A study takes stock of how the uptake of carbon by phytoplankton should evolve over the century according to different scenarios of global warming. If the conclusions agree on an increase in absorption between now and 2100, they diverge very widely beyond this horizon. The results were published in the journal PNAS on July 11.
Phytoplankton includes all plant organisms proliferating in the surface layer of the ocean. Microscopic in size, there are more than twenty thousand species. It should be remembered that it is of major ecosystem importance since it is at the base of marine food chains. In addition, it participates in the regulation of the global climate by capturing atmospheric carbon dioxide (CO2) which it transforms into dioxygen (O2).
Phytoplankton should provide efficient carbon pumping throughout the century
The latest research indicates that plankton will continue to take up an increasing amount of carbon over the century, accounting for 5% to 16% of total ocean uptake by 2100. The biological carbon pump is at play here. the plankton dies, it falls towards the depths which induces a net retrenchment of carbon from the atmosphere for a few hundred years, or even several tens of millions of years for the small fraction which will reach the sediments and will be transformed into fossil fuels .
With climate change, this circulation of plankton remnants is expected to slow down, implying an increase in the residence time of carbon in the intermediate and deep layers of the ocean. The biological pump is therefore more efficient, hence the increasing carbon absorption simulated by the latest generation of climate models (CMIP6) used in the sixth assessment report of the IPCC, the first part of which was published in August 2019. These results contrast with those obtained by previous work.
The researchers trace the origin of this disagreement to the way in which the effectiveness of the biological pump is evaluated. Indeed, previous studies tended to look at the amount of organic matter exported to the depths, a parameter that actually tends to decline. “Exported production may not be as accurate a measure of the biological carbon pump as previously thought,” said Anna Katavouta, co-author of the study.
In practice, the lower export is compensated by a more efficient percolation of organic matter. Environmental factors such as temperature, the degree of oxygenation of the waters and the activity of phytoplankton consumers will affect the quantity of organic matter able to reach the depths. Together, they therefore represent a key element. “We have demonstrated that the flow of organic matter at a thousand meters is a better predictor of long-term carbon sequestration associated with the biological pump”, notes the researcher.
Uncertainty that explodes after the end of the century
Due to the continuing difficulty of representing these complex processes in models, projections of carbon sequestration after 2100 remain highly uncertain. Some models indicate a reversal of the biological pump with a release of carbon towards the atmosphere which would then reinforce the warming. Others, on the contrary, show an absorption which is maintained at a more or less equal level over time.
“This research demonstrates the critical importance of the twilight zone for biological carbon storage in the ocean,” says Jamie D. Wilson, lead author of the paper. “This part of the ocean is still poorly understood, as it is difficult to observe, but it is also just beginning to come under pressure from environmental change, deep-sea fishing and mining. Understanding how the area twilight biologically controls the amount of carbon stored in the ocean means we could figure out how to avoid the worst impacts of human practices like fishing and mining.”
