Tropical rainforests have become carbon *producers*, not carbon sinks.

Nature, reporting on the effects of droughts and logging, reveals that the world’s jungles are now producing more CO2 than they’re socking away out of the atmosphere:

Whereas earlier estimates based on measurements of atmospheric carbon flows suggested that tropical forests might be carbon neutral or even a net sink, more-recent studies — including ones based on data from NASA’s Orbiting Carbon Observatory-2 satellite — agree broadly with this recent paper, says David Schimel, an ecologist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California. He suspects that human activities such as starting fires and natural factors including droughts have dealt a severe blow to forests’ ability to store carbon.

The study authors estimate that the world’s tropical forests release approximately 425 million tonnes of carbon annually, equivalent to roughly 5% of the globe’s annual fossil-fuel emissions, and about five times more than an estimate in a highly cited 2011 paper that relied on ground-based forest inventories.

The research team first travelled to forests throughout the tropics to measure tree diameters and heights. The scientists then fed those measurements into species-specific equations to estimate how much carbon the trees stored. Next, they used those estimates to ground-truth data collected by NASA’s Ice, Cloud, and Land Elevation Satellite (ICESat), a laser-equipped satellite that from 2003 to 2010 gathered data on forest height and vegetation layers around the globe.

Finally, the researchers used a machine-learning algorithm to translate measurements from the Moderate-Resolution Imaging Spectrometer (MODIS) instruments — part of NASA’s Terra and Aqua satellites that image Earth’s entire surface every one to two days — into data they could compare to the ICESat numbers. By extrapolating this comparison to MODIS images for the entire tropics, the team tracked how much carbon tropical forests gained and lost between 2003 and 2014.

Because degradation and natural disturbances often leave forest canopies mostly intact, most previous satellite studies have failed to account for their impact on carbon emissions, says Alessandro Baccini, a remote-sensing scientist at the Woods Hole Research Center in Falmouth, Massachusetts, who led the work. Yet the researchers calculated that these processes accounted for more than two-thirds of forests’ carbon emissions. “We were surprised how much of the emissions were a result of degradation,” he says.

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