Forests have become more efficient in their use of water after atmospheric levels of carbon dioxide have been on the rise, according to a Harvard study.

The study, published Wednesday in the journal Nature, suggested that forests around the world have become more efficient in their use of water over the last two decades. Trevor Keenan, research associate, led the study and was assisted by members of Harvard's department of Earth and Planetary Sciences, the U.S. Forest Service, Ohio State University, Indiana University and the Karlsruhe Institute of Germany.

"This could be considered a beneficial effect of increased atmospheric carbon dioxide," Keenan told the Harvard Gazette. "What's surprising is we didn't expect the effect to be this big. A large proportion of the ecosystems in the world are limited by water. They don't have enough water during the year to reach their maximum growth. If they become more efficient at using water, they should be able to take more carbon out of the atmosphere due to higher growth rates."

Andrew Richardson, assistant professor of organismic and evolutionary biology, said heightened carbon dioxide levels are healthy for forests in the short term, but would be a detriment if they continue to rise.

"We're still very concerned about what rising levels of atmospheric carbon dioxide mean for the planet," Richardson cautioned. "There is little doubt that as carbon dioxide continues to rise - and last month we just passed a critical milestone, 400 ppm, for the first time in human history - rising global temperatures and changes in rainfall patterns will, in coming decades, have very negative consequences for plant growth in many ecosystems around the world."

Keenan and his team gathered data from forests in the Northeastern United States and from around the world. He and Richardson discovered greater efficiency than most computer-generated models had predicted.

Keenan used photosynthesis to explain how plants were becoming more efficient in water use. Plants open tiny pores called stomata to take in their needed amount of carbon dioxide, simultaneously releasing water vapor. With more carbon dioxide in the atmosphere, plants do not need to open their stomata quite as wide and can preserve more water vapor.

"We went through every possible hypothesis of what could be going on, and ultimately what we were left with is that the only phenomenon that could cause this type of shift in water-use efficiency is rising atmospheric carbon dioxide," Keenan said. "This larger dataset will help us to better understand the extent of the response we observed."