By Hauwa Ali
A recent research study has found that phytoplankton stores roughly double the amount of carbon
dioxide that is currently in our atmosphere in the deep ocean with a consistent increase in the carbon
stored in the ocean by the biological carbon pump over the 21st century.
The study which was conducted by researchers at the University of Bristol and the National
Oceanography Centre (NOC) was published recently in the journal PNAS (Proceedings of the National
Academy of Sciences).
Using the latest IPCC models (Intergovernmental Panel on Climate Change), the team expects the
“biological pump”—a process where microscopic plants, often called phytoplankton, take up carbon and
then die and sink into the deep ocean where carbon is stored for hundreds of years—to account for
between 5 and 17% of the total increase in carbon uptake by the oceans by 2100.
However the research study found out that the phytoplankton now takes up twice the amount of
carbon dioxide into the ocean with a slow export chain to rid the ocean of excessive carbon dioxide
which is warming it.
“The biological pump stores roughly double the amount of carbon dioxide that is currently in our
atmosphere in the deep ocean. Because plankton are sensitive to climate change, this carbon pool is
likely to change in size, so we set out to understand how this would change in the future in response to
climate change by looking at the latest future projections by IPCC models.” Lead author Dr. Jamie
Wilson, of the University of Bristol’s School of Earth Sciences, explained.
Microscopic organisms called plankton, living in the sunlit surface of the ocean, use carbon dioxide
during photosynthesis. When these plankton die, their remains rapidly sink down through the “Twilight
Zone” of the ocean (200–1000m), where environmental factors, such as temperature and oxygen
concentration, and ecological factors, such as being eaten by other plankton, control how much reaches
the deep ocean where the carbon from their bodies is stored away from the atmosphere for hundreds
to thousands of years. Warming of the oceans slows down the circulation, increasing the time that
carbon is stored in the deep ocean.
“Our research found a consistent increase in the carbon stored in the ocean by the biological carbon
pump over the 21st century in the latest IPCC model projections. In contrast, we found a decline in the
global export production (the amount of organic matter, such as dead plankton, sinking below the ocean
surface), which suggests that export production may not be as accurate a metric for the biological
carbon pump than previously thought. Contributing author Dr. Anna Katavouta, who worked alongside
early-career scientist Dr. Chelsey Baker, both from the National Oceanography Centre, said.
“We demonstrated that the organic matter flux at 1000 meters is instead a better predictor of long-term
carbon sequestration associated with the biological carbon pump. This outcome will help us to better
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understand the processes that control the biological carbon pump and to predict more reliably how
much of the carbon released due to human activity will be stored in the ocean in the future.” He added.
However, the IPCC models have no consistent representation of the environmental and ecological
processes in the Twilight pump Zone, which leads to a large uncertainty in how much carbon dioxide
originating from the atmosphere the biological will store beyond the end of the century. In theory, after
2100, carbon storage by the biological pump could stall and instead may start acting as a source of
carbon dioxide to the atmosphere, which could exacerbate climate change further.
Dr. Wilson added, “This research demonstrates the crucial importance of the Twilight Zone region of the
ocean for biologically-driven carbon storage in the ocean. This part of the ocean is still poorly
understood because it is so hard to observe but it is also just now starting to come under pressures of
environmental change, fishing and deep-sea mining.
“Understanding how the Twilight Zone controls how much carbon is stored by biology in the ocean
means we can figure out how to avoid the worst impacts from human practices like fishing and mining.”
The team will now work towards figuring out which processes in the Twilight Zone are the most
important for biologically-driven carbon storage and updating ocean models so they can reliably predict
future changes.
