Scientists have found that the impact of the 2015-16 El Nino-related heat and drought occurring in tropical regions of South America, Africa and Indonesia was responsible for the largest annual increases in atmospheric carbon dioxide (CO2) concentration seen in at least 2,000 years.
El Nino is a cyclical warming pattern of ocean circulation in the central and eastern tropical Pacific Ocean that can affect weather worldwide.
The findings, published in the journal Science as part of a collection of five research papers, are based on analysis of the first 28 months of data from NASA's Orbiting Carbon Observatory-2 (OCO-2) satellite.
"These three tropical regions released 2.5 gigatonnes (a billion tonnes) more carbon into the atmosphere than they did in 2011," said lead author of the study Junjie Liu of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California.
"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," Liu added.
In 2015 and 2016, OCO-2 recorded atmospheric carbon dioxide increases that were 50 per cent larger than the average increase seen in recent years preceding these observations.
That increase was about three parts per million of carbon dioxide per year -- or 6.3 gigatonnes of carbon. In recent years, the average annual increase has been closer to two parts per million of carbon dioxide per year -- or four gigatonnes of carbon.
These record increases occurred even though emissions from human activities in 2015-16 are estimated to have remained roughly the same as they were prior to the El Nino.
Using OCO-2 data, Liu's team analysed how Earth's land areas contributed to the record atmospheric carbon dioxide concentration increases.
The team compared the 2015 findings to those from a reference year -- 2011 -- using carbon dioxide data from the Japan Aerospace Exploration Agency's Greenhouse Gases Observing Satellite (GOSAT).
In 2011, weather in the three tropical regions was normal and the amount of carbon absorbed and released by them was in balance.
"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," said OCO-2 Deputy Project Scientist Annmarie Eldering of JPL.
"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," Eldering added.
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