The Impact of Rainfall on the Ocean’s Carbon Sink

The ocean plays a crucial role in the global carbon cycle, absorbing approximately 25% of the carbon emissions produced by human activities each year. Recent research published in Nature Geoscience, co-authored by a University of Hawai’i at Mānoa oceanographer, has shed light on the significant contribution of rainfall to the ocean’s carbon uptake. This study emphasizes the importance of understanding the relationship between rainfall and air-sea CO2 fluxes for a comprehensive understanding of the ocean carbon sink.

Rainfall influences the exchange of CO2 between the atmosphere and the ocean in three primary ways. Firstly, as rain falls on the ocean surface, it creates turbulence that enables deeper water layers to interact with the atmosphere, facilitating carbon exchange. Secondly, rainfall dilutes the seawater at the surface, altering the chemical equilibrium within the oceanic carbon cycle and enhancing the ocean’s capacity to absorb CO2. Lastly, raindrops directly introduce CO2 absorbed during their descent through the atmosphere into the ocean, further contributing to the carbon sink.

A recent study led by Laetitia Parc, a doctoral student at Ecole Normale Supérieure (ENS; France), provided the first global estimate of the three effects of rain on the ocean’s carbon sink. By analyzing satellite observations and global climate and weather data from 2008 to 2018, the research team determined that rainfall enhances the oceanic carbon sink by 140 to 190 million tons of carbon annually. This increase represents a 5% to 7% boost in the 2.66 billion tons of carbon absorbed by the oceans yearly.

The study identified regional differences in the impact of rainfall on the ocean’s carbon sink. Turbulence and seawater dilution, induced by heavy rainfall events in tropical regions with weak winds, play a significant role in increasing the CO2 sink. In contrast, direct injection of dissolved carbon by raindrops is more pronounced in regions with substantial precipitation, including tropical zones, storm tracks, and the Southern Ocean. These findings underscore the need to explicitly consider the effect of rainfall when estimating the global carbon budget, which integrates anthropogenic emissions, atmospheric CO2 growth, and natural carbon sinks.

As climate change continues to alter rainfall patterns, understanding the impact of precipitation on the ocean’s carbon sink is crucial for predicting future changes in the global carbon cycle. By incorporating rainfall effects into carbon budget calculations, scientists can more accurately assess the role of the ocean in mitigating CO2 levels in the atmosphere. This research highlights the interconnectedness of atmospheric, oceanic, and terrestrial processes in regulating the Earth’s carbon balance and underscores the importance of holistic approaches to carbon cycle studies.