STANFORD, Calif., May 4 (UPI) -- U.S. biologists say trees and other plants help keep the planet cool, but rising levels of atmospheric carbon dioxide are negating that effect.
Researchers at the Carnegie Institution for Science say they've determined in some regions more than a quarter of the warming from increased carbon dioxide is due to its direct impact on vegetation. That warming is in addition to carbon dioxide's better-known effect as a heat-trapping greenhouse gas.
Senior scientist Ken Caldeira of the institution's department of global ecology said plants have a very complex and diverse influence on the climate system.
"Plants take carbon dioxide out of the atmosphere, but they also have other effects, such as changing the amount of evaporation from the land surface," Caldeira said. "It's impossible to make good climate predictions without taking all of these factors into account."
The researchers said plants give off water through tiny pores in their leaves in a process that cools the plant. On a hot day, a tree can release tens of gallons of water into the air, acting as a natural environmental air conditioner. But when carbon dioxide levels are high, the leaf pores shrink, causing less water to be released and diminishing the tree's cooling power.
The study is detailed in the early online edition of the Proceedings of the National Academy of Sciences.