The research from the Carnegie Institution's Department of Global Ecology and the Lawrence Livermore National Laboratory was to be presented Wednesday during the American Geophysical Union's fall meeting in San Francisco.
The researchers used complex climate modeling software to simulate changes in forest cover and then examined the effects on global climate.
"We were hoping to find that growing forests in the United States would help slow global warming," study co-author Ken Caldeira of the Carnegie Institution said.
But the researchers found that while tropical forests help keep Earth cool by evaporating a great deal of water, northern forests tend to warm the Earth because they absorb sunlight without losing much moisture. In one computer simulation, the scientists covered much of the northern hemisphere with forests and saw a jump in surface air temperature of more than 6 degrees Fahrenheit.
The study -- authored by Caldeira, Seran Gibbard, Govindasamy Bala, Thomas J. Phillips, and Michael Wickett -- will appear online Dec. 8 in the journal Geophysical Research Letters.