Five new studies highlight results from NASA's Orbiting Carbon Observatory-2 (OCO-2) mission, an endeavor to map out the world's carbon cycle from space. The data provide valuable insights, particularly in terms of understanding the effects of the El Niño weather pattern, a periodic fluctuation in sea surface temperature and air pressure in the Pacific Ocean that causes climate variability over the course of years or even decades. The first study, by Annmarie Eldering et al., describes the overall mission and its initial results. Launched in July 2014, OCO-2 has been consistently gathering data of carbon patterns around the globe over the course of 16-day cycles, collecting roughly 2 million estimates of carbon levels each month. The aim is to understand how natural land and ocean sinks vary from year-to year, and even season-to-season. The data reveal a striking change in the carbon cycle in the Northern Hemisphere across seasons, where in the spring there's a dramatic uptake of carbon by terrestrial plants. During the winter, however, carbon uptake by plants is minimal while the breakdown or decay of plant material feeds carbon back into the atmosphere. As a result of this cycle, together with the continual emissions from fossil fuel burning (particularly over China, Europe, and the southeast United States), carbon levels reach a seasonal maximum in the Northern Hemisphere during April, just before terrestrial plants begin to soak up more carbon.
A study by Junjie Liu et al. highlights ways in which the 2015 El Niño altered the net flux of carbon from terrestrial vegetation, or net biosphere exchange (NBE), in the tropics. The authors report that the increase of NBE in the tropics resulted in the release of about 2.5 gigatons more carbon into the atmosphere in 2015 than in 2011. Even though the three tropical continents, Asia, Africa and South America, had comparable NBE anomalies in 2015 relative to 2011, different processes drove these changes in each region. The researchers found that increased carbon release from biomass burning in tropical Asia, lower 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 the role of tropical land as a buffer for fossil fuel emissions may be reduced in the future.
In a different study, Abhishek Chatterjee et al. use OCO-2 data from above the Pacific and Atlantic oceans to pinpoint the magnitude and timing of El Niño in influencing the carbon cycle - a phenomenon that is widely inferred but not directly observed. The authors combined in situ data from buoys and OCO-2 data to detect a near-total shutdown of sea-to-air flux of carbon during the 2015-2016 boreal winter, which was affected by El Niño, relative to the neutral 2014-2015 boreal winter. The authors also detected El Niño-driven fluctuations in carbon associated with terrestrial sources, such as drought and fires.
A study by Florian Maximilian Schwandner et al. demonstrates the ability of the OCO-2 to track carbon emissions from individual cities and volcanoes. As part of an scheme with a set of 233 orbit paths that repeat in 16-day cycles, the path of the OCO-2 regularly goes above Los Angeles, providing scientists the opportunity to directly observe carbon emission from the megacity, revealing variations between the urban and suburban areas, as well as seasonal variations in anthropogenic carbon levels. The researchers also used OCO-2 to track carbon emissions from a volcano, Yasur, in Vanuatu, finding that local emission measurements confirmed the tool's efficacy.
A final study by Ying Sun et al. discusses how a tool onboard the OCO-2 helps better constrain the relationship between a common indicator for photosynthesis and plant biomass production. Solar-induced chlorophyll fluorescence (SIF) is a technique to quantify photosynthesis. Previous studies have suggested that SIF is correlated with Gross Primary Production (GPP), the amount of biomass created by plants via photosynthesis, yet this relationship has been difficult to prove. The authors compare SIF data with ground-based and airborne observations of terrestrial vegetation at several test locations, finding that the data do a good job at tracking GPP.