Contrary to traditional theories, growing trees in temperate regions may absorb sunlight and just warm the Earth in long term, US scientists reported on Tuesday.

Growing a forest might sound like a good idea to combat global warming, since trees are capable of absorbing the carbon dioxide emitted by the burning of fossil fuels.

But forests in the mid-latitude regions of the Earth affect the climate in a more complicated way, according to researchers from the Carnegie Institution and the Lawrence Livermore National Laboratory. Planting trees may just outweigh the positive effects of sequestering that carbon dioxide, they noted.

The researchers used complex climate modeling software to simulate changes in forest cover and then examined the effects on global climate. Their surprising findings will be presented at the American Geophysical Union meeting in San Francisco.

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 a lot of sunlight without losing much moisture.

In one simulation, the researchers covered much of the northern hemisphere with forests and saw a jump in surface air temperature of more than 3 degrees Celsius. Covering the entire planet's land mass with trees led to a more modest increase of about 1 degree Celsius.

When the scientists restricted the simulation to middle latitudes such as the continental United States, the picture was not quite so clear.

At first, cooling due to the uptake of carbon dioxide would offset warming from sunlight absorption. But after several decades, carbon dioxide would begin diffusing from the ocean into the atmosphere, diminishing the cooling effect and warming the Earth in the long term, they found.

"We were hoping to find that growing forests in the United States would help slow global warming," said Ken Caldeira, a lead researcher from the Carnegie Institution. "But if we are not careful, growing forests could make global warming even worse."

The researchers warned against planting forests on abandoned croplands as a strategy to combat global warming, which some have recommended. But they also recognized the importance of forests.

"Forests provide good habitats for plants and animals, and tropical forests are good for climate, so we should be particularly careful to preserve them," Caldeira said.

"But in terms of climate change, we should focus our efforts on things that can really make a difference, like improving efficiency and developing new sources of clean energy."

Source: Xinhua