'Plant a tree, help the planet' is popular wisdom about preserving the Earth's climate.
But a team of international scientists this week challenged that assumption, saying that planting more trees in high latitudes such as Canada, Siberia and Scandinavia could actually contribute to global warming.
At the same time, however, they emphasized that current forest reserves in those regions should be maintained, not removed, because of the need to keep ecosystems intact. And they suggested that planting more trees in tropical areas would in fact help cool the Earth.
The research, led by Govindasamy Bala of the US government's Lawrence Livermore National Laboratory at the University of California, was published this week online in the Proceedings of the National Academy of Sciences.
"The study concludes that by the year 2100, forests in mid- and high- latitudes will make some places up to 10 degrees Fahrenheit (5.5 degrees Celsius) warmer than would have occurred if the forests did not exist," according to a press release about the study.
The UN-backed Intergovernmental Panel on Climate Change (IPCC) is set to release a third report on global warming in May - the work of hundreds of scientists around the world - dealing specifically with techniques to combat the Earth's rising global temperature.
The new study's researchers explained that most climate mitigation strategies have failed to take into account an important fact about trees - that they are dark and absorb sunlight, warming the Earth.
Most mitigation proposals focus only on two beneficial properties - the ability of trees to absorb greenhouse gases like carbon dioxide that trap heat in Earth's atmosphere; and the evaporation of water from trees that increases "cloudiness" and keeps the planet cool.
In the colder latitudes of Canada, Siberia and Scandanavia, the researchers suggested that the warming from the light absorption cancels out or exceeds the net cooling from carbon absorption and evaporation.
But conserving and expanding tropical rainforests would be "strongly beneficial in helping slow down global warming," Bala said. "It is a win-win situation in the tropics because trees in the tropics, in addition to absorbing carbon dioxide, promote convective clouds that help to cool the planet."
The study used data from large-scale deforestation.
The authors warned that deforestation outside the tropics "should not be viewed as a strategy for mitigating climate change."
"Preservation of ecosystems is a primary goal of preventing global warming, and the destruction of ecosystems to prevent global warming would be a counterproductive and perverse strategy," said Ken Caldeira, from the Carnegie Institution and a co-author of the report.