Since the Industrial Revolution in the early 1800s, atmospheric carbon dioxide (CO2) concentrations have been steadily increasing, but 2015 and 2016 saw an unprecedented spike. A NASA study has now analyzed data gathered by the atmosphere-monitoring satellite, the Orbiting Carbon Observatory-2 (OCO-2), over more than two years and pinpointed the cause: the El Nino weather effect caused certain tropical regions to release far more CO2 than they normally would.
Although there's been some huge efforts to reduce the amount of CO2 produced through human activity, the amount of the gas pumped into the atmosphere has still increased by an average of 2 parts per million (about 4 gigatons of carbon) annually, in recent years. But 2015 and 2016 broke the trend with the largest spikes on record: up to 3 parts per million, amounting to 6.3 gigatons of carbon. Emissions from human activity stayed roughly the same in those years, so where was it all coming from?
The climate cycle El Nino was a prime suspect, but it wasn't clear exactly how. This phenomenon occurs over the Pacific Ocean every few years, when warmer water from near the Phillipines and Indonesia drifts east towards South America, and the effects can be strong enough to alter weather across the entire planet. Warmer waters at the surface of the ocean drag the rains with it, lowering precipitation and causing droughts in areas like Australia, India, Southeast Asia, Indonesia, northeastern South America, while increasing rainfall in places like Peru, Chile and Ecuador.
The El Nino event in 2015 was one of the strongest since the 1950s, so it's no coincidence that 2016 was the hottest year ever recorded. To study what effects the event may have had on atmospheric CO2 concentrations, NASA researchers analyzed 28 months of data gathered by the OCO-2 satellite, which can take thousands of readings of carbon dioxide levels per day in a given area, as well as measure how well vegetation is processing the gas via photosynthesis.
The team compared that data to 2011 as a reference year, when weather and carbon cycle processes were normal. Their conclusion? The increase was due to warmer-than-average temperatures and drought in tropical parts of South America, Africa and Indonesia, which in turn were caused by El Nino.
"These three tropical regions released 2.5 gigatons more carbon into the atmosphere than they did in 2011," says Junjie Liu, lead author of the study. "Our analysis shows this extra carbon dioxide explains the difference in atmospheric carbon dioxide growth rates between 2011 and the peak years of 2015-16. OCO-2 data allowed us to quantify how the net exchange of carbon between land and atmosphere in individual regions is affected during El Nino years."
The researchers combined the OCO-2 data with that gathered by other satellites, to figure out the specific processes in each of those regions that were contributing to the extreme increase in CO2. Drought ravaged eastern and southeastern tropical areas of South America, bringing about the driest year in the last three decades. Coupled with higher than average temperatures, vegetation in these regions were stressed and as such, photosynthesis slowed, meaning the plants plucked less carbon from the atmosphere.
Meanwhile, tropical Asia suffered through its second-driest year in 30 years, which increased the severity of forest fires that in turn pumped more carbon into the air. During the same time, tropical Africa endured hotter temperatures but no drought, which sped up the rate of decomposition of dead trees and plants, resulting in more CO2 emissions.
"We knew El Ninos were one factor in these variations, but until now we didn't understand, at the scale of these regions, what the most important processes were," says Annemarie Eldering, Deputy Project Scientist on the OCO-2 mission. "Understanding how the carbon cycle in these regions responded to El Nino will enable scientists to improve carbon cycle models, which should lead to improved predictions of how our planet may respond to similar conditions in the future. The team's findings imply that if future climate brings more or longer droughts, as the last El Nino did, more carbon dioxide may remain in the atmosphere, leading to a tendency to further warm Earth."
These naturally-occurring processes may be seen by some as evidence against human-induced climate change, but they're snowballing symptoms of bigger anthropogenic causes. After all, increased carbon levels are believed to contribute to more frequent and severe El Nino events, which in turn can speed up these "natural" processes and exponentially alter the Earth's climate.
The research was published in the journal Science.