Careful Where You Put That Tree

Forget planting trees to negate your SUV's contribution to global warming -- according to Stanford University atmospheric scientist Ken Caldeira, forests in the wrong location can actually make the Earth hotter.

Plants absorb large amounts of carbon dioxide during photosynthesis, so scientists and policy makers have long assumed new forest growth helps combat global warming. At an American Geophysical Union conference in San Francisco earlier this month, however, Caldeira rolled out a provocative new finding: Trees may be good at capturing carbon dioxide from the atmosphere, but their dark leaves are also very efficient at soaking up sunlight, which is later released as heat. At certain latitudes, the net effect of these two processes is warming, rather than cooling.

"Forests do store carbon, and as a result, the planet initially cools a little -- maybe tenths of degrees," Caldeira said. "But over the long term, trees' heat absorption warms things up more."

Caldeira and colleagues at California's Lawrence Berkeley National Laboratory created a computer simulation showing that if most land areas in northern latitudes were covered with forests, the planet would be six degrees warmer than it is today. Forest growth in equatorial areas, on the other hand, reduced global temperatures in the simulation because the warmer air in these regions allows more moisture to evaporate from the leaves of trees. This produces substantial cooling that cancels out the effects of heat absorption.

These seemingly maverick ideas have met with serious interest among some climatologists. "Planting trees definitely sequesters carbon dioxide, which tends to lower temperatures," said Eric Adams, an ecologist in Massachusetts Institute of Technology's environmental engineering department. "But the trees also do absorb light that might otherwise be reflected, which causes warming."

"It's very interesting that changing land use -- whether that means growing trees or cutting them down -- can have an effect on climate," added David Erickson, director of the Climate and Carbon Research Institute at Oak Ridge National Laboratory in Tennessee. "That effect is working in conjunction with the impact of greenhouse gases."

If future studies confirm Caldeira's findings, his work could have a substantial impact on environmental policy. Currently, programs like Carbonfund and the Chicago Climate Exchange support the planting of temperate forests in various regions of the United States in order to reduce global warming.

In the United Kingdom, for-profit Climate Care offers customers the chance to "cancel out" the carbon-dioxide emissions they produce by donating to a fund that supports reforestation efforts. Its Stratus package, which costs about $130, is billed as making one person "completely climate-neutral for the whole year."

Caldeira's research suggests efforts like these are off base. "Organizations should not be giving these kinds of credits," he said. "Planting forests in mid-latitudes should not be considered equivalent to using renewable resources."

Carbonfund spokesman Craig Coulter, however, urged caution. "If scientific consensus shows that this study is valid, then of course we'd have to take that into account," he said. "But there's always been tit-for-tat among academics about different methods for calculating the impact of reducing carbon, and I'd want to see more studies along these lines before making policy changes." He also pointed out that planting trees has a variety of environmental benefits unrelated to global warming, such as restoring threatened animal habitats and preventing the erosion of topsoil.

Caldeira stressed that lawmakers shouldn't advocate chopping down swaths of forest in hopes of reducing global temperatures a few degrees. He thinks investing in new sources of clean energy, like hydrogen and biofuel, is a better way to address the global-warming problem.

"Earth systems are very complicated -- you might be able to reduce warming by cutting down some trees, but that wouldn't be good for the environment overall," he said. "The less we interfere with the system, the more likely we are to have a healthy planet."

Special Thanks to Wired