A huge spike in carbon emissions seen in the past couple of years has puzzled scientists, since there was no evidence of a rise in human activities, like fossil fuel burning, that might explain it. But new satellite data shows that the weather phenomenon El Nino is to blame, because it led to dry spells that put stress on plants and trees across the tropics, and made it harder for them to perform their important role of absorbing carbon dioxide from the atmosphere.
Experts warn that in the coming decades, climate change could lead to even more such warming in the future, as severe droughts and heat waves become more common across the planet. The 2015-16 El Nino was one of the strongest on record, and led to the biggest increase in annual concentrations of carbon released into the atmosphere in some 2,000 years, according to the NASA-led study published on Thursday in the journal Science.
In those two years, the tropical regions of Africa, South America and Asia released 2.5 billion tons more carbon into the atmosphere than they did in 2011, it said. These increases in atmospheric carbon dioxide were 50 percent larger than the average increase seen in recent years.
Excess heat and drought related to El Nino in the tropics of South America, Africa and Indonesia "were responsible for the record spike in global carbon dioxide," NASA said in a statement. Key drivers of this change in carbon emissions were lower precipitation in South America and increased temperatures in Africa.
"These drier and hotter conditions stressed vegetation and reduced photosynthesis, meaning trees and plants absorbed less carbon from the atmosphere," NASA said. In tropical Asia, the increased carbon release was mostly due to biomass burning.
Satellite data
Carbon dioxide is a leading byproduct of fossil fuel burning, and its accumulation in the atmosphere heats up the Earth, hence the name "greenhouse gas." Scientists have suspected El Nino - a weather pattern that causes sea surface temperature and air pressure in the tropical Pacific Ocean to fluctuate, and may last years at a time - might wield an influence on the balance of carbon in the atmosphere.
But 28 months of data from a NASA satellite - called the Orbiting Carbon Observatory-2 (OCO-2) and launched in 2014 - have clarified its role. The satellite's mission is to examine how carbon dioxide moves across the Earth and how it changes over time. Scientists compared 2015-16 data from the NASA satellite in recent years to 2011 data from the Japan Aerospace Exploration Agency's Greenhouse Gases Observing Satellite (GOSAT), because 2011 was a normal year, weather-wise, with no El Nino.