Rain Helps the Ocean Absorb 13.6% More Carbon Than Wind Alone, Columbia Study Finds
Carbon budgets may undercount ocean uptake by roughly 5% to 15% because they miss the thin, freshened layers that rainfall creates at the sea surface.
Global carbon budgets may be underestimating how much carbon dioxide the ocean absorbs by roughly 5% to 15%, because current estimates do not fully account for the effect of rain on the sea surface. A new analysis from researchers at Columbia's Lamont-Doherty Earth Observatory estimates that rain increases global ocean carbon uptake by 13.6% compared with calculations based on wind-driven exchange alone.
"When we do global carbon accounting, we need to account for rain," said Carson Witte, a postdoctoral research scientist at Lamont-Doherty, part of the Columbia Climate School. "It's a big enough effect that getting it wrong biases our ocean carbon budgeting."
Rain boosts CO2 uptake in several ways. It dilutes the surface water, making it less salty and shifting its chemistry in ways that favor absorbing more CO2. It carries dissolved CO2 down from the air, a process called wet deposition. And it creates turbulence that speeds gas exchange between ocean and atmosphere. In the analysis, dilution accounts for the largest share, about 8% relative to wind-driven exchange alone. Wet deposition adds 3.8%. Turbulence contributes little by itself but amplifies dilution when the two happen together.
To reach the 13.6% figure, the team used a model called RIM-3, which simulates how rainfall changes sea-surface salinity over time, tracking recent rain and the evolution of the freshened layer. The model has been validated with satellite salinity observations. The researchers applied it to rainfall and wind data from the year 2000 for ocean regions between 60 degrees south and 60 degrees north.
A simpler method run on the same data produced 8.4%, close to the 7.7% upper estimate from a 2024 study, suggesting that simpler approaches underestimate rain's role by failing to capture how rain dilutes surface water. The main calculation also leaves out rain's cooling effect, which the team estimates could add up to about 2 percentage points more. Rain matters most in the tropical Indian Ocean, where wind-driven CO2 uptake is comparatively small, and across the tropics it offsets some of the CO2 the ocean would otherwise release.
Measurements are part of the problem. The freshened layers are typically 1 to 3 meters deep and can last for hours near the surface unless winds mix them downward. Nearly 95% of the measurements in SOCAT, a leading ocean carbon database, are taken at 5 meters, below the zone where rain's influence is strongest. The researchers estimate current observations capture only about 30% of rain's combined effects on CO2 exchange.
The authors caution that the 5% to 15% range is a general guide rather than a precise correction. It reflects the 2024 research and other uncertainties, especially how rain dilutes surface water, which cannot yet be quantified with statistical confidence. The 13.6% figure compares the new model with wind-driven exchange alone, while the 5% to 15% estimate accounts for rain effects already captured in existing measurements.
Coauthors Christopher Zappa, a Lamont research professor and project lead, and Wade McGillis of the University of Notre Dame, who was at Lamont at the time, began studying rain's effect on gas exchange in 2003. Experiments at Biosphere 2 in Arizona in 2003 and 2004 gave early evidence, and later work in the western equatorial Pacific laid the groundwork for the new global estimate.
