文章来源:未知 文章作者:meng 发布时间:2010-10-11 02:49 字体: [ ]  进入论坛

The soils in large areas of the Southern Hemisphere, including major portions of Australia, Africa and South America, have been drying up in the past decade, a group of researchers conclude in the first major study to ever examine "evapotranspiration(蒸发蒸腾) " on a global basis. Most climate models have suggested that evapotranspiration, which is the movement of water from the land to the atmosphere, would increase with global warming. The new research, published online this week in the journal Nature, found that's exactly what was happening from 1982 to the late 1990s.

But in 1998, this significant increase in evapotranspiration – which had been seven millimeters per year – slowed dramatically or stopped. In large portions of the world, soils are now becoming drier than they used to be, releasing less water and offsetting1(抵消,弥补) some moisture(水分,湿度) increases elsewhere.

Due to the limited number of decades for which data are available, scientists say they can't be sure whether this is a natural variability or part of a longer-lasting global change. But one possibility is that on a global level, a limit to the acceleration2 of the hydrological cycle(水循环) on land has already been reached.

If that's the case, the consequences could be serious.

They could include reduced terrestrial(地球的,陆地的) vegetation growth, less carbon absorption, a loss of the natural cooling mechanism3 provided by evapotranspiration, more heating of the land surface, more intense heat waves and a "feedback loop" that could intensify4 global warming.

"This is the first time we've ever been able to compile observations such as this for a global analysis," said Beverly Law, a professor of global change forest science at Oregon State University. Law is co-author of the study and science director of the AmeriFlux network of 100 research sites, which is one major part of the FLUXNET synthesis that incorporates data from around the world.

"We didn't expect to see this shift in evapotranspiration over such a large area of the Southern Hemisphere," Law said. "It is critical to continue such long-term observations, because until we monitor this for a longer period of time, we can't be sure why this is occurring."

Some of the areas with the most severe drying include southeast Africa, much of Australia, central India, large parts of South America, and some of Indonesia. Most of these regions are historically dry, but some are actually tropical rain forests.

The rather abrupt5 change(突变,陡变) from increased global evapotranspiration to a near halt in this process coincided with a major El Nino event in 1998, the researchers note in their report, but they are not suggesting that is a causative mechanism for a phenomenon that has been going on for more than a decade now.

Greater evapotranspiration was expected with global warming, because of increased evaporation6 of water from the ocean and more precipitation(沉淀,坠落) overall. And data indeed show that some areas are wetter than they used to be.

However, other huge areas are now drying out, the study showed. This could lead to increased drought stress on vegetation and less overall productivity, Law said, and as a result less carbon absorbed, less cooling through evapotranspiration, and more frequent or extreme heat waves.

Some of the sites used in this study are operated by Law's research group in the central Oregon Cascade7 Range in the Metolius River watershed8, and they are consistent with some of these concerns. In the last decade there have been multiple years of drought, vegetative stress, and some significant forest fires in that area.

Evapotranspiration returns about 60 percent of annual precipitation back to the atmosphere, in the process using more than half of the solar energy absorbed by land surfaces. This is a key component9 of the global climate system, linking the cycling of water with energy and carbon cycles.

Longer term observations will be needed to determine if these changes are part of decadal-scale variability or a longer-term shift in global climate, the researchers said.


1 offsetting c005dfe34c894146c623d2dd18e54b55     
n.偏置法v.抵消( offset的现在分词 );补偿;(为了比较的目的而)把…并列(或并置);为(管道等)装支管
  • Dealers, having concluded a forward contract, should always hedge with an offsetting contract. 外汇经营商在签订了一项远期合同之后总是应进行套头交易签订一项相抵合同。 来自辞典例句
  • Where does Germany think offsetting shifts into greater external deficits might occur? 在德国看来,这么大的外部赤字应该转移到哪里? 来自互联网
2 acceleration ff8ya     
  • All spacemen must be able to bear acceleration.所有太空人都应能承受加速度。
  • He has also called for an acceleration of political reforms.他同时呼吁加快政治改革的步伐。
3 mechanism zCWxr     
  • The bones and muscles are parts of the mechanism of the body.骨骼和肌肉是人体的组成部件。
  • The mechanism of the machine is very complicated.这台机器的结构是非常复杂的。
4 intensify S5Pxe     
  • We must intensify our educational work among our own troops.我们必须加强自己部队的教育工作。
  • They were ordered to intensify their patrols to protect our air space.他们奉命加强巡逻,保卫我国的领空。
5 abrupt 2fdyh     
  • The river takes an abrupt bend to the west.这河突然向西转弯。
  • His abrupt reply hurt our feelings.他粗鲁的回答伤了我们的感情。
6 evaporation Pnoxc     
  • Be careful not to lose too much liquid by evaporation.小心不要因蒸发失去太多水分。
  • Our bodies can sweat,thereby losing heat by evaporation.我们的身体能出汗,由此可以蒸发散热。
7 cascade Erazm     
  • She watched the magnificent waterfall cascade down the mountainside.她看着壮观的瀑布从山坡上倾泻而下。
  • Her hair fell over her shoulders in a cascade of curls.她的卷发像瀑布一样垂在肩上。
8 watershed jgQwo     
  • Our marriage was at a watershed.我们的婚姻到了一个转折关头。
  • It forms the watershed between the two rivers.它成了两条河流的分水岭。
9 component epSzv     
  • Each component is carefully checked before assembly.每个零件在装配前都经过仔细检查。
  • Blade and handle are the component parts of a knife.刀身和刀柄是一把刀的组成部分。
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