Tropical forests have flipped from sponges to sources of carbon dioxide

The world’s tropical forests are exhaling — and it’s not a sigh of relief. Instead of soaking up climate-warming gases on balance, these so-called “lungs of the planet” are beginning to release them.

A new study based on analyses of satellite imagery of tropical Asia, Africa and the Americas suggests that tropical forests contribute more carbon dioxide to the atmosphere than they remove. Much of that carbon contribution is due to deforestation, the conversion of forests to urban spaces such as farms or roads. But more than two-thirds comes from a less visible source: a decline in the number and diversity of trees in remaining forests, researchers report online September 28 in Science.

Tropical forests are a bulwark against rising carbon dioxide in the atmosphere, taking up carbon and storing it as stems, leaves and roots. Deforestation’s effect is clear; it cuts the number of trees that take up CO₂. But even seemingly intact forests can be degraded or disturbed by selective logging, environmental change, wildfires or disease.

Unlike deforestation, which is easy to see in satellite images, these other kinds of degradation can be hard to spot, says study lead author Alessandro Baccini, a forest ecologist and remote sensing specialist at the Woods Hole Research Center in Falmouth, Mass. A degraded forest still looks like forest — it’s just less dense, containing less carbon biomass.

“Carbon density is a weight,” Baccini says. “The problem is that there is no satellite in space that can give an estimate of weight.”

So Baccini and his colleagues devised their own. First, they came up with a way to calibrate satellite images of the tropics using field observations and NASA Light Detection and Ranging Data, or lidar. Then, they created an algorithm that compares 500-meter-square parcels of each image from each year from 2003 to 2014 to calculate gains and losses in carbon density.

In total, the researchers found, tropical forests emit 862 teragrams of carbon to the atmosphere annually — more than all cars in the United States did in 2015 — and absorb only 436 teragrams of carbon each year. Of that net loss of carbon to the atmosphere, 69 percent is from degraded forests and the rest from deforestation.

Some 60 percent of those carbon emissions came from tropical America, including the Amazon Basin. Africa’s tropical forests were responsible for about 24 percent of the carbon loss, and Asia’s forests for 16 percent.

The results are a wake-up call that there is an opportunity for improvement, says study coauthor Wayne Walker, also a forest ecologist and remote sensing specialist at Woods Hole Research Center. “Forests are low-hanging fruit. Maintaining forests intact, and attempting to restore forests where they might have been lost, is relatively straightforward and inexpensive.”

Nancy Harris, the research manager of the forest program at the World Resources Institute in Washington, D.C., also sees opportunity in these findings. “We’ve known for a long time that forest degradation is happening, but haven’t had a good way to measure it,” Harris says. “This paper goes a long way to capturing it in an empirical way.”

But terrestrial ecosystem scientist Joshua Fisher of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., notes that this forest biomass study doesn’t fully gel with atmospheric observations of carbon emissions from tropical forests, which still show that the forests are taking up more carbon than they are emitting overall. That may be because the new study focuses on aboveground biomass and doesn’t include what’s absorbed in soils, he says.

Still, Fisher says, the study shows how important it is to include forest degradation as well as deforestation in studies of climate change. “It’s a good first step for future analyses,” he adds.