Fires, drought and warmer temperatures were to blame for excess carbon dioxide in the atmosphere during the 2015-16 El Nino, scientists with NASA's Orbiting Carbon Observatory-2 say.
The findings, part of five papers published in the journal Science, shed light on the mechanisms through which Earth "breathes" carbon dioxide, a potent greenhouse gas, and reveal how those mechanisms affect climate change.
Global temperatures have been on the rise, thanks largely to the human-driven increase in greenhouse gases like carbon dioxide. But not all of the carbon dioxide produced each year ends up in the atmosphere. Some of it ends up trapped in the ocean, or locked on land thanks to plants that use the gas during photosynthesis.
"We know how much we're emitting when we burn fossil fuel, and we see that about half of it stays in the atmosphere and the other half appears to go get absorbed into the land and the ocean," said Jet Propulsion Laboratory atmospheric scientist Annmarie Eldering, the mission's deputy project scientist. "But there are still these questions of which parts of the land are doing that."
And on top of that, the amount that gets pulled out of the atmosphere shifts dramatically from year to year, from about as little as 20 percent to as much as 80 percent.
"Why is it that there's a lot of variability from year to year?" Eldering said. "We didn't understand why that was."
Orbiting Carbon Observatory-2, or OCO-2, was launched in July 2014 to help discover those mechanisms and solve that mystery. Because the spacecraft was launched prior to the 2015-16 El Nino season, it allowed the scientists to get a glimpse of the effect that the weather pattern had on the Earth's ability to store carbon.
"You can think of it as like a big natural experiment where you had a lot of heat and a lot of drought," Eldering said. "So we could start investigating, how do plants respond when these conditions happen?"
OCO-2 near-infrared sensors revealed that normal carbon sinks _ forests in tropical South America, tropical Africa and Indonesia _ weren't pulling as much carbon down as they had in the past. But they were all doing so for different reasons.
In South America, a long drought was slowing down the growth of trees and other plants, which meant they were taking up carbon dioxide more slowly. In Africa, temperatures were higher, which could mean that dead plant matter was decomposing faster than usual, allowing carbon dioxide to escape. And in Indonesia, a rash of wildfires burned through trees, releasing their stored carbon, while also leaving fewer plants to pull that carbon down.
"Now we can see that the tropical forest and plants didn't absorb as much carbon as they usually do and that's what caused this big increase in that time period," Eldering said.
Drought and higher temperatures have been linked to the climate change fueled by greenhouse gases. Now, it seems that there could be a vicious cycle at work.
"The projections of climate suggest there will be more heat and there will be more drought in the future," Eldering said. "This would suggest that with more warmth and more heat, we'll have more carbon left in the atmosphere, so that would even accelerate the growth rate of carbon dioxide."
The results should help experts develop more effective strategies to deal with climate change in the future, Eldering said.
"If you want to make a good plan, you've got to have some good information," she said. "This is going to add to that information, and hopefully be reflected in a better plan down the road."
The findings come a few months after President Donald Trump's budget plan proposed to cut OCO-3, a follow-up mission that would continue OCO-2's work.
