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Oceans Carbon Sink or Source
Do oceans remove carbon dioxide from the atmosphere or contribute to it?
The answer is crucial for climate change. Dr.
Mae-Wan Ho
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version of this article is posted on ISIS members’ website. Membership details
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NASA’s ocean watch
NASA’s SeaWiFS
project (Sea-viewing Wide Field-of View Sensor)(SeaWiFS) has been watching
the oceans twenty-four hours a day every day since September 1997 [1]. It
provides quantitative data on the biological state of the global oceans through
remote colour sensing. The colour in most of the world’s oceans varies with
the concentration of chlorophyll and other plant pigments contained in the
phytoplankton, the greater the concentration of plant pigments, the greener
the water. As an orbiting sensor can view every square kilometre of cloud-free
ocean every 48 hours, satellite-acquired ocean colour data are valuable for
determining the abundance of ocean life and to assess the ocean’s role in
climate change.
One big question the SeaWiFS project wants to answer is whether the oceans
are a carbon source that adds carbon dioxide to the atmosphere, or a carbon
sink that removes it from the atmosphere, which is crucial to monitoring climate
change and taking appropriate action. The oceans not only contain 97 percent
of all the water on earth, they are also the biggest carbon reservoir, and hence
a major player in climate and climate change (Oceans
and global warming, this series).
Passive and active carbon exchanges
Carbon dioxide in the atmosphere can dissolve in water,
and the colder and more turbulent regions of the oceans tend to absorb carbon
dioxide, while the warmer and less turbulent regions release it, carbon dioxide
being less soluble in warm water than in cold water. In the early 1990s, the
oceans were thought to be a net carbon sink [2], with the North
Atlantic Ocean accounting for 60 percent of the carbon dioxide absorbed by
the world’s oceans, which amounted to about 2 Gt a year [3].
The passive uptake and release of carbon dioxide is not
as important, however, as the active uptake of carbon dioxide by the phytoplankton
in photosynthesis and its active release in respiration by the whole community
of marine organisms, which amount to about 100 Gt a year each way (Oceans
and global warming, this series).
Phytoplankton consists of microscopic green algae that
grow at prodigious rates in the surface layers of the oceans, feeding an enormous
marine food web that has nine times the biomass of the terrestrial food web.
A thriving phytoplankton population would tend to remove more carbon dioxide
through photosynthesis than is returned through respiration by the entire
community (phytoplankton plus zooplankton and other organisms living in the
surface layers), and the ocean works as an effective carbon sink. Conversely,
if respiration of the community were to exceed photosynthesis, more carbon
dioxide would be generated than is fixed, and the ocean becomes a carbon source.
Monitoring how well the ocean’s plankton is doing is therefore very
important for predicting climate and climate change
North East Atlantic a carbon source
Scientists at several Spanish universities
used data from nine Spanish cruises conducted between 1991-2000 in the subtropical
NE Atlantic
to calculate gross primary production (photosynthesis) and respiration [4].
They found that two-thirds of the 33 stations investigated had respiration
rate greater than photosynthesis. That meant the phytoplankton was not fixing
carbon dioxide fast enough, and the oceans, or at least the northeast Atlantic Ocean could be a carbon source instead of a carbon
sink.
In photosynthesis, carbon dioxide
and water are combined to make sugar (carbohydrates) with the evolution of
oxygen; conversely, oxygen is consumed to oxidise sugars back to carbon dioxide
and water in respiration. So a convenient way to estimate the rates of photosynthesis
and respiration is to measure the oxygen produced (in the light) and consumed
(in the dark).
The researchers found that the
average photosynthesis over all the marine stations in northeast Atlantic was 2 600 + 271 mg O2/m2/day,
while the average community respiration was 3 821 + 276 mg O2/m2/day.
Clearly, respiration rate was far in excess of photosynthesis. Additional
evidence indicated that over the period of a year, respiration still exceeded
gross production.
The study concentrated
on the water column from a depth at which one percent of the sunlight has
penetrated up to the surface with full sunlight, and did not include the respiration
of organisms living at greater depths, where no photosynthesis could take
place. If that were included, the deficit in gross production would be even
bigger. The scientists estimated that 0.5 Gt of carbon is released per year just by the plankton
community covering the 5.26 million square kilometres of the subtropical NE Atlantic.
Other evidence has come to light since indicating that the increase of carbon
dioxide in the atmosphere and global warming are undermining the conditions
of growth for phytoplankton, which has the potential to wipe out the marine
biota at its very basis and to aggravate global warming (Shutting
down the oceans, this series).
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