Rice yields 'to fall' under global warming

Global warming is set to cut rice yields in Asia, research suggests.

Scientists found that over the last 25 years, the growth in yields has fallen by 10-20% in some locations, as night-time temperatures have risen.

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.

This is the latest study to suggest that climate change will make it harder to feed the world's growing population.

In 2004, other researchers found that rice yields in the Philippines were dropping by 10% for every 1C increase in night-time temperature.

That finding, like others, came from experiments on a research station.

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.

Describing the findings, which are published in Proceedings of the National Academy of Sciences (PNAS), lead researcher Jarrod Welch said:

"We found that as the daily minimum temperature increases, or as nights get hotter, rice yields drop."

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.

Although yields have risen as farming methods improved, the rate of growth has slowed as nights have grown warmer.

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.

However, if temperatures continue to rise as computer models of climate project, Mr Welch says hotter days will eventually begin to bring yields down.

"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.

"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.

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".

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.

Richard.Black-INTERNET@bbc.co.uk

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Correction 12th August: this story has been amended to reflect the fact that it is the rate of growth in yields that has fallen, not the yields themselves.