured that because the ground is frozen solid during the cold months, methane emissions practically shut down for the winter.

 

"Virtually all the climate models assume there's no or very little emission⁶ of methane when the ground is frozen," Oechel said. "That assumption is incorrect."

 

The water trapped in the soil doesn't freeze completely even below zero degrees Celsius²⁰, he explained. The top layer of the ground, known as the active layer, thaws in the summer and refreezes in the winter, and it experiences a kind of sandwiching effect as it freezes. When temperatures are right around zero degrees Celsius -- the so-called 'zero curtain' -- the top and bottom of the active layer begin to freeze, while the middle remains²¹ insulated. Microorganisms in this unfrozen middle layer continue to break down organic matter and emit methane many months into the Arctic's cold period each year.

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