Rice is arguably the world's most important food source and helps feed about half the globe's people. But yields in many areas will drop as the globe warms in future years, a review of studies on rice and climate change suggests.

The poorest parts of the world, including Africa, will probably be hardest hit, the study says. Rice harvests already need to increase by about a third just to keep up with global population growth.

Predicting how a changing climate will affect crop yields is notoriously difficult. Temperature, carbon dioxide concentration and ozone levels all have a big impact on growth. Yet most studies look at just one of these factors, making it difficult to know what the combined effect will be.

It is also hard to know whether results from experiments in greenhouses with artificial climates will hold true in the real world. But when the evidence from some 80 different studies is combined, the outlook is bleak, says Elizabeth Ainsworth of the University of Illinois, Urbana-Champaign.

Troubling temperatures

In regions where the average daily temperatures are expected to rise above 30ºC, rice yields will start to fall off, and the impact will get worse as the temperature increases.

The drop in yield caused by rising temperatures can be counteracted by the boost to photosynthesis provided by the increased levels of carbon dioxide driving climate change. But when Ainsworth pooled the studies, she found that effect is not strong enough to counteract the stress plants suffer at high temperatures.

Harvests will also be reduced by rising ground-level ozone concentrations. They are caused by nitrogen oxides (NOX) from power stations that catalyse the formation of ozone in warm and sunny conditions. Ainsworth’s review found that ozone concentrations of around 60 parts per billion, which have already being recorded on farms in China and the United States, cause yields to drop by 14%.

Experiments on the effect of ozone using greenhouses containing artificial atmospheres are still crude, so other rice researchers are urging caution in interpreting Ainsworth's results. For example, many experiments use fixed levels of ozone, but outdoors levels fluctuate daily and plants can use the low points to recover from brief periods of high concentrations.

"Better breeds needed"

In general, however, critics agree with Ainsworth’s conclusion that new varieties of rice, bred to tolerate high ozone and increased temperatures, are urgently needed.

She points out that tropical regions need these varieties most, as temperatures there are already close to the maximum that traditional types of rice can withstand. And these many of those areas, including parts of Africa, already suffer regular food shortages.

"This won’t affect the planet equally," says Ainsworth. "In places where the demand for food is already too great, things are going to get worse."

Agricultural scientists say it is still too early to say for sure how climate change will affect yields. Very little is known about the combined effect of high ozone levels and increased carbon dioxide, for example, since the two factors are usually studied independently.

"In the real world, it's still pretty hard to know how these factors will stack up," says Daniel Taub of Southwestern University in Georgetown, Texas, US.

But he adds that Ainsworth's study, together with her previous field experiments, have all but wiped out early hopes that increased carbon dioxide might be enough to overcome the other factors and boost yields. "Considering that we're likely to see an increase in population, if one doesn't see an increase in yields that's worrisome," Taub told New Scientist.

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Journal ref: Global Change Biology (DOI: 10.1111/j.1365-2486.2008.01594.x)