Rainfall significantly increases the absorption rate of carbon dioxide in the ocean

Recent research is shedding new light on how rainfall affects the ocean’s ability to absorb carbon dioxide (CO₂) from the atmosphere – a factor that scientists have often overlooked when assessing how well our oceans act as a ‘carbon sink’. This latest research shows that rainfall can increase the ocean’s CO₂ absorption by 5 to 7 percent, meaning an additional 140 to 190 million tons of CO₂ are absorbed by the ocean annually. The research, led by David Ho, an oceanographer from the University of Hawai’i at Manoa, builds on work he began nearly three decades ago. In those early days, Ho set up two children’s pools to investigate how rain affected CO₂ transfer between air and water. His recent work now provides the first global estimate of the influence of rainfall on ocean surface CO₂ levels.

Traditionally, scientists have focused on measuring CO₂ in deeper ocean samples, collected 5 to 7 meters below the surface. These measurements ignore the direct impact of rain on the surface layer, where gas exchange with the atmosphere takes place most intensively.

How rain drives carbon uptake in the ocean

The study identifies three major ways rainfall increases CO₂ absorption in the ocean: turbulence, dilution, and wet deposition. First, raindrops falling on the ocean surface create turbulence, increasing the interaction between seawater and the atmosphere. Second, rainwater dilutes the salty ocean water, changing the CO₂ concentration gradient and releasing more CO₂ into the ocean. Finally, rain can transport CO₂ from the atmosphere as it falls, a process called wet deposition, which releases CO₂ directly into the ocean.

Laetitia Parc, who led this research as part of her doctoral studies at Sorbonne Université, emphasizes the importance of understanding this effect. Quantifying how rainfall affects carbon uptake in the ocean can improve the accuracy of models that track carbon exchange between the ocean and the atmosphere.

Implications for climate models

The team developed a model to monitor how rainfall affects the salinity of the ocean at the surface. They discovered that turbulence and dilution play a major role in tropical regions, where warm water absorbs more CO₂. Meanwhile, wet deposition is especially important in areas of heavy rainfall, such as storm tracks and the Southern Ocean.

Tatiana Ilyina, an earth scientist at the University of Hamburg, notes that this study makes a valuable contribution to understanding the global carbon cycle. As climate change is expected to alter global rainfall patterns, the effect of rain on ocean carbon uptake may also change, with important implications for climate models and forecasts.