The biological productivity of the Southern Ocean in the summertime is substantially greater than many previous estimates have suggested, according to new airborne research by the U.S. National Science Foundation National Center for Atmospheric Research (NSF NCAR).
The findings provide new insight into the global carbon cycle and point to a reason why Earth system models have struggled to accurately capture the role of the Southern Ocean: Models that underestimate the ocean's biological productivity also tend to underestimate the ocean's capacity to uptake carbon. Additionally, the new research is important for better understanding the entire marine food web and could help improve the models used to predict changing fisheries.
The study, published in the journal Nature Geoscience, relies on atmospheric measurements of carbon dioxide and oxygen taken by research aircraft flying over the Southern Ocean during multiple field campaigns spanning nearly a decade.
"By estimating the biological productivity of the ocean using atmospheric oxygen measurements, this study helps explain why most of the models are not accurately capturing how much carbon is being drawn down by the Southern Ocean," said Yuming Jin, a postdoctoral researcher at the University of California, Santa Barbara, who led the work while participating in NSF NCAR's Advanced Study Program. "We've developed a novel technique to analyze data collected by research aircraft, which improves our understanding of crucial interactions between the ocean and atmosphere in this unique and important part of the world."
The Southern Ocean plays a central role in regulating global climate. Its circulation determines how heat is absorbed and redistributed, controls the supply and distribution of nutrients that fuel marine productivity worldwide, and drives the formation of deep water masses that store carbon for centuries.
The study was funded by NSF and relied on observations supported by NSF, NASA, and NOAA.
A new technique to measure ocean productivity
The ocean's primary biological productivity — the rate at which algae and other microorganisms grow through photosynthesis — is linked to the carbon cycle because the process transforms carbon dioxide into biomass. But biological productivity is not the only factor that impacts how much carbon dioxide the ocean absorbs.
The temperature of the water also plays an important role. Warmer water can hold less carbon dioxide than cooler water. This means that, in the summer, surface water must discard some of its dissolved carbon dioxide as it warms. This excess carbon dioxide can either be taken up through photosynthesis or released into the air. The balance of these two processes determines whether the Southern Ocean becomes a net carbon dioxide source (when biological productivity is weak, carbon dioxide is released into the atmosphere) or sink (when biological productivity is strong, carbon dioxide is actually pulled from the atmosphere to replenish the dissolved carbon dioxide taken up by photosynthesis).
Scientists know from both airborne observations and observations taken at the ocean surface that the Southern Ocean absorbs more carbon dioxide in the Southern Hemisphere summer than it releases. But models have struggled to accurately simulate not just the degree of that net uptake but also whether net uptake is happening at all.
"In some models, the errors are so large that they simulate the Southern Ocean releasing carbon dioxide in the summer, when observations clearly show it is absorbing carbon dioxide," Jin said. "We needed a way to separate the biological and thermal contributions to these biases."
For the new study, Jin and his coauthors developed a new technique that allowed them to make that distinction by focusing on airborne measurements of oxygen rather than solely carbon dioxide. In the summer, oxygen is released during photosynthesis and also (like carbon) by the ocean surface as waters warm. In the winter, oxygen decreases as biological activity falls off, and simultaneously, the ocean surface absorbs more oxygen (like carbon) from the air as it cools.
"Warming and biology reinforce each other in oxygen fluxes rather than oppose each other as they do for carbon dioxide," Jin said. "And because the thermal component of oxygen flux can be estimated more reliably from ocean temperature, the total oxygen measurement allows us to cleanly isolate the biological signal."
Using the new technique, the scientists estimated that the Southern Ocean's biological productivity transformed 6.5 billion metric tons of carbon a year into biomass, a number substantially higher than most previous estimates based on model output and satellite data. While this number is large, the carbon dioxide is not permanently removed from the carbon cycle. The carbon stored in biomass is eventually converted back to carbon dioxide when the algae and other microorganisms die, sink, and decompose, eventually returning that carbon dioxide back to the atmosphere at other times of year and locations.
The unique value of airborne measurements
This study was only possible because research aircraft have taken measurements of the atmosphere above the Southern Ocean. Surface-based observations are important but they aren't able to give researchers a big-picture view of the exchange of gases with the ocean surface. Measurements taken from ships or robotic buoys in the water, for example, are difficult to use to generalize large-scale conditions because the ocean mixes slowly, which allows for a significant amount of variability from point to point. Observations of the air from ships or on land give us useful data about what is happening at the surface, but not what is happening overhead.
In contrast, research aircraft can cover a lot of ground — flying both near the surface and thousands of feet above the ground — to paint a whole picture. And because the atmosphere mixes quickly the measurements may be generalized over an entire ocean basin.
The observations used for this study were collected during ten separate campaigns conducted under three projects: the NSF-funded HIAPER Pole-to-Pole Observations (HIPPO) project, which ran from 2009–2011; the NSF-funded O2/N2 Ratio and CO2 Airborne Southern Ocean (ORCAS) study, which ran in 2016; and NASA's Atmospheric Tomography Mission (ATom), which ran from 2016-2018. HIPPO and ORCAS used the NSF NCAR Gulfstream V aircraft , while ATom used the NASA DC-8. The data collected during these campaigns have been the basis for hundreds of peer-reviewed publications and have led to significant progress in our understanding of the carbon cycle and atmospheric chemistry.
"High-performance research aircraft allow us to make critical measurements of the atmosphere that we can not get any other way," said NSF NCAR scientist Britton Stephens, a co-author of the study. "The return on investment from these airborne measurement campaigns has been immense."
About the article
Title: Atmospheric oxygen constraints on Southern Ocean productivity and drivers of carbon uptake
Authors: Yuming Jin, Britton B. Stephens, Matthew C. Long, Manfredi Manizza, Nicole S. Lovenduski, Cynthia Nevison, Eric J. Morgan, and Ralph F. Keeling
Journal: Nature Geoscience
This material is based upon work supported by the NSF National Center for Atmospheric Research, a major facility sponsored by the U.S. National Science Foundation and managed by the University Corporation for Atmospheric Research. Any opinions, findings and conclusions or recommendations expressed in this material do not necessarily reflect the views of NSF.
