Forests are a powerful, troubled ally in the struggle against climate change. They soak in 29% of the carbon dioxide humanity emits every year—a feat that has kept temperatures from spiking higher than the 1.1°C that they already have. But tropical deforestation gnaws away at this benefit, pushing CO₂ levels higher.
This binary model—carbon in, carbon out—frames many debates about land management in climate policies. It may be too simple, according to a new study in the journal Frontiers in Forests and Global Change. That’s because it leaves out other critical but overlooked effects that have an important, perhaps 0.5°C cooling effect on the global climate, a monumental figure given that every 0.1°C matters. Uncurbed deforestation puts this benefit in jeopardy, too.
“Forests are not just carbon sponges. They—their physical structure—interact with the atmosphere to cool the surface of the Earth,” said Deborah Lawrence, an environmental scientist at the University of Virginia and the lead author of the paper.
Trees shift heat from ground level skyward by using solar energy to vaporize liquid water, a process called evapotranspiration that’s like natural air conditioning. A forest’s canopy also helps keep heat away from the surface, where people and ecosystems are. The more bumpy the top of a forest is, the more air turbulence it creates, and the more heat is pushed away from the ground.
There are two other important factors. One is albedo, or the reflectivity of the planet’s surface. Arctic sea ice bounces sunlight right back up to space, for example. Tropical forests have a low albedo—they absorb heat instead of reflect it. That’s counteracted to some extent, though, by chemicals that trees produce. These aerosol particles (think of the “smoke” of the Great Smoky Mountains) both reflect sunlight and help high-albedo clouds form.
All of these processes, well known for decades, have critical and immediate local effects, where they keep areas cool. What the new study brings is a global accounting of how they work with or against forests’ carbon-storage potential at various latitudes.
With carbon offset markets already under growing scrutiny, the new finding that forests cool the planet more than was thought adds further complexity to the question of how to account for their climate impact—and what that might be worth in a carbon market.
“Clearly, the value of climate stabilization by tropical forests is under-valued,” Lawrence said.
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Eric Roston in New York at email@example.com
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