Three decades ago, David Ho, then a young NOAA technician, set up an unusual experiment involving two dinosaur-patterned kiddie pools in the NOAA parking lot to investigate the role of rain in CO2 exchange between air and water. Ho’s daily routine involved waiting for Miami’s frequent thunderstorms to sample the rain-affected pools, enduring months of drenching for science. The initial results showed that rainfall significantly enhances the transfer of carbon dioxide (CO2) from the atmosphere to the ocean, setting the stage for Ho’s lifetime research into ocean carbon uptake. Now an oceanographer at the University of Hawai‘i at Manoa, Ho’s recent study provides a global assessment of this process, revealing rain’s critical impact on the ocean’s carbon sink.
The study, led by PhD student Laetitia Parc, estimates that rainfall boosts ocean carbon uptake by 140–190 million metric tons annually, approximately 5%–7% of global CO2 absorption by the oceans. This effect occurs through three primary mechanisms: turbulence, dilution, and wet deposition. When a raindrop strikes the ocean surface, it generates turbulence, increasing the water-atmosphere interface area and enhancing CO2 exchange. The rain’s relatively fresh water dilutes the surface, shifting the air-sea CO2 concentration gradient, which further encourages uptake. Finally, wet deposition allows raindrops to absorb CO2 from the air and deliver it directly into the ocean upon contact. Each mechanism contributes uniquely to the increase in ocean carbon uptake across different regions, depending on rainfall intensity and wind conditions.
Ho and his team found that the impact of these mechanisms varies significantly by region. In tropical zones with high rainfall and calm winds, turbulence and dilution have substantial effects on CO2 absorption. Wet deposition also plays a large role in high-precipitation areas like storm tracks and the Southern Ocean, expanding the geographical influence of rain on ocean carbon uptake beyond tropical latitudes. This broad impact came as a surprise to the researchers, as they initially expected rainfall to increase carbon uptake only in the tropics.
Hugo Bellenger, a climate modeler at the French National Centre for Scientific Research (CNRS), developed a model to track rain-induced salinity changes on the ocean’s surface. He noted that while models have historically focused on temperature changes, salinity shifts from rain have long been overlooked, despite their significance. This new modeling approach helped pinpoint where and how rain amplifies CO2 absorption, highlighting the strong influence in regions with both frequent and intense rainfall.
The study underscores the role of precipitation as a critical but previously unmeasured variable in the global carbon cycle. As Earth’s climate warms, shifts in precipitation patterns are expected to intensify rainfall over oceans, likely amplifying rain’s effect on ocean carbon uptake. Already, researchers observed a gradual increase in this effect between 2008 and 2018, driven by changing rainfall patterns linked to climate change. The implications are profound for carbon-cycle models, which must now incorporate rainfall’s contribution to accurately estimate the ocean’s carbon sink potential.
Tatiana Ilyina, a carbon cycle modeler at Universität Hamburg, emphasizes that this research should drive a rethinking of global carbon models. She argues that understanding rain’s role is crucial to improve the accuracy of these models, which have traditionally struggled with capturing the nuances of precipitation’s influence on carbon dynamics. This study not only advances insights into ocean carbon uptake but also highlights the intricate ways climate change affects carbon cycling between the ocean and atmosphere.
Three decades ago, David Ho, then a young NOAA technician, set up an unusual experiment involving two dinosaur-patterned kiddie pools in the NOAA parking lot to investigate the role of rain in CO2 exchange between air and water. Ho’s daily routine involved waiting for Miami’s frequent thunderstorms to sample the rain-affected pools, enduring months of drenching for science. The initial results showed that rainfall significantly enhances the transfer of carbon dioxide (CO2) from the atmosphere to the ocean, setting the stage for Ho’s lifetime research into ocean carbon uptake. Now an oceanographer at the University of Hawai‘i at Manoa, Ho’s recent study provides a global assessment of this process, revealing rain’s critical impact on the ocean’s carbon sink.
The study, led by PhD student Laetitia Parc, estimates that rainfall boosts ocean carbon uptake by 140–190 million metric tons annually, approximately 5%–7% of global CO2 absorption by the oceans. This effect occurs through three primary mechanisms: turbulence, dilution, and wet deposition. When a raindrop strikes the ocean surface, it generates turbulence, increasing the water-atmosphere interface area and enhancing CO2 exchange. The rain’s relatively fresh water dilutes the surface, shifting the air-sea CO2 concentration gradient, which further encourages uptake. Finally, wet deposition allows raindrops to absorb CO2 from the air and deliver it directly into the ocean upon contact. Each mechanism contributes uniquely to the increase in ocean carbon uptake across different regions, depending on rainfall intensity and wind conditions.
Ho and his team found that the impact of these mechanisms varies significantly by region. In tropical zones with high rainfall and calm winds, turbulence and dilution have substantial effects on CO2 absorption. Wet deposition also plays a large role in high-precipitation areas like storm tracks and the Southern Ocean, expanding the geographical influence of rain on ocean carbon uptake beyond tropical latitudes. This broad impact came as a surprise to the researchers, as they initially expected rainfall to increase carbon uptake only in the tropics.
Hugo Bellenger, a climate modeler at the French National Centre for Scientific Research (CNRS), developed a model to track rain-induced salinity changes on the ocean’s surface. He noted that while models have historically focused on temperature changes, salinity shifts from rain have long been overlooked, despite their significance. This new modeling approach helped pinpoint where and how rain amplifies CO2 absorption, highlighting the strong influence in regions with both frequent and intense rainfall.
The study underscores the role of precipitation as a critical but previously unmeasured variable in the global carbon cycle. As Earth’s climate warms, shifts in precipitation patterns are expected to intensify rainfall over oceans, likely amplifying rain’s effect on ocean carbon uptake. Already, researchers observed a gradual increase in this effect between 2008 and 2018, driven by changing rainfall patterns linked to climate change. The implications are profound for carbon-cycle models, which must now incorporate rainfall’s contribution to accurately estimate the ocean’s carbon sink potential.
Tatiana Ilyina, a carbon cycle modeler at Universität Hamburg, emphasizes that this research should drive a rethinking of global carbon models. She argues that understanding rain’s role is crucial to improve the accuracy of these models, which have traditionally struggled with capturing the nuances of precipitation’s influence on carbon dynamics. This study not only advances insights into ocean carbon uptake but also highlights the intricate ways climate change affects carbon cycling between the ocean and atmosphere.
www.eos.org/articles/rainfall-makes-the-ocean-a-greater-carbon-sink