The clear-cutting of giant swathes from the globe's tropical forests has long been understood to be a major force behind global warming, but new research finds that smaller-scale forest loss—from minor logging and fires—is an even more powerful driver of climate change.
On Thursday, scientists at the Woods Hole Research Center and Boston University published a study in the journal Science that says the planet's tropical forests are releasing more carbon dioxide than they can store, mostly due to "fine scale" degradation and disturbance that previous studies haven't captured.
The finding means tropical forests may not act as carbon "sinks" unless both deforestation writ large and this more subtle degradation is stopped or slowed.
The researchers looked at tropical forests across Asia, Africa and Latin America using a trio of tools—remote sensing, field observations and satellite imagery—that gave them a more comprehensive and detailed picture over a period of years, from 2003 to 2014.
This approach allowed them to measure not just large-scale deforestation, largely from agriculture, but smaller-scale degradation and disturbances that have, until now, been especially difficult to gauge.
"Collectively, these fine-scale losses have been very difficult to quantify," said Wayne Walker, an associate scientist at Woods Hole and one of the report's authors. "While they don't seem like much in any given place, when you add them up across an areas as big as the tropics, they can be huge."
69 Percent of Loss from Small-Scale Damage
Walker and his colleagues found that forests lost more carbon across every continent, with the average loss across the tropics of about 425 million metric tons a year—nearly a tenth of the annual U.S. carbon footprint. Of the total carbon loss, the researchers found that 69 percent came as a result of this smaller-scale degradation and disturbance.
"With degradation, you lose a few trees here and there—from selective logging, from people relying on wood for fuel, people foraging and collecting," Walker said. "But you also have natural disturbance from drought, and increasingly, with climate change, you have fire where you didn't before."
Their research concluded that total yearly losses were about 862 million metric tons of carbon, while gains were about 437 million metric tons of carbon across the tropics. Most of the loss was attributable to Latin America—home to the Amazonian rainforest—and nearly 24 percent to Africa and 16 percent to Asia.
Biggest Contributors Vary by Location
The study found that in Africa and the Americas, most of the carbon loss was due to fine-scale degradation and disturbance, while in Asia—a victim of wide-scale deforestation due to forest-clearing for palm oil—less than half came from degradation and disturbance.
Walker and his colleagues hope the research will present a more accurate picture of tropical forests' carbon contributions, particularly given that forest conservation measures are a key component of national emissions-reduction commitments under the Paris climate agreement. The United Nations REDD+ program—Reducing Emissions from Deforestation and Forest Degradation—was included in the Paris agreement, the first time it was part of a major international climate deal. But, the program, in which developing countries are paid to leave forests intact, depends on more accurate reporting of forest carbon loss.
"At the end of the day, whether we're talking about larger losses or small losses, deforestation versus degradation, we need to continue to look for ways to address the drivers," Walker said. "And in doing so, leave forests intact and, at the same time, look for opportunities where we can put forests back."