But the study concludes that planting new trees in certain parts of the planet may actually warm the Earth.

The new study, which combines climate and carbon-cycle effects of large-scale deforestation in a fully interactive three-dimensional climate-carbon model, confirms that planting more tropical rainforests could help slow global warming worldwide.

The research, led by Lawrence Livermore National Laboratory atmospheric scientist Govindasamy Bala, appears in the April 9-13 online edition of the Proceedings of the National Academy of Sciences.

According to the study, new forests in mid- to high-latitude locations could actually create a net warming. Specifically, more trees in mid-latitude locations like the United States and most of Europe would only create marginal benefits from a climate perspective. But those extra trees in the boreal forests of Canada, Scandinavia and Siberia could actually be counterproductive, Bala said.

Forests affect climate in three different ways: they absorb the greenhouse gas - carbon dioxide - from the atmosphere and help keep the planet cool; they evaporate water to the atmosphere and increase cloudiness, which also helps keep the planet cool; and they are dark and absorb sunlight (the albedo effect), warming the Earth. Previous climate change mitigation strategies that promote planting trees have taken only the first effect into account.

"Our study shows that only tropical rainforests are 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. In other locations, the warming from the albedo effect either cancels or exceeds the net cooling from the other two effects."

Other researchers from the Carnegie Institution, Stanford and Universite Montpellier II, France also contributed to the report.

The study concludes that by the year 2100, forests in mid- and high-latitudes will make some places up to 10 degrees Fahrenheit warmer than would have occurred if the forests did not exist.

The authors caution that the cooling from 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 this report.

"Apart from their role in altering the planet's climate, forests are valuable in many other aspects," Bala said. "Forests provide natural habitat to plants and animals, preserve the biodiversity, produce economically valuable timber and firewood, protect watersheds and indirectly prevent ocean acidification.

"In planning responses to global challenges, it is important to pursue broad goals and avoid narrow criteria that may lead to environmentally harmful consequences," Caldeira said.

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Tropical Forests Air Condition Planet Earth
Stanford CA (SPX) Apr 10 - Planting and protecting trees-which trap and absorb carbon dioxide as they grow-can help to remove carbon dioxide from the atmosphere. But a new study suggests that, as a way to fight global warming, the effectiveness of this strategy depends heavily on where these trees are planted. In particular, tropical forests are very efficient at keeping the Earth at a happy, healthy temperature.

The researchers, including Ken Caldeira of Carnegie's Department of Global Ecology and Govindasamy Bala at Lawrence Livermore National Laboratory, found that because tropical forests store large amounts of carbon and produce reflective clouds, they are especially good at cooling the planet.

In contrast, forests in snowy areas can warm the Earth, because their dark canopy absorbs sunlight that would otherwise be reflected back to space by a bright white covering of snow.

The work simulates the effects of large-scale deforestation, and accounts for the positive and negative climate effects of tree cover at different latitudes.

The result, which appears in this week's early online edition of the Proceedings of the National Academy of Sciences, makes a strong case for protecting and restoring tropical forests.

"Tropical forests are like Earth's air conditioner," Caldeira said. "When it comes to rehabilitating forests to fight global warming, carbon dioxide might be only half of the story; we also have to account for whether they help to reflect sunlight by producing clouds, or help to absorb it by shading snowy tundra."

Forests in colder, sub-polar latitudes evaporate less water and are less effective at producing clouds. As a result, the main climate effect of these forests is to increase the absorption of sunlight, which can overwhelm the cooling effect of carbon storage.

However, Caldeira believes it would be counterproductive to cut down forests in snowy areas, even if it could help to combat global warming. "A primary reason we are trying to slow global warming is to protect nature," he explains. "It just makes no sense to destroy natural ecosystems in the name of saving natural ecosystems."

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DOE/Lawrence Livermore National Laboratory
Carnegie Institution
Save the Forests at Wood Pile