Marine bacteria produce methane where it was not expected. This mechanism, now absent from climate models, could already intensify.
The oceans absorb a significant amount of atmospheric CO2. But they could also, silently, amplify the warming they are experiencing. A study published in the Proceedings of the National Academy of Sciences by Thomas Weber, associate professor at the Department of Earth and Environmental Sciences at the University of Rochester, and his colleagues, lifts the veil on a long underestimated mechanism: the production of methane in the open ocean, where water is nevertheless rich in oxygen. Their results suggest that this phenomenon could double within a century under the effect of warming. Serious projections, but which still call for inclusion in major climate models.
A paradox solved by the hunger of bacteria
The mechanism is not entirely new. Laboratory studies, then in real marine environments, had already shown that some bacteria produce methane by degrading an organic compound, methylphosphonate, when they lack phosphate. « It is a relatively well-known microbial process, » confirms Weber contacted by email. What his team, including his students Shengyu Wang and Hairong Xu, brings again is twofold.
First, the demonstration that this mechanism is widespread throughout the tropical and subtropical oceans, and not limited to a few isolated areas. Then, the precise quantification of associated global emissions and their sensitivity to climate change. To do this, the researchers crossed a global data set with a computer model of marine biogeochemical cycles.
2 teragrams per year, but a worrying trajectory
This process now represents about 2 teragrams of methane per year, or 2 million tons. This remains modest compared to terrestrial wetlands, which emit about 100 teragrams annually. But Weber invites us not to stop at this comparison. « What matters from a climate point of view is the ability of a source to change. » However, the projections of the study indicate that these marine emissions could double by the end of the century. The mechanism is as follows: warming heats the ocean from above, strengthening the stratification of water and slowing the rise of nutrients, including phosphate, from the depths. Surface waters that are increasingly poor in phosphate favor methane-producing bacteria. The latter release more greenhouse gases, which further warms the ocean. The loop will close.
And she could already be in action. An independent study, published at the same time, shows that surface phosphate levels have already declined in the tropical Ocean over the past few decades. « It is therefore likely that open ocean methane emissions have already increased compared to their pre-industrial level, » says Weber. The doubling he predicts would add to an increase already underway. « I hope that by the end of my career, we will be able to detect significant increases in marine methane emissions, » he adds.
A blind spot of climate models
Unfortunately, this mechanism is now absent from major climate projection models. The reason is simple. Marine methane emissions were considered, until now, to be negligible compared to terrestrial sources. However, all these models already simulate the cycles of oceanic nutrients, including phosphate. Because they control the growth of phytoplankton and the CO2 cycle.
Weber therefore offers a concrete and accessible first step. « It would be enough to implement a flow of methane into the atmosphere, linked to the simulated concentration of surface phosphate. Our study provides all the information necessary to do so. » In the longer term, he calls for models capable of explicitly simulating the marine cycle of methane, to capture not only this feedback, but also those related to the warming of the seabed and the deoxygenation of the oceans. « This is an exciting avenue for research, which will lead to more robust climate simulations, » he concludes.
source : science et vie
