Tropical cyclones release carbon, but warming could reverse the trend
Tropical cyclones, while destructive, play a role in the oceanic carbon cycle. A new study suggests their impact could reverse by 2035, shifting them from a carbon sink to a source, with major implications for the climate.
Oceans, the true blue lungs of our planet, absorb about 20 to 30% of anthropogenic CO2 emissions annually, amounting to 1 to 3 billion tons. At the heart of this complex mechanism, tropical cyclones, storms of devastating power, exert a profound influence on the upper layers of the ocean. Their exact role in the global carbon cycle has long been debated: do they contribute to net CO2 absorption or its release? A recent study, published in the journal Nature Climate Change, sheds new and potentially alarming light on this issue: not only do cyclones release carbon, but climate warming could reverse their role by 2035, transforming these meteorological giants into net CO2 sources.
Tropical Cyclones, Disruptors of the Carbon Cycle
Tropical cyclones are weather systems of phenomenal intensity, characterized by violent winds, torrential rains, and devastating swells. Their passage over warm ocean waters has direct consequences on oceanic processes, particularly water mass circulation and gas exchange at the air-sea interface. Traditionally, research has focused on the impact of cyclones on marine ecosystems and coastlines. However, their influence on the carbon cycle, this constant flow of carbon between the atmosphere, oceans, soils, and biosphere, had begun to attract interest. Global climate models already incorporate the ocean's role as a major carbon sink, but the specific contribution of extreme events like cyclones remained unclear.
The new study, based on satellite data and sophisticated ocean models, reveals that tropical cyclones currently act as disruptors that, overall, lead to the release of carbon from the ocean to the atmosphere. When these storms cross the oceans, they churn the water layers. This mixing brings deeper waters, richer in dissolved CO2, to the surface. This dissolved CO2 is then more easily exchanged with the atmosphere. Furthermore, the strong agitation of the ocean surface promotes CO2 degassing. Thus, paradoxically, these destructive events contribute to the release of this greenhouse gas. Researchers estimate that this current release is compensated, on a global scale, by the ocean's absorption of other CO2 sources, particularly of human origin. However, the scenario could dramatically reverse. Projections indicate that with continued ocean warming, the amount of dissolved CO2 in deep waters will increase, and the stratification of surface layers will become more pronounced. These conditions will make the upwelling of deep waters during a cyclone's passage even more effective in terms of CO2 release. The study predicts that by 2035, the net effect of tropical cyclones could shift from marginal absorption to a net CO2 source for the atmosphere.
How Cyclones Release Carbon
The mechanism is twofold. Firstly, the passage of a tropical cyclone induces deep water upwelling. Deep waters are naturally richer in carbon dioxide (CO2) because carbon is trapped there for long periods, away from atmospheric exchange. The intense churning caused by the cyclone's violent winds and waves brings these CO2-laden waters to the surface. Once at the surface, this dissolved CO2 is in direct contact with the atmosphere and can escape more easily.
Secondly, the agitation of the ocean surface by extreme winds increases the contact area between water and air. This turbulence promotes the diffusion of gases, including CO2, from the water to the atmosphere. It's a bit like shaking a soda bottle: it releases the contained gas. In the case of cyclones, it's on a gigantic scale.
Finally, cyclones can disrupt the biological carbon pump. Phytoplankton, these marine microorganisms that absorb CO2 through photosynthesis, are essential for carbon absorption. Cyclones can damage these communities or alter the availability of nutrients necessary for their growth, thus affecting their ability to capture CO2.
If tropical cyclones become a net source of CO2, this will have significant consequences for the global carbon balance. It could accelerate global warming by adding an extra source of greenhouse gases, thus complicating the achievement of global temperature limitation goals. Current climate models, which estimate the ocean's role as a carbon sink, will need to be revised to incorporate this new dynamic. The ECMWF (European Centre for Medium-Range Weather Forecasts) and other modeling centers will need to integrate these effects into their future simulations. The use of increasingly precise satellite data and the application of machine learning could help to better quantify and predict these complex exchanges. Predictive models will need to refine their ability to simulate not only the trajectory and intensity of cyclones but also their impact on biogeochemical cycles, such as the carbon cycle. This represents a major challenge for atmospheric and oceanographic science.
Why This Discovery is Crucial Today
This study underscores the urgency of acting on climate change. It highlights complex and potentially dangerous feedback loops that we do not yet fully control. The ocean, while a valuable ally in the fight against warming by absorbing a large portion of our emissions, could see its role altered by the very impacts of the warming it seeks to mitigate. Understanding and precisely quantifying these interactions is essential for refining our climate projections and guiding mitigation and adaptation policies. Atmospheric data collected by programs like Copernicus, combined with advances in neural networks for analyzing this massive data, are indispensable for deciphering these phenomena. The forecasting uncertainty regarding these future interactions highlights the need for continued research.
In conclusion, our understanding of the role of tropical cyclones in the carbon cycle is rapidly evolving. Current findings suggest a worrying trend reversal within about fifteen years, transforming these natural phenomena into net contributors to warming. This discovery reinforces the imperative to drastically reduce our greenhouse gas emissions to limit ocean warming and, consequently, the climate feedbacks that could result.
