Satellite data shows largest CO2 increase comes from Earth’s tropics
Data collected by NASA’s Orbiting Carbon Observatory-2 (OCO-2) satellite, launched in 2014 to measure changing levels of atmospheric carbon dioxide (CO2) worldwide, indicates that Earth’s tropics have been the largest sources of recent CO2 emissions.
The findings were announced at a NASA teleconference on Thursday, October 12, held to coincide with the publication of five papers on OCO-2 data published Friday, October 13, in the journal Science.
Panelists at the teleconference included Michael Freilich, director of the Earth Science Division at NASA’s Washington, DC, headquarters; Annmarie Eldering, OCO-2 project scientist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California; Junjie Liu, JPL research scientist; and Scott Denning, professor of atmospheric science at Colorado State University in Fort Collins, CO.
OCO-2 measured record CO2 increases in 2015 and 2016, which coincided with one of the largest ever El Niño events. El Niño is a cyclic phenomenon in which a band of warm ocean water develops in the central and eastern equatorial regions of the Pacific Ocean and impacts weather globally.
Many scientists suspected the phenomenon played a major role in the emissions spike during this time and sought OCO-2 data to better understand the nature of its impact.
Based on data collected by the satellite over its first 28 months, scientists identified three tropical regions – parts of South America, Africa, and Indonesia – as the biggest sources of annual CO2 emission increases on the planet.
The increases in these regions’ emissions were driven by heat and drought caused by El Niño.
“These three tropical regions released 2.5 gigatons [one gigaton equals one billion tons] more carbon into the atmosphere than they did in 2011. 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 Niño years,” said Liu, who is the lead author of one of the studies.
OCO-2 data showing CO2 emission level increases 50 percent larger than average are corroborated by measurements conducted independently by the National Oceanic and Atmospheric Administration (NOAA).
NOAA measured an increase of about 6.3 gigatons of CO2 during this time, up from an average of four gigatons per year.
The increase occurred in spite of CO2 emissions generated by human activity has remained the same both before and during the recent El Niño.
Together, land areas in the three tropical regions with the highest emissions contributed an additional three gigatons of CO2 to the atmosphere in 2015 alone, 80 percent of which were caused by natural processes in tropical forests.
South America’s tropical regions experienced abnormal heat and severe drought in 2015, which was their driest year of the past three decades. As a result, vegetation suffered, with plants and trees reducing their photosynthesis and absorbing less CO2 from the atmosphere.
African tropical areas had normal rainfall levels in 2015, but hotter than normal temperatures led to an increased decay of dead trees and plants, thereby releasing CO2.
Like South America’s tropics, Asia’s tropical regions were also extremely dry during that year, leading to more forest and peat fires, which produced more CO2. The majority of CO2 released in Asia came from Indonesia.
Eldering emphasized that OCO-2 is making it possible for scientists to identify the specific El Niño-influenced processes causing increased CO2 in each of the regions, something that previously was not possible.
Tropical rainforests are difficult to access from the ground, as they are remote and difficult for scientists to reach, Freilich said. Additionally, frequent thunderstorms in these areas distort ground-based CO2 measurements.
OCO-2’s greatest contribution is its ability to study the movement of CO2 on regional levels. The satellite makes approximately 100,000 CO2 measurements each day in regions around the world.
Levels of atmospheric CO2 change constantly. Because plants and trees actively conduct photosynthesis during the summer, CO2 levels typically drop during that season and increase in the winter. However, no two years are alike; CO2 levels depend on the balance of activity in the atmosphere, on the ground, and in the oceans.
Natural processes can remove anywhere between 20 and 80 percent of CO2 emitted by human activity during any given year.
Global average CO2 emissions have been increasing annually since the start of the Industrial Revolution in the early 1800s. Earth’s atmosphere today contains approximately 850 gigatons of CO2 in contrast to approximately 595 gigatons before the start of the industrial age.
“OCO-2 has given us two revolutionary new ways to understand the effects of drought and heat on tropical forests: directly measuring carbon dioxide over these regions thousands of times a day; and sensing the rate of photosynthesis by detecting fluorescence from chlorophyll in the trees themselves,” Denning said. “We can use these data to test our understanding of whether the response of tropical forests is likely to make climate change worse or not.”
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.