If humans don't curb use of fossil fuels, the planet will warm 14.5 degrees Fahrenheit by the year 2300. The polar ice caps will disappear and oceans will rise 23 feet (7 meters).
That's the conclusion of climate simulations released today by scientists at the Lawrence Livermore National Laboratory.
At the poles, the average temperature would rise more than 36 degrees Fahrenheit (20 degrees Celsius), the model predicts.
"The temperature estimate is actually conservative because the model didn't take into consideration changing land use such as deforestation and build out of cities into outlying wilderness areas," said Govindasamy Bala of the Laboratory's Energy and Environment Directorate.
The forecast is based on estimated changes in atmospheric carbon dioxide, a so-called greenhouse gas that acts like a blanket to trap heat. Carbon dioxide is a byproduct of coal-fired power planets, automobile engines and other machines that burn fossil fuels.
Today's level of atmospheric carbon dioxide is 380 parts per million (ppm). By the year 2300, the model predicts that amount would nearly quadruple to 1,423 ppm.
Forests would cover much of the now-frozen regions in the Arctic and Antarctica.
The carbon dioxide eventually ends up in the oceans, which would become more acidic as a result, the scientists say.
"The doubled-CO2 climate that scientists have warned about for decades is beginning to look like a goal we might attain if we work hard to limit CO2 emissions, rather than the terrible outcome that might occur if we do nothing," said Ken Caldeira, of the Department of Global Ecology at the Carnegie Institution and one of the other authors.
The most drastic changes would come during the 22nd Century, the model predicts. Sea ice cover almost completely disappears by 2150.
Critics charge that computer models cannot accurately predict how much the planet will warm. Few scientists disagree that a warming trend is underway, however. And other studies have found that even if all fossil fuel burning stopped today, temperatures would still rise for at least many decades.
Other studies have generated dire predictions, but few are as dramatic as the new forecast.
"We definitely know we are going to warm over the next 300 years," Bala said. "In reality, we may be worse off than we predict."