Oceans absorb 6 percent more CO2 thanks to rain

September 11 () –

Approximately 6% of the total absorption of carbon dioxide (CO2) by the ocean is due to rainfall, according to a study recently published in Nature Geoscience.

“The impact of rainfall on air-sea CO2 fluxes has not been systematically examined, but understanding it gives us a more complete picture,” he said. in a statement David Ho, co-author of the study and a professor at the University of Hawaii Manoa’s School of Ocean and Earth Science and Technology. “This is especially important since rainfall patterns over the ocean are expected to change with climate change, and that could affect the ocean’s carbon sink“, he added.

The ocean plays an important role in the global carbon cycle, absorbing about a quarter of the carbon emitted by human activities each year.

Exchanges between the ocean and the atmosphere are governed by chemical, physical and biological properties and processes. Rain alters these properties of the ocean surface, promoting the exchange of CO2 between the air and the sea.

Rain affects this carbon exchange in three different ways. First, as it falls on the ocean surface, it creates turbulence that allows water just below the surface to come into contact with the atmosphere. Second, it dilutes seawater at the surface, altering the chemical balance within the ocean carbon cycle and allowing seawater to absorb greater amounts of CO2. Finally, raindrops directly inject the CO2 absorbed during their fall through the atmosphere into the ocean.

The new study, led by Laetitia Parc, a PhD student at the Ecole Normale Supérieure (ENS; France), is the first to provide a global estimate of these three rainfall effects. The research team relied on an analysis of satellite observations and a new analysis of global climate and weather data over an 11-year period, from 2008 to 2018.

Their research showed that rainfall increases the ocean carbon sink by 140 to 190 million tons of carbon per year. This represents a 5% to 7% increase in the 2.66 billion tons of carbon absorbed annually by the oceans. Increased surface exchanges due to turbulence and seawater dilution play an order of magnitude role. comparable to direct injection of dissolved carbon in raindrops.

However, the regions where these processes are significant differ. Turbulence and dilution primarily increase the CO2 sink in tropical regions characterized by strong rainfall events associated with weak winds, inducing notable salinization and dilution of CO2. In contrast, raindrop deposition is significant in all regions with heavy rainfall – the tropics, of course, but also storm routes and the Southern Ocean.

The results of this study suggest that the effect of rainfall should be explicitly included in the estimates used to construct the global carbon budget, which is compiled annually and integrates anthropogenic emissions, atmospheric CO2 growth, and natural carbon sinks.