The darkness of trees helps to absorb heat that empty snow fields would reflect.Punchstock

Large-scale deforestation — long fingered as a contributing factor in climate change — could cool Earth, say the researchers behind one of the first attempts to model the phenomenon at a global scale.

Logging is often attacked because living trees help to mop up carbon dioxide, thereby buffering rises in greenhouse gases. But deforestation has different effects in different parts of the world.

In high latitudes, for example, removing the forests could help to cool these regions. This is because the trees, which absorb sunlight, would be replaced by snow-covered fields in winter that reflect the light. But in tropical regions, cutting back on forests would mean that less water is transferred from soils into the atmosphere, meaning fewer clouds and a warmer planet.

Ken Caldeira of the Carnegie Institution of Washington in Stanford, California, and his colleagues have now compared these two effects and declared that the effect of boreal deforestation dominates. Removing all the forests would put a slight brake on global warming, they predict — enough to leave the world 0.3 °C cooler in 2100, they report in Proceedings of the National Academy of Sciences¹.

Total and sudden deforestation is unrealistic, Caldeira acknowledges. But he says that modelling such an extreme approach allows him to separate the effect of forests on climate from other influences, such as rising greenhouse gas levels.

The finding should be taken with caution, warn other Earth system modellers. Colin Prentice of the University of Bristol, UK, says that models do a poor job of quantifying how sunlight is reflected back into space. Until a new generation of simulations arrives, in which the model's parameters have been matched more closely to observations, he questions whether the overall result can be taken as correct.

Still, others add that Caldeira's study is a useful confirmation of earlier work that had focused on either boreal or tropical forests individually.

Its message, though, might not be popular. After writing about his work in the New York Times, for example, Caldeira received at least one anonymous ResearchPhone.php">phone message accusing him of aiding the timber industry. He also says that some researchers might prefer that he didn't promote his message: "There is a sense among some colleagues that one should keep quiet about this."

But he doesn't think that his work provides justification for chopping down forests. "One main reason to fear global warming is the need to protect ecosystems," he says. "To destroy forests would confuse the narrow goal [of fighting climate change] with the broader goal of protecting the environment."

To move forward, he and his colleagues want to run more detailed simulations. The type of forest matters, for instance, because the dark needles of pine trees can have more of a warming impact that the light leaves of aspens. He also wants to run simulations that track the tropical deforestation projected to occur in the scenarios used by the Intergovernmental Panel on Climate Change.

Wolfgang Cramer, a modeller at of the Potsdam Institute for Climate Impact Research in Germany, adds that more background work needs to be done to understand how the biosphere interacts with the climate. That type of information can then be factored into the models, he says, and perhaps produce results that are useful for policy makers.

Visit our destructionofforestsp.html">newsblog to read and post comments about this story.  

• References

  1. Bala G., et al. Proc. Natl. Acad. Sci., 104 . 6550 - 6555 (2007).