More Trees, Less Global Warming, Right? -- Not Exactly

A 150-year simulation of worldwide deforestation finds that tropical forests are carbon sinks and boreal forests contribute to warming
snowy forest

Before compact fluorescent light bulbs and ethanol, the first line of defense against global warming was planting trees.

Forests, after all, cool the atmosphere by drinking in carbon dioxide from the air. A new study, however, published in Proceedings of the National Academy of Sciences reports that forests' other climatic effects can cancel out their carbon cleaning advantage in some parts of the world. Using a three-dimensional climate model, the research team mimicked full global deforestation and also studied the effects of clear-cutting in different regions of latitude, such as the tropics and boreal zones. Apparently, these natural carbon sinks only do their job effectively in tropical regions; in other areas, they have either no impact or actually contribute to warming the planet. In fact, according to this model, by the year 2100, if all the forests were cut and left to rot, the annual global mean temperature would decrease by more than 0.5 degree Fahrenheit.

"I'm not sure the slight amount of cooling is necessarily significant, but that removing all the forest produced little change" on temperature is, says study co-author Ken Caldeira, an ecologist at the Carnegie Institution of Washington's Department of Global Ecology in Stanford, Calif. "I think what's interesting is this global cancellation was a product of very different responses at different latitudes."

Trees perform three major climate functions: They absorb carbon, which they pull from the atmosphere, creating a cooling effect; their dark green leaves absorb light from the sun, heating Earth's surface; and they draw water from the soil, which evaporates into the atmosphere, creating low clouds that reflect the sun's hot rays (a mechanism known as evotranspiration that also leads to cooling). These three factors—the second two being largely ignored in climate models up to this point, according to Caldeira—taken together created very different results in the primary latitudes studied: the equatorial tropic zone; the midlatitudes that include most of the U.S.; and the boreal areas, which are subarctic and include much of Canada, Russia and the northern extremities of the U.S.

In all three regions, forests dutifully perform their task of sucking carbon dioxide from the air, but light absorption and evotranspiration vary wildly. In tropical zones, forests have a significant, overall cooling effect. The soil is very wet and, so, via evotranspiration, the trees are covered by low-lying clouds that create a small albedo (power of light that is reflected by a surface). In nontropical areas, Caldeira explains, "the real significant factor is whether there's snow on the ground in the winter." If a forest covers a snowy expanse, "that has a strong warming influence," he notes, because of little cloud cover resulting from less efficiency in evaporating water. The poor cloud formation coupled with the intense absorption of light by the trees "far overwhelms the cooling influence of the carbon storage," he says.

"In midlatitudes, we got that it was basically a wash—the carbon dioxide effects were pretty much directly balanced by the physical effects," Caldeira says. He attributes this to the low contrast between light absorption from trees and from grass in pastures, though he notes that because there are some areas with wintry snow cover, the loss of a forest will probably have a slight, if any, cooling effect. He uses this example to point out the relative influence of the different forest functions. Whereas carbon levels can affect warming on a global scale, the effects of increased albedo and poor evotranspiration would affect temperatures only on a regional level. For instance, he says, "if you remove all the forest in the U.S., it would probably heat up the world, but have a slight cooling influence on the U.S., itself."

Navin Ramankutty, an assistant professor of geography and Earth system sciences at McGill University in Montreal, says this study is the first to take a comprehensive look at the consequences of deforestation on the entire world. "You can't just blindly go ahead and reforest and that will tackle climate change," he says, pointing out a key finding in the study. "If you think about conservation groups, they're all talking about planting trees. We should be protecting tress for other reasons."

Caldeira agrees, saying that protecting the forest should be part of an effort to sustain the world's biodiversity. He also adds that the findings do not endorse clear-cutting or destroying wildlife habitats. "I think that it's important to look at preventing climate change as a means rather than an end in itself," he says. "Too narrow a focus on global warming and a loss of the broader focus of protecting life on this planet can lead to perverse outcomes." Rather than looking to forests to solve the current climate crisis by capturing carbon dioxide, he suggests targeting our "energy system," which continues to create the pollutant.

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