In the document, the Bush administration endorsed for the first time the widely held view of scientists that U.S. greenhouse gas emissions will rise significantly over the next two decades due mostly to human activities.

The White House had previously said there was not enough scientific evidence to blame oil refining, power plants and automobile emissions for global warming. The administration has rejected participating in the international Kyoto treaty to reduce greenhouse gases, saying a less-costly and effective approach is to encourage voluntary emission cuts by industry.

Although climate change could threaten U.S. barrier islands and mountain meadows, the EPA report said most American farmers had little to worry about.

"Based on studies to date, unless there is inadequate or poorly distributed precipitation, the net effects of climate change on the agricultural segments of the U.S. economy over the 21st century are generally projected to be positive," the report said.

The United States has more than 1 billion acres (420 million hectares) of farmland, pastures and grazing land.

The rising concentration of carbon dioxide in the soil is beneficial to many crops and should help continue decades of rising crop yields, the report said.

Higher concentrations of carbon dioxide in the soil and slightly warmer temperatures should benefit crops such as cotton, corn, soybeans, sorghum, barley, sugar beets and citrus fruits, it said.

"In general, northern areas such as the Midwest, West and Pacific Northwest are projected to show large gains in yields, while influences on crop yields in other regions vary more widely," the report said.

For example, some scientific studies cited by the report estimated soybean crop yields could climb by 23 percent to 40 percent, depending on the specific scenario of rainfall and temperature. Orange crop yields could soar by 13 percent to 120 percent.

However, corn yields would climb by a more modest 1 percent to 9 percent, the report said.

More carbon also helps the soil hold water and plants grow faster, so that farmers will need less irrigation water, the EPA report said.

However, climate change also means many growers will need to use more pesticides, the report said without elaboration.

The EPA acknowledged that scientific analyses have not considered all of the consequences of climate change on crop pests, diseases and extreme weather.

"Agricultural technology is currently undergoing rapid change, and future production technologies and practices seem likely to be able to contain or reduce these impacts," the EPA report said.

Farmers in the Southeast face more uncertainties linked to climate change.

The EPA report said a change in planting dates and genetically altered crop varieties will be important for the Southeast in case summer rainfall declines.