The ocean is an important carbon sink that absorbs 20~30% of the total anthropogenic CO2 emissions in the industrial era (1.0-3.0 Pg annually, 1 Pg = 1015 g). Tropical cyclones are among the most devastating weather systems that profoundly disturb the upper ocean. However, their role in the global carbon cycle has been controversial: do tropical cyclones lead to net carbon absorption or release by the ocean, and does it matter?
Now, an international team has synthesized various observations to construct a daily available dataset of global air-sea CO2 flux, based on which the evolving role of tropical cyclones in the global carbon cycle under global warming has been quantified.
Here's what they've found:
▸ Tropical cyclones cause net ocean carbon outgassing. Strong winds of tropical cyclones greatly enhance sea-to-air CO2 transfer, although the sea surface cooling left behind by tropical cyclones contributes to ocean carbon uptake and partly offsets the carbon outgassing.
▸ The net outgassing is decreasing. Tropical cyclones contribute about 16% of the global annual carbon flux during 1993–1997, but that fraction decreases to merely 4.5% during 2016-2020. This decrease is because global warming leads to a sharpened vertical temperature gradient in the upper ocean, so that stronger sea surface cooling occurs after tropical cyclones and causes larger ocean carbon uptake.
▸ Carbon mitigation faces mounting pressure. If anthropogenic carbon dioxide emissions remain high, tropical cyclones could reverse their role after about 2035—switching from ocean carbon outgassing to carbon uptake, thereby accelerating ocean acidification.
The study, published in Nature Geoscience and led by researchers at the National University of Defense Technology, Chinese Academy of Sciences, the NSF National Center for Atmospheric Research, and the GEOMAR Helmholtz Centre for Ocean Research Kiel, overcame the limitations of sparse CO2 measurements. It reveals that tropical cyclones contribute about 9% to 23% of ocean carbon outgassing in the main basins since 1993 on average. However, this contribution has decreased significantly in recent decades, falling to more than half of that level by the 2010s. If anthropogenic CO2 emissions remain at a high level in the future, tropical cyclones might instead cause net ocean uptake.
The vertical gradient of upper-ocean temperature under global warming is increasing stratification as the main contributor. Under global warming, the sea surface warms faster than the subsurface water. With tropical cyclones at the same intensities, wind forcing causes stronger sea surface cooling in so-called cold wakes in a warmer climate, which can persist for weeks to over one month after the passage of a tropical cyclone. Consequently, the carbon disequilibrium at the air-sea interface increases, and the role of ocean carbon uptake is enhanced.
"The role of tropical cyclone in the global carbon cycle has long been obscure, owing to sparse observations during and after tropical cyclones. This work provides a sophisticated global air-sea carbon flux dataset, enabling exploration of the tropical cyclone contribution in the context of global warming."
— Prof. Zhanhong Ma, National University of Defense Technology; co-author of the study.
The author team stresses the importance of future carbon mitigation. Whether tropical cyclones remain a carbon source or sink depends on how human emissions into the atmosphere change. If anthropogenic carbon dioxide emissions remain high, the role of tropical cyclones could reverse from boosting ocean carbon outgassing to enhancing ocean carbon uptake. The outcome is even more severe for ocean acidification and chemical changes in seawater, leading to habitat shrinkage for marine species.
If emissions are controlled immediately, the downward trend in cyclone-driven carbon flux would be unlikely to reverse until around the 2040s. Then it would take until the end of this century for carbon uptake to return to current levels.
