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Climate change in the United States has had a greater influence on crop yields than previously thought, a new study suggests. The results have global implications.

The research, published in this week's issue of Science, analysed climate trends from 1982 to 1998. During this time corn and soybean yields rose by 30 per cent, an increase that has until now been attributed to better farming practices.

But by looking at regions that have experienced the same changes in technology — but different changes in climate — scientists from the Carnegie Institute of Washington and Stanford University found that 20 per cent of the total gains were actually due to a cooling climate over this period. The researchers suggest that increases in temperature due to climate change will have the reverse effect, with global implications.

"Our results suggest that global warming will affect food production," says Gregory Asner who led the study. "We can expect a 17 per cent decline in yield of these crops for a one degree increase in growing-season temperature."

These results are likely to spur interest in developing countries, many of which are already experiencing warming temperatures.

"Agricultural production in the developing world is even more vulnerable to global warming than mid- and high-latitude places like the United States," says Cynthia Rosenzweig of Columbia University and NASA's Goddard Institute for Space Studies in New York. "Temperatures are already high and developing countries usually have a less robust research infrastructure, meaning that they will be less able to generate differently-adapted crops."

Along with colleagues at NASA, as well as in Uruguay and Egypt, Rosenzweig has been investigating whether increasing temperatures are holding back crops in the developing world. Lower yields would put a strain on ecosystems by expanding agriculture into marginal and potentially more environmentally sensitive areas.

But David Lobell, co-author of the study, notes that lack of records in developing countries makes such research more difficult. "The first step is to try and get a measurement of crop yield," he says. He is using satellite records from the past 20 years to generate spatial maps of crop yield.

One area Lobell has already looked at is north-west Mexico. Here, he says, almost 100 per cent of gains in wheat yields have been due to a drop in temperature, rather than to better farming practices. However, he also points out that improved agricultural techniques have also had an important beneficial effect.

"In such countries, the impact of disease and pests are high and a lot of effort goes into just trying to maintain current yields," he says.

Source: SciDev.Net 2003