The amount of atmospheric carbon dioxide (CO₂) in our world is likely to top 410 parts per million (ppm) this year. The CO₂ measured at the Mauna Loa Baseline Atmospheric Observatory in Hawaii hit 405.1 ppm in 2016 - an increase of 3 ppm over the previous year, marking five consecutive years of an unprecedented rate of growth, according to the National Oceanic and Atmospheric Administration.

CO₂ is the most prominent, abundant and measurable greenhouse gas, responsible for keeping the world warm enough for life. It is also capable of making the atmosphere too warm when its concentration becomes too great.

The increased burning of fossil fuels since the industrial revolution has driven up the amount of CO₂ held in the atmosphere to unprecedented levels. However, a new NASA study provides space-based evidence that Earth's tropical regions were the source of the largest annual increases in atmospheric carbon dioxide concentration in at least 2 000 years.

The 2015-16 El Nino - one of the largest on record - caused abnormal, but expected, heat and drought in the tropical regions of South America, Africa and Indonesia. "These three tropical regions released 2.5 gigatons more carbon into the atmosphere than they did in 2011," said Junjie Liu of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, who is the lead author of the study.

Warming trend

NASA's Orbiting Carbon Observatory-2 mission, (OCO-2), launched in 2014, gathers global measurements of atmospheric CO₂ with the resolution, precision and coverage needed to map the movement and concentration of CO₂ around the world.

OCO-2 recorded atmospheric CO₂ increases that were 50% larger than the preceding non-El Nino years and equate to about 3 ppm or 6.3 gigatons of carbon. This sudden increase occurred even though emissions from human activities in 2015-16 were estimated to have remained roughly the same as they were before the El Nino event.

OCO-2 Deputy Project Scientist Annmarie Eldering of JPL said: "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."

The reasoning behind this argument is the altered natural processes that take place in these normally carbon-sink areas of the earth's green clothing: In tropical South America, severe drought (driest in 30 years) and higher-than-normal temperatures stressed vegetation and reduced photosynthesis. Thus, vegetation absorbed less carbon from the atmosphere, and the effect was to increase the net amount of carbon released into the atmosphere. Rainfall in tropical Africa was at normal levels, but ecosystems endured higher-than-normal temperatures. Dead trees and plants decomposed more, resulting in more carbon being released into the atmosphere. Tropical Asia had the second-driest year in the past 30 years. Its increased carbon release, primarily from Indonesia, was mainly due to increased peat and forest fires, also measured by satellite instruments.

On average, Earth's land and ocean absorb about half the CO₂ released by humans, with the other half leading to increasing atmospheric concentrations. There is natural variation, and in some years the absorption is as little as 20% of human emissions, while in other years as much as 80% can be scrubbed from the atmosphere.

Deforestation and the burning of fossil fuels remain humanity's biggest contributions to CO₂ increase, and therefore climate change. It is notable though that each El Nino event makes another irregular but significant injection.