新托福阅读材料：Riceyieldsfallingunderglobalwarming

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如果说十年前大家还对全球气候变暖说法保持怀疑的话，近两年世界各地的各类气候灾害频发已经给我们人类敲响了警钟。而据美国科学家最新研究结果表明，全球气候变暖也与亚洲的一些主要大米出产国的耕地面积减少有关。  
0 o8 _1 J/ C+ j# u3 a　　Global warming is cutting rice yields in many parts of Asia, according to research, with more declines to come.

1 b0 \, T" b2 r) U　　Yields have fallen by 10-20% over the last 25 years in some locations.

　　The group of mainly US-based scientists studied records from 227 farms in six important rice-producing countries such as Thailand, Vietnam, India and China.
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/ }$ I6 O4 Y& B. I. m5 e& L' X　　This is the latest in a line of studies to suggest that climate change will make it harder to feed the world's growing population by cutting yields.
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　　In 2004, other researchers found that rice yields in the Philippines were dropping by 10% for every 1C increase in night-time temperature.
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% ^+ P1 @$ D. H9 [& W& ]. ?" ?9 H  l( x1 x　　That finding, like others, came from experiments on a research station.
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0 W6 b1 n. l( t3 p% t　　The latest data, by contrast, comes from working, fully-irrigated farms that grow "green revolution" crops, and span the rice-growing lands of Asia from the Indian state of Tamil Nadu to the outskirts of Shanghai.

& G0 ]3 I% X& b  ^+ k# _　　Describing the findings, which are published in Proceedings of the National Academy of Sciences (PNAS), lead researcher Jarrod Welch said:

6 I& y7 b6 N+ {1 m. C　　"We found that as the daily minimum temperature increases, or as nights get hotter, rice yields drop."
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+ c9 Z( w3 x4 Q0 B　　The mechanism involved is not clear but may involve rice plants having to respire more during warm nights, so expending more energy, without being able to photosynthesise.
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5 ]8 U9 }# r  y) }) Z0 ?　　By contrast, higher temperatures during the day were related to higher yields; but the effect was less than the yield-reducing impact of warmer nights.
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2 M) X' A/ ?0 P+ j0 I　　However, if temperatures continue to rise as computer models of climate project, Mr Welch says hotter days will eventually begin to bring yields down.
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　　We see a benefit of [higher] daytime temperatures principally because we haven't seen a scenario where daytime temperatures cross over a threshold where they'd stop benefiting yields and start reducing them," he told BBC News.

( l3 L9 l" t: k　　"There have been some recent studies on US crops, in particular corn, that showed the drop-off after that threshold is substantial," said the University of California at San Diego researcher.
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　　The 2007 assessment of climate impacts from the Intergovernmental Panel on Climate Change (IPCC) concluded that although a modest temperature rise could increase crop yields in some regions, for "temperature increases more than 3C, average impacts are stressful to all crops assessed and to all regions".
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0 G7 s) u( b; v1 ?9 |　　A study published at the begining of last year concluded that half of the world's population could face a climate-induced food crisis by 2100, with the most extreme summers of the last century becoming routine towards the end of this century.