The ocean plays a crucial role in the global carbon cycle, acting as a sink for approximately one-quarter of the carbon dioxide emitted by human activities each year. Recently, a study published in Nature Geoscience, with a co-author from the University of Hawai’i at Mānoa, highlighted the significance of rainfall in this process. Understanding the impact of rain on air-sea CO2 fluxes is essential for gaining a comprehensive understanding of the ocean carbon sink, especially in light of changing rainfall patterns due to climate change.
The study revealed that about 6% of the total uptake of carbon dioxide by the ocean can be attributed to rainfall. Rainfall affects the exchange of CO2 between the air and the sea in three main ways. Firstly, when rain falls on the ocean surface, it creates turbulence that promotes the contact between the water just below the surface and the atmosphere. This facilitates the exchange of CO2 between the two mediums. Secondly, rain dilutes the seawater at the surface, leading to alterations in the chemical equilibrium within the oceanic carbon cycle. This allows the seawater to absorb higher quantities of CO2. Lastly, raindrops directly inject CO2 absorbed during their descent through the atmosphere into the ocean.
Led by Laetitia Parc, a doctoral student at Ecole Normale Supérieure (ENS; France), the research team conducted a comprehensive study to estimate the effects of rain on the ocean carbon sink. Through an analysis of satellite observations and global climate and weather data over an 11-year period, the study found that rain increases the oceanic carbon sink by 140 to 190 million tons of carbon per year. This equates to a 5% to 7% increase in the total carbon absorbed by the oceans annually.
The study also highlighted that the impact of rain on the ocean carbon sink varies across different regions. Turbulence and seawater dilution primarily enhance the CO2 sink in tropical regions with heavy rainfall events and weak winds, leading to significant salinity and CO2 dilution. On the other hand, direct injection of dissolved carbon by raindrops is more prominent in regions with heavy precipitation, including the tropics, storm tracks, and the Southern Ocean.
The results of the study suggest that the effect of rain on the ocean carbon sink should be explicitly incorporated into estimates used to construct the global carbon budget. The annual compilation of the carbon budget takes into account anthropogenic emissions, atmospheric CO2 growth, and natural carbon sinks. By including the impact of rainfall, a more accurate representation of the ocean’s role in the global carbon cycle can be achieved.
Rain plays a crucial yet often overlooked role in the ocean carbon sink. As climate change continues to alter rainfall patterns, understanding the influence of rain on the exchange of CO2 between the atmosphere and the ocean is paramount. Further research in this area is essential for developing effective strategies to mitigate the impact of human activities on the global carbon cycle.