Moreover, the impact is unambiguously negative. There is little chance that a significant rise in global temperature could benefit U.S. agriculture, reported the UC Berkeley scientists at the annual meeting in Chicago of the American Agricultural Economic Association.

They estimate that a five degree temperature rise -projected to occur in the next 30-50 years at current rates of carbon dioxide accumulation in the atmosphere - could result in $15 billion to $30 billion in annual damage to American crops.

"People have postulated a wide range of possible impacts on agriculture from global warming. Some even believe there might be benefits. But our results show we can expect damage, not benefits," said Anthony Fisher, chair of UC Berkeley's Department of Agricultural & Resource Economics.

Fisher said mistakes have been made because people did not factor in the cost of providing a water supply in areas of the country that depend on natural rainfall for growing crops. Some two thirds of American counties, mainly in the eastern and midwestern parts of the country, do not have irrigation systems for agriculture.

Past projections also have been based on the value of agricultural land that is close to urban areas. That value goes up as people optimize choices during rising temperatures, said Fisher.

By analyzing agricultural values in non-irrigated, rural areas of the country, the UC Berkeley team reached quite different, more certain conclusions, about the damage from global warming, he said.

"Non-irrigated U.S. agriculture is unambiguously damaged under the CO2 doubling scenario, and the damages are quite large relative to other estimates," the team concluded in a summary to the paper.

The paper was presented in Chicago by UC Berkeley doctoral student Wolfram Schlenker. It was co-authored by Michael Hanemann, UC Berkeley professor of agricultural and resource economics.

The analysis was based on the 1982 and 1987 agricultural censuses of 3,000 U.S. counties which contain an estimation of the relationship between farmland values, climate and other variables. This data, on which most projections are based, overlooks the cost of providing an irrigated water supply in regions that don't now have it, said Fisher.

In California, for instance, the historic cost of providing irrigation was borne by government primarily and now affects land value in ways that no longer reflect the true costs of creating new irrigated systems, he said.

"We found greater damage to the agricultural economy because we did not assume that providing the water to compensate for higher temperatures would be subsidized or cost the farmers nothing," said Fisher.

The UC Berkeley team is currently analyzing the impact of global warming on California agriculture and is fairly sure the effects will also be negative. Fisher explained that higher temperatures
would reduce the snow pack in the Sierras, leading to earlier runoff and less water in late spring and summer.

He added that an 8 percent projected increase in rainfall that would also come with a global rise in temperature would not apparently offset the impact of greater heat in general.