Earth’s Tropical Forests Sprung a Major CO2 Leak During Last Year's El Niño

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

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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.

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.

“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.

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

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.

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

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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.

Maddie Stone is a freelancer based in Philadelphia.

DISCUSSION

dnapl
Dense non aqueous phase liquid

Now that was good hard-science distillation. I have more unread peer reviewed academic studies than read ones downloaded. Like ten to one. And the one’s read were skimmed at best.

My only quibble is no mention of the US Midwest. That’s always my quibble - it comes from living in flyoverville. And Midwestern insecurities. As far as the biosphere fertility goes, it’s even more intense than the Amazon, “according to scientists.” Anyway, it’s the soil. Good soil. That an roundup ready corn grown under no-till farming practices and shitloads of nutrient application. But that’s another discussion.

Speaking about earth’s biosphere respiration and how it controls atmospheric CO2 concentration, here’s an awesome free government data set and interactive from NOAA’s Earth Systems Research Laboratory (ESRL) Global Monitoring division. For example, here are all the major and minor met stations for measuring GHGs and physical properties like temp and humidity and shit.

All right let’s nerd out by looking at how Midwestern respiration with respect to CO2 concentration stacks up compared to say, American Samoa down somewhere in the Pacific. The length between seasonal peaks tells the story of respiration intensity and supports Maggie’s point on why it’s important when the tropics belch more CO2 than normal, since it doesn’t go through as intense seasonal growth/death cycles (maybe?):

Iowa: big swing in CO2 concentration as measured at a met station somewhere in fucking Iowa:

That’s some serious CO2 sucking by tightly planted corn and soybean for gasoline blending ethanol. Now CO2 swings in Samoa over the period from 2004 to now:

Not as big of a seasonal CO2 changes.

Just nerding out. This is a blog for nerds, reportedly. I’m not sure if any of this comment means anything, I just find it interesting. And this free government data might be behind a paywall after Trump gets through gutting science through government agency. Those business friendly folks don’t want scientists working without at least five layers of douchebag business majors overseeing their oompa loompa asses.