Ocean uptake of carbon dioxide could drop as carbon emissions are cut
Shrinkage due to COVID-19 may provide case in point

Date:
June 3, 2020
Source:
Earth Institute at Columbia University
Summary:
The ocean is so sensitive to declining greenhouse gas emissions
that it immediately responds by taking up less carbon dioxide,
says a new study.

The authors say we may soon see this play out due to the COVID-19
pandemic lessening global fuel consumption; they predict the ocean
could take up less carbon dioxide in 2020 than in 2019.



FULL STORY ========================================================================== Volcanic eruptions and human-caused changes to the atmosphere strongly influence the rate at which the ocean absorbs carbon dioxide, says a
new study.

The ocean is so sensitive to changes such as declining greenhouse gas
emissions that it immediately responds by taking up less carbon dioxide.


==========================================================================
The authors say we may soon see this play out due to the COVID-19
pandemic lessening global fuel consumption; they predict the ocean will
not continue its recent historic pattern of absorbing more carbon dioxide
each year than the year before, and could even take up less in 2020 than
in 2019.

"We didn't realize until we did this work that these external forcings,
like changes in the growth of atmospheric carbon dioxide, dominate the variability in the global ocean on year-to-year timescales. That's a real surprise," said lead author Galen McKinley, a carbon cycle scientist at Columbia University's Lamont-Doherty Earth Observatory. "As we reduce our emissions and the growth rate of atmospheric carbon dioxide slows down,
it's important to realize that the ocean carbon sink will respond by
slowing down." The paper, published today in the journal AGU Advances,
largely resolves the uncertainty about what caused the ocean to take up
varying amounts of carbon over the last 30 years. The findings will enable
more accurate measurements and projections of how much the planet might
warm, and how much the ocean might offset climate change in the future.

A carbon sink is a natural system that absorbs excess carbon dioxide
from the atmosphere and stores it away. Earth's largest carbon sink
is the ocean. As a result, it plays a fundamental role in curbing the
effects of human-caused climate change. Nearly 40 percent of the carbon
dioxide added to the atmosphere by fossil fuel burning since the dawn
of the industrial era has been taken up by the ocean.

There's variability in the rate at which the ocean takes up carbon
dioxide, which isn't fully understood. In particular, the scientific
community has puzzled over why the ocean briefly absorbed more carbon
dioxide in the early 1990s and then slowly took up less until 2001,
a phenomenon verified by numerous ocean observations and models.



========================================================================== McKinley and her coauthors addressed this question by using a diagnostic
model to visualize and analyze different scenarios that could have driven greater and lesser ocean carbon uptake between 1980 and 2017. They found
the reduced ocean carbon sink of the 1990s can be explained by the slowed growth rate of atmospheric carbon dioxide early in the decade. Efficiency improvements and the economic collapse of the Soviet Union and Eastern
European countries are thought to be among the causes of this slowdown.

But another event also affected the carbon sink: The massive eruption of
Mount Pinatubo in the Philippines in 1991 caused the sink to temporarily
become much larger coincident with the eruption.

"One of the key findings of this work is that the climate effects of
volcanic eruptions such as those of Mount Pinatubo can play important
roles in driving the variability of the ocean carbon sink," said
coauthor Yassir Eddebbar, a postdoctoral scholar at Scripps Institution
of Oceanography.

Pinatubo was the second-largest volcanic eruption of the 20th century. The estimated 20 million tons of ash and gases it spewed high into the
atmosphere had a significant impact on climate and the ocean carbon
sink. The researchers found that Pinatubo's emissions caused the ocean
to take up more carbon in 1992 and 1993. The carbon sink slowly declined
until 2001, when human activity began pumping more carbon dioxide into
the atmosphere. The ocean responded by absorbing these excess emissions.

"This study is important for a number of reasons, but I'm most interested
in what it means for our ability to predict the near-term, one to ten
years out, future for the ocean carbon sink," said coauthor said Nicole Lovenduski, an oceanographer at the University of Colorado Boulder. "The
future external forcing is unknown. We don't know when the next big
volcanic eruption will occur, for example. And the COVID-19-driven
carbon dioxide emissions reduction was certainly not anticipated very
far in advance." Investigating how the Pinatubo eruption impacted
global climate, and thus the ocean carbon sink, and whether the drop
in emissions due to COVID-19 is reflected in the ocean are among the
research team's next plans.



==========================================================================
By understanding variability in the ocean carbon sink, the scientists
can continue to refine projections of how the ocean system will slow down.

McKinley cautions that as global emissions are cut, there will be an
interim phase where the ocean carbon sink will slow down and not offset
climate change as much as in the past. That extra carbon dioxide will
remain in the atmosphere and contribute to additional warming, which
may surprise some people, she said.

"We need to discuss this coming feedback. We want people to understand
that there will be a time when the ocean will limit the effectiveness
of mitigation actions, and this should also be accounted for in
policymaking," she said.

The study was coauthored by Amanda Fay and Lucas Gloege of Columbia University's Lamont-Doherty Earth Observatory.


========================================================================== Story Source: Materials provided by
Earth_Institute_at_Columbia_University. Original written by Rebecca
Fowler. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Galen A. McKinley, Amanda R. Fay, Yassir A. Eddebbar, Lucas Gloege,
Nicole S. Lovenduski. External Forcing Explains Recent Decadal
Variability of the Ocean Carbon Sink. AGU Advances, 2020; 1 (2)
DOI: 10.1029/2019AV000149 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200603130016.htm https://www.sciencedaily.com/releases/2020/06/200603130016.htm

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