A region of the Amazon forest in Brazil, the world’s largest tropical forest. Image: Wikimedia Commons

Earth’s tropical forests hoard huge amounts of carbon, but as temperatures rise and weather patterns shift due to climate change, scientists worry that their lush green canopies will start leaking more CO2. That concern got some serious validation this week, when a new study concluded that the recent El Niño period caused the tropics to spew an extra 2.5 billion tons of carbon into the air in a single year. The researchers say this CO2 pulse could be a preview of what’s to come in a hotter future.

The 2015-2016 El Niño, one of the strongest on record, led to record high temperatures and low precipitation across Earth’s tropical belt. While El Niño was ongoing, scientists suspected the climate pattern could be throwing the tropical carbon cycle out of whack—hotter temperatures speed up soil carbon decomposition and plant metabolism, two processes that send CO2 into the air. Drought, meanwhile, reduces photosynthesis, decreasing the amount of carbon plants soak up.

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A leakier tropics could help explain why the 2015-2016 El Niño period saw the largest jump in atmospheric CO2 in the past 2,000 years. But it was hard to be sure the tropics (in addition to humanity’s ever-present carbon pollution) played a role, because we simply don’t have a lot of environmental monitoring equipment on the ground in the Amazon, equatorial Africa, or Southeast Asia.

Lucky for scientists, the 2015-2016 El Niño happened to fall shortly after the launch of NASA’s Orbiting Carbon-2 Observatory (OCO-2), a satellite designed to measure CO2 dynamics in our planet’s atmosphere with unprecedented sensitivity. In results published in Thursday’s Science, researchers from NASA’s Jet Propulsion Laboratory and elsewhere used OCO-2 data to confirm that tropical forests soaked up 2.5 billion tons (Gt) less carbon in 2015 than they did in the reference year of 2011.

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“That’s almost a third of all the CO2 emitted from human activities in that same time period,” OCO-2 deputy project scientist and study author Annmarie Eldering said in a press briefing on Thursday. Combined with another 0.5 Gt of CO2 released from land surfaces outside the tropics during El Niño, it accounts for the CO2 spike.

The world’s three major tropical regions—the Amazon, Central Africa, and Southeast Asia—all became poorer carbon sinks for different reasons.

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In the northern and southeastern Amazon, a severe El Niño-induced drought reduced forest growth, leading to less CO2 uptake. In tropical Africa, rainfall was fairly normal, but temperatures were elevated, speeding soil carbon decomposition and CO2 release. In Southeast Asia, wildfires—sparked by humans and driven by drought—led to additional carbon emissions. The main culprit was a spate of devastating peat fires that raged across tropical Indonesia in the fall of 2015.

Effects of the 2015-2016 El Nino on climate and carbon cycling across Earth’s three major tropical forest regions. Image: NASA/JPL-Caltech

Taken together, a less effective tropical carbon sink meant more of humanity’s carbon emissions remaining in the air. Unfortunately, the researchers say El Niño’s climate effects could foretell a leakier future for Earth’s tropical forests.

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“If the future climate is more like climate during El Niño, the trouble is Earth might lose some of the carbon removal services from these tropical forests,” study co-author Scott Denning said in the press briefing.

The good news is there are obvious steps we can take to prevent the tropics from becoming a carbon-spewing, climate change-aggravating headache. For one, better management. Another recent study found that even without El Niño, the tropics have become a net carbon source in recent years thanks to forest degradation. Policy measures that slow or reverse tropical deforestation—including returning land rights to indigenous communities—will help keep tropical carbon out of the atmosphere.

Of course, we could also try to do something about the 10 billion tons of fossil carbon we emit every year. Here’s how you can help on that front.

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