Data retrieved from NASA’s Orbiting Carbon Observatory-2 (OCO-2) satellite show that the impacts of El Niño-related heat and drought occurring in tropical regions in South America, Africa and Indonesia, were responsible for the largest annual increases in atmospheric carbon dioxide concentration seen in at least 2,000 years.
“Tropical forests are known to play a vital role in absorbing carbon dioxide from the atmosphere. This new research suggests that the 2015-16 El Niño hampered their ability to do that,” says Debra Wunch, assistant professor in the Department of Physics in the Faculty of Arts & Science, who played a key role on the team behind the discovery.
The findings are published in “Contrasting carbon cycle responses of the tropical continents to the 2015-16 El Niño,” – part of a collection of five research papers based on OCO-2 data published in Science today.
Lead author Junjie Liu of the Jet Propulsion Laboratory (JPL) and colleagues describe how, compared to 2011, three gigatons more carbon was released to the atmosphere from all land areas in 2015-16 due to El Niño. Most of that increase – eighty per cent or 2.5 gigatons – came from natural processes in the tropical forests of South America, Africa and Indonesia. Emissions from human activities in 2015-16 are estimated to have been roughly the same as they were prior to the El Niño event. 2011 is used as a base year for comparison because weather in the three tropical regions was normal and the amount of carbon absorbed and released by them was in balance at that time.
The team found that while each of the three tropical areas released roughly the same amount of carbon dioxide, they did so for different reasons.
In eastern and southeastern tropical South America, including the Amazon rainforest, severe drought spurred by El Niño made 2015 the driest year in the last 30 years with higher than normal temperatures. These drier and hotter conditions stressed vegetation and reduced photosynthesis: trees and plants absorbed less carbon from the atmosphere thus increasing the net amount of carbon released into the atmosphere.
In contrast, tropical Africa had normal rainfall but hotter-than-normal temperatures leading to more decomposition of dead trees and plants and more carbon released into the atmosphere.
During the same time period, tropical Asia had its second-driest year in the past 30 years. It also released more carbon, primarily from Indonesia and mainly due to increased peat and forest fires.
“The team’s findings imply that if future climate brings more or longer droughts, as the last El Niño did, more carbon dioxide may remain in the atmosphere, leading to a tendency to further warm Earth,” said OCO-2 deputy project scientist Annmarie Eldering in a news release from the JPL.
Wunch is a deputy chair of the Total Carbon Column Observing Network (TCCON), the ground-based validation network of 25 observatories around the world that was charged with ensuring that the OCO-2 satellite’s CO2measurements are accurate. Two of these ground-based observatories are located in Canada: one in Saskatchewan run by Wunch and the other in Eureka, Nunavut, run by U of T physicist Kimberly Strong, director of the School of the Environment in the Faculty of Arts & Science.
“Our role is to ensure that what OCO-2 is measuring from space is the same as what we measure from the ground,” says Wunch, who also holds an appointment with the School of the Environment. “Being able to corroborate the OCO-2 data is key to ensuring its accuracy.”
The new information could help scientists develop better carbon cycle models, leading to “improved predictions of how our planet may respond to similar conditions in the future,” Eldering says.
OCO-2, launched in 2014, gathers global measurements of atmospheric carbon dioxide with the resolution, precision and coverage needed to understand how this important greenhouse gas – the principal human-produced driver of climate change – moves through the Earth at regional scales, and how it changes over time. From its vantage point in space, OCO-2 is able to make roughly 100,000 measurements of atmospheric carbon dioxide each day, around the world.
In addition to members from the Jet Propulsion Laboratory, California Institute of Technology, and the University of Toronto, the team included researchers from the National Center for Atmospheric Research in Boulder, Colorado, Colorado State University, and Arizona State University in Tempe. This work was supported by NASA and the National Science Foundation.
With files from the Jet Propulsion Laboratory.