Every two to seven years, abnormally warm water in the Pacific Ocean causes an atmospheric disturbance called El Niño. It often makes extreme weather worse in various places around the world: greater floods, tougher droughts, more wildfires. Now scientists have new evidence indicating El Niño conditions might also add extra carbon dioxide to the atmosphere as well as lessen the ability of trees to absorb the greenhouse gas.

By certain measures, the most recent El Niño, which held sway in 2015 and 2016, was one of the three strongest on record, along with episodes in 1982–1983 and 1997–1998. Although its impacts on land were not clearly stronger than those of the other events, it appears it was the major culprit for a record increase in CO2 during its reign. “CO2 emissions from fossil fuels and industry did not really change from 2014 to 2016,” says climate scientist Pierre Friedlingstein at the University of Exeter in England, and an author of the 2017 carbon budget report released by the Global Carbon Project in November. So the dramatic increase, he says, must be due to how land and sea responded to El Niño.

A recent article in Science about satellite measurements made during El Niño by NASA’s Orbiting Carbon Observatory-2 showed most of the extra CO2 originated in the tropics. It also suggested each tropical region contributed a similar amount of CO2 as in other strong El Niño years, each in its own way. In South America's Amazon, for example, slower-growing plants absorbed less CO2, whereas in Africa, plants and soils released more of the gas.

The observations are based on satellite readings of CO2, carbon monoxide (which is released by wildfires) and other factors like the fluorescence of the chlorophyll in plant tissues (which reflects growth). But scientists would want some ground-truthing to prove El Niño conditions in the tropics boosted atmospheric CO2 levels. In the past, field data from plants and soils during an El Niño has been thin, but in 2015 researchers were better prepared.

Ecosystem scientist Yadvinder Malhi, at the University of Oxford, and colleagues analyzed data from 54 tropical forest plots in Ghana, Gabon, Malaysia, Brazil and Peru, which they presented at a recent meeting Mahli organized in London. The ground data confirmed what the satellite data had suggested: the tropics are key. The amount of CO2 released from the sites predicted the atmospheric levels “surprisingly well,” according to Malhi. “In contrast to long-term climate change,” he said at the meeting, “everybody agrees that El Niño makes the amount of CO2 in the atmosphere go up faster than usual.”

The next question was how much each continent contributed. Malhi says the Amazon appears to be the biggest player. “Africa always emits a lot of CO2, since it has a lot of wildfires, especially in the savannas. But its tropical forests and savannas didn’t behave very differently from other years,” he says. In the Amazon, however, “the emissions seemed largely due to respiration: CO2 released from plants, decaying logs and the soil.”

Further analysis will allow Malhi’s team to distinguish between those different sources, which is important for predicting longer-term impacts. If, for example, it turns out the extra CO2 released during El Niño is largely due to microbial activity in soils and decomposing plant material, Malhi says, “I wouldn’t expect it to last” as the world warms further. If the increase is coming from the plants because they are respiring more, that might be a signal of long-term stress that may continue to raise CO2 levels.

As a carbon booster, El Niño could hasten rising temperatures, bringing the world to dangerous thresholds sooner than thought. It could also enhance feedbacks between climate and vegetation that could reduce plants’ ability to absorb CO2 in non-Niño years as well. If bad droughts or wildfires kill many trees, for example, forests and their carbon sequestering potential may take centuries to recover, if ever.

Wildfires are a big concern. They were not quite as pervasive in 2015–2016 as they were in 1997–1998, when they were the most important global source of extra CO2. In 2015–2016, “a bit of rain at just the right moment prevented the Southeast Asian peat forest fires from spreading as widely as they did two decades ago,” says climate scientist Guido van der Werf at Vrije University Amsterdam. “Nevertheless, fire is a growing problem in the region.” Drought and heat make the forests more vulnerable. Wildfires release a lot of CO2, and they also eliminate the trees that might store more CO2 in the future.

Even in the absence of fire, El Niño might be killing trees. In northern Australia's remote Gulf of Carpentaria, more than 18,200 acres of mangrove vegetation died during the last El Niño due to a convergence of severe drought and unprecedented high temperatures. Death by drought may threaten trees in the Amazon as well.

To transport water from roots up to leaves, trees depend on tension created by the difference in water availability in the soil and the atmosphere, says plant physiologist Lucy Rowland at Exeter. “If it is very dry, however, that tension can get too high,” she says. “If that happens, the column of water in the trees’ transport vessels breaks. Then air gets in, blocking water flow like a bubble in your central heating system does.”

The only way trees can fix this problem is by growing new vessels—if the lack of water does not kill them first. Rowland is also investigating whether severe drought hampers the ability of tropical vegetation to absorb more CO2 as the atmospheric concentration rises. Right now, she says, the way climate models incorporate vegetation’s response to drought is too simplistic.

For now tropical trees appear to have returned to normal, their ecosystems absorbing more CO2 than they emit, Friedlingstein concludes. Unfortunately, man-made emissions have returned to their old status as well. “After two years of stability,” he adds, “they are going up again.”

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