If global warming is fact (and it's by no means universally accepted, even among respected scientists), agriculture will face even more uncertainty in years ahead.

This year broke 500-year heat records in much of Europe, resulting in thousands of deaths and decimating crop production. In the United States, corn and soybean yields in many Midwest areas are best described as pitiful as a result of a dry summer and a dearth of snow last winter; several million acres have been affected. Over the past five years, drought has been a major problem in the Southwest and in the Southeast (where things were so bad the state of Georgia last year paid farmers who would agree not to irrigate).

While total agricultural production in the United States has increased steadily since the 1970s, there has also been greater variability in production, partly climate-related.

A just-released study on the potential impact of climate change on U.S. agriculture notes that the most recent climate models predict that U.S. grain-producing regions will heat up, on average, more than the rest of the world, and that despite an increase in high-intensity rainfall events, those areas will experience more droughts.

"While some farmers in the United States, growing some crops in some years, may prosper because of warmer temperatures, more precipitation, and carbon dioxide fertilization, agriculture in general is likely to become increasingly unstable, and farmers may find it hard to plan what crops to plant and when."

The white paper, prepared for the Center for Health and Global Environment at Harvard Medical School, notes that such events "have already taken a heavy toll on U.S. crop yields."

This year's drought was "in large part, responsible for a 94 million-bushel decrease in end-season U.S. wheat stocks, and based on figures as of Sept. 1, national average soybean yields are estimated at only 36.4 bushels per acre, the lowest levels since 1996. Some Kansas farmers have reported corn harvests of 30 to 40 bushels per acre, compared to typical averages of around 150 bushels per acre." Similar stories abound in Minnesota, not usually thought of as a droughty region.

"Crop modeling results paint a consistent picture of yields being lower than today," says William Easterling, Penn State University agronomy professor.

Based on historic trends, warmer temperatures and extreme weather events will also increase crop losses from insects, diseases, and weeds, says X.B. Yang, Iowa State University associate professor of plant pathology. In just three growing seasons, from 1988 through 1990, pests destroyed approximately 37 percent ($22 billion) of the total North American crop value. Warming allows more pest breeding cycles, expands the range of those pests, and increases overwinter survival. Soybeans, the report notes, are "now under threat from twice the number of diseases as in the mid-1980s," costing $2 billion in 2002.

Agriculture can help be a part of the solution, the scientists say, through wider use of no-till cropping and programs such as carbon sequestration in soils, which could reduce carbon emissions in the U.S. by 15 percent. Trading of these "carbon credits" could generate payments to farmers of from $1 billion to $5 billion over per year for the next 20 to 40 years, they say.

e-mail: hbrandon@primediabusiness.com