Rainforests apparently aren’t riding to the global-warming rescue. A story in the latest issue of Harvard Magazine notes that research by the Center for Tropical Forest Science shows that two tropical forests on opposite sides of the globe did not increase their growth as carbon dioxide levels rose over the last 25 years. In fact, because of complex relationships among temperature, cloud cover and CO2 levels, the forests have actually slowed their growth and, consequently, their ability to act as a carbon “sink.”

Here’s how the magazine explains the counterintuitive results of the research:

What accounts for the slowing of growth that CTFS data revealed to (researchers)? Even though increases in atmospheric CO₂ provide fertilizer that can stimulate growth by aiding photosynthesis, they found that over the period of the study, the number of rainy days had increased at both sites, meaning that less sunlight was available to fuel photosynthesis. Moreover, nighttime temperatures had increased at the sites, and higher temperatures mean higher respiration rates. When trees have less energy coming in and more going out, they have less for growth, as appears to have happened in these two forests.

“Temperature and CO₂ may operate in opposite directions on tree growth,” says Stuart Davies, Ph.D. ’96, a coauthor of the paper. “That’s what we’re arguing, but we have to be honest and say that we don’t know the answer yet.”

The story on tropical forest growth rates is a sidebar accompanying a profile of the amazing Peter Shaw Ashton, a cofounder of the Center for Tropical Forest Science who recently won the Japan Prize, awarded by the Science and Technology Foundation of Japan in the category of “Science and Technology of Harmonious Co-Existence.” Though stodgily told here, the story of Ashton’s life is worth a look, too.