Scientists have found that global warming could decrease crop yields, not increase them as once thought. VOA's Frank Ling reports that this finding suggests future crop estimates may have to be lowered.

At the same time, they point out that extra CO-2 also hurts plants. They say plant growth is slowed by higher temperatures and lower soil moisture caused by faster evaporation.

The United Nations Intergovernmental Panel on Climate Change, the world's leading authority on global warming, has concluded that these two trends balance each other, so global warming was not expected to hurt agriculture overall.

U.S. government and university experiments carried out in greenhouses have supported this view.

"What they do is put one kind of plant in two different greenhouses that are right next to each other and then they put higher CO-2 levels in one of the greenhouses and have regular atmospheric level of CO-2 in the other greenhouse," said Myron Ebell from the Competitive Enterprise Institute in Washington, a pro-business group that opposes government efforts to limit greenhouse gas emissions.

He says more carbon dioxide is good for farming.

"The results of these studies going back half a century or more are stunning because almost every single study show not only do all kinds of plants grow more quickly with higher levels of CO-2, but they are also much hardier," he said. "They are more resistant to things like drought."

But a new study shows differently when crops are grown outdoors. University of Illinois agriculture expert Stephen Long and colleagues report in the journal "Science" that the benefits of raised CO-2 levels in global warming do not balance the harmful effects.

"The two things were very roughly thought to counteract each other," he said. :However, the CO-2 fertilization until recently has been studied only in greenhouses and other enclosed environments. If you raise the CO-2 level under fully open conditions, do you see this large fertilization of crop yield? Roughly what we found was that under open air conditions, that increase appears only to be half of what was expected."

The experiments were carried out using five different crops around the world. All showed considerable growth reductions outdoors. Long says that in the tropics, carbon dioxide increases may not help the growth of crops like corn and sorghum at all.

The findings suggest that without changes in the way crops are planted, future yields will drop with increasing carbon dioxide levels.

"We also simulated that rise in our experiment," he said. "That reduces the yield of soybean by about 20 percent, which is a very large yield decrease, and this has not been taken account in future projections on food supply."

But Myron Ebell of the Competitive Enterprise Institute maintains that higher carbon dioxide concentrations benefit agriculture.

"There is a lot of satellite evidence that the Earth is in a period of rapid greening right now and that is probably due to higher CO-2 levels," he said.

Researchers on both sides of the debate agree that more studies are needed to understand the effects of climate change and greenhouse gases on crops. http://www.voanews.com/english/archive/2006-06/2006-06-30-voa64.cfm?moddate=2006-06-30