The amount of carbon stored by plankton is going to increase over the next 100 years, according to researchers based at the University of Bristol and the National Oceanography Centre (NOC), but it’s difficult to predict beyond this point. These findings were published in the journal PNAS.
Relying on the latest models from the Intergovernmental Panel on Climate Change, the team calculated that the biological pump — a process whereby microscopic plants take up carbon and then sink to the bottom of the ocean when they die, storing carbon for hundreds of years — will represent up to 17% of the total increase in carbon uptake by 2100.
“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,” explained Dr. Jamie Wilson, from the University of Bristol.
Plankton living on the surface of the ocean use carbon dioxide during photosynthesis. When these microscopic organisms die, they sink and pass the “Twilight zone” (around 200-1000m), where factors such as temperature and oxygen concentration, as well as how much is eaten by other plankton, control what reaches the deep ocean. When they reach the bottom, carbon dioxide stays in their bodies and is stored for hundreds or even thousands of years. It turns out that warming the oceans slows down this 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. 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,” said Dr. Anna Katavouta from the National Oceanography Centre. This outcome will help us to better 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.”
However, researchers cannot predict what’s going to happen beyond the end of the century. The problem is that the models used in this study can’t predict what happens in the Twilight Zone, leaving researchers guessing how much carbon dioxide from the atmosphere will reach the bottom of the ocean in the future.
It’s even possible that — after 2100 — the carbon storage with the biological pump will stall or start releasing carbon dioxide, which would worsen climate change. “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,” said Dr. Wilson. “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 now wants to understand what processes in the Twilight Zone are essential for this carbon storage mechanism. The aim is to update ocean models and make them more reliable to predict future changes caused by climate change.
Wilson J, Andrew’s O, Katavouta A et al. (2022) The biological carbon pump in CMIP6 models: 21st century trends and uncertainties. PNAS, 119 (29) e2204369119, https://doi.org/10.1073/pnas.2204369119