The Impact of Rainfall on the Global Carbon Cycle

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. Recent research published in Nature Geoscience has shed light on a previously overlooked factor in this process – rainfall. The study, co-authored by a University of Hawai’i at Mānoa oceanographer, highlights how rainfall can affect the exchange of CO2 between the atmosphere and the ocean.

The Role of Rainfall

While exchanges between the ocean and the atmosphere are influenced by various chemical, physical, and biological factors, rainfall has been shown to play a significant role in the process. Rainfall impacts the oceanic carbon sink in three main ways. Firstly, as rain falls on the ocean surface, it creates turbulence, allowing for greater interaction between the water just below the surface and the atmosphere. Secondly, rain dilutes the seawater at the surface, changing the chemical equilibrium and enabling the ocean to absorb more CO2. Finally, raindrops directly inject CO2 into the ocean, enhancing the carbon sink.

Research Findings

A recent study led by Laetitia Parc, a doctoral student at Ecole Normale Supérieure (ENS; France), provided a global estimate of the impact of rainfall on the oceanic carbon sink. Using satellite observations and climate data from 2008 to 2018, the research team found that rain increases the oceanic carbon sink by 140 to 190 million tons of carbon per year. This represents a significant increase of 5% to 7% in the total amount of carbon absorbed by the oceans annually.

The study also highlighted regional variations in the impact of rainfall on the carbon cycle. Turbulence and dilution, which increase the CO2 sink, were found to be most significant in tropical regions with heavy rainfall and weak winds. On the other hand, the direct injection of CO2 by raindrops was more prominent in areas with heavy precipitation, including the tropics, storm tracks, and the Southern Ocean.

The findings of this research have significant implications for our understanding of the global carbon budget. The authors suggest that the effect of rainfall should be explicitly considered in estimates used to calculate the carbon budget each year. With climate change predicted to alter rainfall patterns over the ocean, the impact of rain on the oceanic carbon sink is likely to become even more significant in the future.

The study highlights the importance of considering all factors, including rainfall, in our efforts to understand and mitigate the effects of human activities on the global carbon cycle. As we continue to study and address climate change, it is essential to take into account the complex interactions between different components of the Earth system, such as the role of rainfall in enhancing the ocean’s capacity to absorb CO2.