A new study based on global airborne surveys provides more clarity about the amount of carbon dioxide that the world's forests and other vegetation on land are taking up (or releasing) throughout the year, information that is crucial to better understanding the world's carbon cycle.
The research, led by the U.S. National Science Foundation National Center for Atmospheric Research (NSF NCAR) and published in the Proceedings of the National Academy of Sciences (PNAS), found that forests in the tropics take up less carbon than Earth system models typically predict. The research also concludes that models struggle to simulate carbon dioxide uptake in forests north and south of the tropics, either because the forests are taking in more carbon dioxide than previously thought or because carbon dioxide emissions from burning fossil fuels are being overestimated, or a combination of both.
The work highlights the incredible value of global-scale airborne carbon dioxide measurements, which are not routinely taken, and suggests that regular airborne missions going forward could narrow the uncertainty further. This would allow scientists to more definitively determine how carbon dioxide from both human-produced and natural sources move through the Earth system.
"There are big uncertainties in our understanding of the natural carbon cycle at the largest scales," said NSF NCAR scientist Britton Stephens, who led the study. "By refining our understanding of how much carbon dioxide is taken up and released by the oceans and the land, we can more accurately track where emissions are going and the impacts of changing emissions on the Earth system."
The work was funded by NASA, NOAA, and the U.S. National Science Foundation.
Reducing model uncertainties
The concentration of carbon dioxide in the atmosphere is the balance of how much carbon is released into the air by "sources" and how much carbon is taken out by "sinks." Scientists know that atmospheric carbon dioxide concentrations have steadily risen (from 280 to more than 430 parts per million) owing to industrial emissions. About half of these industrial emissions remain in the atmosphere, driving up the concentration of carbon dioxide, but the rest is absorbed by natural carbon sinks. However, where these sinks are and the rate at which individual sinks are absorbing carbon dioxide is not well understood.
For example, to what extent is vegetation growth (a carbon sink) increasing as fossil fuel emissions (a carbon source) also increase? How much carbon dioxide is emitted due to tropical deforestation? What regions of the world's oceans act as sources and sinks, and how big are their impacts?
Scientists have attempted to answer these questions and others using atmospheric models to estimate the patterns of carbon emissions and uptake that could have produced the carbon dioxide concentrations observed in the atmosphere at different locations around the globe today. But those models can differ significantly in their results, including which regions of the world serve as sources and sinks and how big those sources and sinks are.
In the new study, scientists use airborne observations taken during NASA's Atmospheric Tomography Mission (ATom) between 2016 and 2018 to reduce the uncertainty among the models by as much as half. In particular, the measurements indicate that the distribution of carbon dioxide above the surface differs from what Earth system and atmospheric models suggest, with relatively more carbon dioxide observed in the atmosphere above the tropics compared to elsewhere. This implies much less uptake of carbon dioxide in tropical forests than models suggest, and in the regions north and south of the tropics, either more carbon dioxide uptake than models suggest or less fossil fuel emissions than previously thought.
The unique value of airborne observations
Scientists routinely measure atmospheric carbon dioxide using instruments based at the surface as well as onboard satellites, but both present challenges. For example, scientists cannot easily extrapolate large-scale trends from surface measurements, because the network of measurement stations is sparse and the way carbon dioxide mixes vertically in the atmosphere is poorly known. Satellite observations give scientists a large-scale view, but the technique for determining carbon dioxide concentrations from remote sensing requires extremely sensitive measurements that are challenging to take and coverage is lacking in cloudy and high-latitude regions.
Airborne measurements, on the other hand, are well suited to give scientists crucial information about where carbon is coming from and where it is going. The aircraft can fly along transects at varying altitudes, sampling the well-mixed atmosphere to provide detailed information about regional patterns. For the ATom mission, flights using the NASA DC-8 were made during all four seasons over a four-year period. The plane took measurements continuously from near the surface to over 40,000 feet in the air, and covered the globe, from the western Arctic to Antarctica over both the Pacific and Atlantic Oceans.
Airborne missions also provide consistency in measurements, since the same set of instruments are used to sample the air across the transects. During the ATom mission, five instruments onboard measured carbon dioxide, vastly reducing measurement errors.
"Our results highlight the unique value of airborne measurements for understanding the carbon cycle," Stephens said. "The aircraft and satellites work together synergistically. Satellite carbon dioxide measurements have transformed how we observe small-scale sources and large-scale interannual variability, but they have struggled when it comes to estimating how carbon dioxide moves through Earth's sinks and sources on average at global scale. By filling in that gap, the aircraft can trigger the scientific payoff from the much larger investment in satellite measurements."
