A new NASA study provides space-based evidence that Earth’s tropical regions were the cause of the largest annual increases in atmospheric carbon dioxide concentration seen in at least 2,000 years.
What was the cause of this?
Scientists suspect that the 2015-2016 El Niño – one of the largest on record – was responsible. El Niño is a cyclical warming pattern of ocean circulation in the Pacific Ocean that affects weather all over the world. Before OCO-2, we didn’t have enough data to understand exactly how El Nino played a part.
Analyzing the first 28 months of data from our Orbiting Carbon Observatory (OCO-2) satellite, researchers conclude that impacts of El Niño-related heat and drought occurring in the tropical regions of South America, Africa and Indonesia were responsible for the record spike in global carbon dioxide.
These three tropical regions released 2.5 gigatons more carbon into the atmosphere than they did in 2011. This extra carbon dioxide explains the difference in atmospheric carbon dioxide growth rates between 2011 and the peak years of 2015-16.
In 2015 and 2016, OCO-2 recorded atmospheric carbon dioxide increases that were 50% larger than the average increase seen in recent years preceding these observations.
In eastern and southern tropical South America, including the Amazon rainforest, severe drought spurred by El Niño made 2015 the driest year in the past 30 years. Temperatures were also higher than normal. These drier and hotter conditions stressed vegetation and reduced photosynthesis, meaning trees and plants absorbed less carbon from the atmosphere. The effect was to increase the net amount of carbon released into the atmosphere.
In contrast, rainfall in tropical Africa was at normal levels, but ecosystems endured hotter-than-normal temperatures. Dead trees and plants decomposed more, resulting in more carbon being released into the atmosphere.
Meanwhile, 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 satellites.
We knew El Niños 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. OCO-2’s geographic coverage and data density are allowing us to study each region separately.
Why does the amount of carbon dioxide in our atmosphere matter?
The concentration of carbon dioxide in Earth’s atmosphere is constantly changing. It changes from season to season as plants grow and die, with higher concentrations in the winter and lower amounts in the summer. Annually averaged atmospheric carbon dioxide concentrations have generally increased year over year since the 1800s – the start of the widespread Industrial Revolution. Before then, Earth’s atmosphere naturally contained about 595 gigatons of carbon in the form of carbon dioxide. Currently, that number is 850 gigatons.
Carbon dioxide is a greenhouse gas, which means that it can trap heat. Since greenhouse gas is the principal human-produced driver of climate change, better understanding how it moves through the Earth system at regional scales and how it changes over time are important aspects to monitor.
