Satellite Shows How Climate Could Alter Global Carbon Cycle

Unusually large amounts of carbon dioxide (reported here in gigatons) were released from the tropics during the 2015 El Niño. | NASA/ JPL/ Caltech

A NASA satellite mission launched to watch the Earth "breathe" has revealed some striking patterns in how the planet's carbon flux changes seasonally and with large weather events such as El Niño, with some troubling implications for future climate change. The mission's findings are highlighted in five studies in the October 13 issue of Science.

NASA's Orbiting Carbon Observatory-2 (OCO-2) mission was launched in July 2014 and has been consistently gathering data on patterns of carbon exchange between the land and the atmosphere around the globe over the course of 16-day cycles, collecting roughly 2 million measurements each month.

"OCO-2 was designed to help locate and identify natural regional processes that serve as sources and sinks of atmospheric carbon dioxide, and how these vary with time and location," explained Annmarie Eldering, a NASA scientist working on the mission.

Eldering notes that the mission was initiated at a critical time — just before the 2015 El Niño began. El Niño is a weather pattern characterized by a periodic fluctuation in sea surface temperature and air pressure in the Pacific Ocean, which causes climate variability over the course of years, sometimes even decades. The El Niño that began in 2015 was one of the strongest in modern history, providing a valuable opportunity to understand how major fluxes in carbon relate to Earth processes.

In one of the five studies, Junjie Liu of the NASA Jet Propulsion Laboratory and colleagues report a major increase in carbon release occurring in the tropics; about 2.5 gigatons more carbon was released from land into the atmosphere in 2015, when El Niño occurred, than in 2011.

Even though the three tropical continents, Asia, Africa and South America, had comparable changes in carbon patterns in 2015 relative to 2011, different processes drove these changes in each region. The researchers found that increased carbon release from vegetation burning in tropical Asia, lower levels of precipitation in South America, and increased temperatures in Africa were key drivers, the latter two of which are hallmarks of El Niño.

The authors note that lower precipitation in South America and higher temperatures in Africa are changes that are expected to occur by the end of this century due to climate change. Therefore, they suggest that although the tropics were thought to be an important sink for carbon emissions in the past, this role may be reduced in the future.

"Our results imply that if future climate brings more or longer periods of drought and heat, as the El Niño did, it may result in more carbon dioxide remaining in the atmosphere, further warming our planet," Liu explained.

Among numerous findings, the data also revealed a major seasonal pattern in the Northern Hemisphere. In the springtime, there is a dramatic removal of carbon from the atmosphere as plants absorb carbon dioxide, using it through photosynthesis to fuel their growth. In the winter, however, there is a release of carbon from decaying plants.

The process is a striking cycle, said Eldering. "We really are watching the planet breathe."

As a result of this annual cycle, together with the continual emissions from fossil fuel burning (particularly over China, Europe, and the southeast United States), carbon levels reach a maximum in the Northern Hemisphere in April, just before terrestrial plants begin to soak up more carbon.

The OCO-2 mission will continue to monitor the Earth's carbon cycle. "Although it was exciting to have a large El Niño and see the response of the tropics, we look forward to collection of data in a more typical weather year," said Eldering. "This will let us test how well our global Earth system models represent the carbon cycle in more frequent conditions."

[Credit for associated image: JPL/ NASA]