The Impact of Rain on the Ocean’s Carbon Cycle

The ocean’s role in the global carbon cycle cannot be understated, with approximately one-quarter of the carbon emissions produced by human activities being absorbed by the ocean each year. A recent study published in Nature Geoscience, co-authored by a University of Hawai’i at Mānoa oceanographer, sheds light on a previously overlooked aspect of this process – the impact of rainfall. According to David Ho, a professor at UH Mānoa, approximately 6% of the total uptake of carbon dioxide (CO2) by the ocean can be attributed to rainfall.

Rainfall affects the exchange of CO2 between the atmosphere and the ocean in three key ways. Firstly, as rain falls onto the ocean surface, it creates turbulence that allows for greater contact between the water just below the surface and the atmosphere. This promotes the exchange of CO2. Secondly, rainwater dilutes the seawater at the surface, altering the chemical equilibrium within the oceanic carbon cycle and enabling the absorption of larger quantities of CO2. Finally, raindrops directly inject CO2 into the ocean as they fall through the atmosphere.

A groundbreaking study led by Laetitia Parc, a doctoral student at Ecole Normale Supérieure in France, provided the first global estimate of rain’s influence on the oceanic carbon sink. The research team analyzed satellite observations and global climate data over an 11-year period from 2008 to 2018. Their findings revealed that rain increases the oceanic carbon sink by 140 to 190 million tons of carbon annually, representing a 5% to 7% increase in the total carbon absorbed by the oceans each year.

The study also highlighted regional differences in how rain impacts the ocean’s carbon sink. Turbulence and dilution, which primarily occur in tropical regions with heavy rainfall events and weak winds, were found to significantly increase the CO2 sink. On the other hand, the direct injection of dissolved carbon by raindrops was more pronounced in regions with heavy precipitation, including the tropics, storm tracks, and the Southern Ocean. These findings suggest that the effect of rain on the ocean’s carbon cycle must be explicitly incorporated into global carbon budget estimates.

Understanding the role of rain in the oceanic carbon cycle is crucial, especially in the context of climate change. With shifting rainfall patterns predicted as a result of climate change, the impact of rain on the ocean’s carbon sink is likely to evolve. By factoring in the influence of rain on carbon exchange, researchers can more accurately predict the future behavior of the global carbon cycle and its implications for climate change mitigation efforts.

Rain may seem like a minor player in the grand scheme of the ocean’s carbon cycle, but its impact is significant. By shedding light on the role of rainfall in facilitating carbon exchange between the atmosphere and the ocean, researchers are gaining a more comprehensive understanding of this intricate process. As we continue to grapple with the challenges of climate change, incorporating rain’s influence into our models and projections is essential for developing effective strategies to mitigate the impacts of rising carbon emissions.