Aircraft reveal a surprisingly strong Southern Ocean carbon sink
New study relies on airborne measurements of carbon dioxide to estimate
ocean uptake

Date:
December 2, 2021
Source:
National Center for Atmospheric Research/University Corporation
for Atmospheric Research
Summary:
The Southern Ocean is indeed a significant carbon sink -- absorbing
a large amount of the excess carbon dioxide emitted into the
atmosphere by human activities -- according to a new study.



FULL STORY ==========================================================================
The Southern Ocean is indeed a significant carbon sink -- absorbing a
large amount of the excess carbon dioxide emitted into the atmosphere by
human activities -- according to a new study led by the National Center
for Atmospheric Research (NCAR).


==========================================================================
The findings provide clarity about the role the icy waters surrounding Antarctica play in buffering the impact of increasing greenhouse gas
emissions, after research published in recent years suggested the Southern Ocean might be less of a sink than previously thought.

The new study, published this week in the journal Science, makes use of observations from research aircraft flown during three field projects
over nearly a decade, as well as a collection of atmospheric models,
to determine that the Southern Ocean takes up significantly more carbon
than it releases.

The research also highlights the power that airborne observations have
to reveal critical patterns in the global carbon cycle.

"You can't fool the atmosphere," said NCAR scientist Matthew Long, the
paper's lead author. "While measurements taken from the ocean surface
and from land are important, they are too sparse to provide a reliable
picture of air-sea carbon flux. The atmosphere, however, can integrate
fluxes over large expanses.

Airborne measurements show a drawdown of CO2 in the lower atmosphere
over the Southern Ocean surface in summer, indicating carbon uptake by
the ocean." The research is funded by the National Science Foundation
(NSF), which is NCAR's sponsor, as well as by NASA and NOAA.

Uncertainty about the role of the Southern Ocean Once human-produced
emissions of CO2 -- from burning fossil fuels and other activities --
enter the atmosphere, some of the gas is taken up by plants and some is absorbed into the ocean. While the overall concentration of CO2 in the atmosphere continues to increase, causing the global temperature to rise,
these land and ocean "sinks" slow the effect.



==========================================================================
A more precise understanding of where carbon sinks exist, how big they
are, and how they may be changing as society continues to emit more CO2
is crucial to projecting the future trajectory of climate change. It is
also necessary for evaluating the impact of potential emission reduction measures and CO2 removal technologies.

Scientists have long thought that the Southern Ocean is an important
carbon sink. In the region around Antarctica, cold water from the deep
ocean is transported to the surface. This upwelling water may not have
seen the surface of the ocean for hundreds of years -- but once in
contact with the atmosphere, it's able to absorb CO2 before sinking again.

Measurements of CO2 and related properties in the ocean suggest that 40
percent of all human-produced CO2 now stored in the ocean was originally
taken up by the Southern Ocean. But measuring the actual flux at the
surface -- the back and forth exchange of CO2 between the water and the overlying air throughout a year -- has been challenging.

In recent years, scientists have used observations of pH taken from
autonomous floats deployed in the Southern Ocean to infer information
about air-sea carbon flux. The results of those efforts suggested
that the carbon sink in the Southern Ocean might be much smaller than previously thought. The possibility that the prevailing understanding
of the role the Southern Ocean plays in the carbon cycle might be wrong generated a lot of discussion within the scientific community and left unanswered questions, including where the excess CO2 is going if not
into the Southern Ocean. Could there be a significant sink on land or
elsewhere in the global oceans that scientists have missed? The value
of atmospheric measurements In the new study, the research team sought
to address the uncertainty by looking at carbon in the air instead of
in the water. The atmosphere and the ocean exist in balance, and they
are constantly exchanging CO2, oxygen, and other gases with each other.



==========================================================================
The research team pieced together airborne measurements from three
different field projects with deployments stretching over nearly a decade:
the HIAPER Pole-to-Pole Observations (HIPPO) project, the O2/N2 Ratio and
CO2 Airborne Southern Ocean (ORCAS) study, and the Atmospheric Tomography (ATom) mission.

While there are also surface monitoring stations that measure CO2 in the atmosphere over the Southern Ocean, these stations are relatively few
and far between, making it difficult to characterize what is happening
across the entire region.

"The atmospheric CO2 signals over the Southern Ocean are small
and challenging to measure, especially from surface stations using
different instruments run by different laboratories," said NCAR scientist Britton Stephens, a co-author of the study who co-led or participated
in all of the field campaigns. "But with the suite of high-performance instrumentation we flew, the signals were striking and unequivocal." Critically, the data from the aircraft campaigns captured the vertical
CO2 gradient. For example, during the NSF-funded ORCAS field campaign,
which took place in January and February 2016, Stephens, Long, and other scientists on board the NSF/NCAR HIAPER Gulfstream V research aircraft
could see a decrease in CO2 concentrations on their instruments as the
plane descended.

"Every time the GV dipped near the surface, turbulence increased --
indicating the air was in contact with the ocean -- at precisely the
moment when all the CO2 instruments registered a drop in concentrations," Stephens said. "You could feel it." The new study finds that this
gradient is quite sensitive to the air-sea carbon flux, offering
researchers an unprecedented opportunity to characterize the Southern
Ocean's carbon uptake.

"We needed observations that included both intensive surveys at a
particular time of the year and that spanned the seasonal cycle," Long
said. "That was the motivation for combining multiple aircraft campaigns
that span roughly a decade. We were able to aggregate them together to
assess the mean seasonal cycle of CO2 variability in the atmosphere."
After piecing together how CO2 typically varies in the atmosphere at
a particular time of the year, the research team turned to a suite of atmospheric models to help them translate their atmospheric profiles into
an estimate of how much CO2 the ocean was soaking up or releasing. Their conclusion was that the Southern Ocean takes in significantly more carbon
in the summer than it loses during the winter, absorbing a whopping 2
billion tons of CO2 over the course of a year. In the summer, blooms of photosynthetic algae, or phytoplankton, play a key role in driving CO2
uptake into the ocean.

The research team noted that a regular program of future airborne
observations over the Southern Ocean could also help scientists understand whether the area's capacity to continue taking up carbon may change
in the future. A similar measurement strategy could yield important
information in other regions of the globe too.

"We've really seen that these observations are hugely powerful,"
Long said.

"Future aircraft observations could yield extremely high
scientific value for the investment. It's critical that we
have a finger on the pulse of the carbon cycle as we enter
a period when global society is taking action to reduce CO2
in the atmosphere. These observations can help us do just that." ========================================================================== Story Source: Materials provided by National_Center_for_Atmospheric_Research/University Corporation_for_Atmospheric_Research. Original written by Laura
Snider. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Matthew C. Long, Britton B. Stephens, Kathryn McKain, Colm Sweeney,
Ralph
F. Keeling, Eric A. Kort, Eric J. Morgan, Jonathan D. Bent, Naveen
Chandra, Frederic Chevallier, Ro'isi'n Commane, Bruce C. Daube,
Paul B.

Krummel, Zoe" Loh, Ingrid T. Luijkx, David Munro, Prabir Patra,
Wouter Peters, Michel Ramonet, Christian Ro"denbeck, Ann Stavert,
Pieter Tans, Steven C. Wofsy. Strong Southern Ocean carbon uptake
evident in airborne observations. Science, 2021; 374 (6572):
1275 DOI: 10.1126/ science.abi4355 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/12/211202141501.htm

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