Writing on Thursday in the journal Science, the researchers said the nations of southern Africa -- Angola, Botswana, Lesotho, Malawi, Mozambique, Namibia, South Africa, Swaziland, Zambia and Zimbabwe -- could lose about 30 percent of their main crop of corn, also known as maize.

Agricultural losses also could be significant in the South Asia region encompassing India, Pakistan, Nepal and Sri Lanka, with a drop-off of at least 5 percent in many regional staples, including millet, maize and rice, the researchers said.

"We still have time to avoid these impacts, but we don't have much time," David Lobell of the Program on Food Security and the Environment at Stanford University in California, who led the research, said in a telephone interview.

"It's certainly our hope not to scare people, but to show them that there is some basis for focusing efforts and trying to get things done in a relatively speedy time frame," Lobell added.

The researchers projected how global warming would affect agriculture in 12 developing regions worldwide, looking at local climate projections and at the sensitivity of key local crops to warming temperatures and rainfall changes. They determined that average temperatures in most of the regions could rise by 1.8 degrees Fahrenheit (1 degree C) by 2030.

"We were surprised by how much and how soon these regions could suffer if we don't adapt," Marshall Burke, another Stanford scientist involved in the study, said in a statement.

Some places could be spared serious problems including China, a generally cooler region where climate change is not projected to be as bad for local crops, the researchers said.

Relatively inexpensive adaptations like planting earlier or later in the season or changing crops could reduce the harm from climate change, but the biggest benefits probably would stem from more expensive steps like developing new crop varieties and expanding irrigation, the researchers said.