Rain may raise ocean carbon uptake by 13.6%, study finds

That comparison suggests simpler approaches may underestimate rain’s contribution because they miss how thoroughly rain dilutes surface water. The main 13.6% calculation also excludes rain’s cooling effect, which could increase uptake further.

In a separate estimate, the researchers put cooling’s potential contribution at up to about 2 percentage points. They consider that value a likely upper bound rather than a central estimate.

Where does rain have the greatest effect?

The model found rain’s largest relative effect in the tropical Indian Ocean, where wind-driven CO₂ uptake is comparatively small. Across the tropics, rain offsets some CO₂ the ocean would otherwise release.

Rain also shifts ocean CO₂ exchange toward greater uptake, except in a small part of the eastern tropical Pacific.

Why do measurements miss part of the effect?

Rain creates fresher surface layers typically 1 to 3 meters, or 3 to 10 feet, deep. They can persist for hours near the surface unless winds mix them downward.

Nearly 95% of measurements in SOCAT, a leading ocean carbon database, come from 5 meters, or 16 feet, below the main zone of rain’s influence. Researchers estimate that current ocean carbon observations capture about 30% of rain’s combined effects on surface CO₂ exchange, including dilution and turbulence.

How certain is the 13.6% estimate?

The authors present 5% to 15% as a general guide for the possible budget underestimation, not a precise correction. The range reflects 2024 findings and other uncertainties, particularly uncertainty over how rain dilutes surface water.

Researchers cannot yet quantify that dilution effect with statistical confidence. The 13.6% figure compares RIM-3 with wind-driven exchange alone, while the 5% to 15% range accounts for rain effects already captured in existing ocean carbon measurements.

How did the work develop?

Project lead Christopher Zappa, a Lamont research professor, and University of Notre Dame’s Wade McGillis, who was at Lamont at the time, began studying ocean-atmosphere gas exchange in 2003. Experiments at Biosphere 2 in Arizona in 2003 and 2004 provided early evidence that rain-induced dilution could raise ocean carbon uptake.

Subsequent work by Zappa and McGillis demonstrated rain’s influence on CO₂ exchange in the western equatorial Pacific. That research laid the groundwork for the new global analysis.

What could change the estimate?

Future shifts in rainfall, wind, ocean CO₂ levels, temperature and salinity could raise or lower rain’s overall contribution, Zappa said. The outcome depends on how those changes overlap geographically.

“The dilution due to rain itself will always work to increase the uptake by the ocean,” Zappa said.

Quantifying rain’s contribution requires better knowledge of how rain and wind mix surface waters and how the layered, fresher water left by precipitation evolves. Zappa said researchers also need to understand longer-term ventilation, meaning the movement of deeper water toward the surface.

More measurements of near-surface salinity and rainfall, especially in equatorial oceans, could narrow those gaps. Direct measurements of air-sea CO₂ exchange during and after rain would also help determine how much the ocean absorbs.

Carson R. Witte and colleagues published “Global carbon dioxide flux into the surface ocean intensified by rain-induced dilution” in Nature Geoscience in 2026. The DOI is 10.1038/s41561-026-02112-z.