Rain May Increase Ocean Carbon Uptake More Than We Thought

Global carbon budgets, which track carbon’s movement among the atmosphere, ocean and land, may underestimate how much carbon the ocean absorbs by roughly 5% to 15% because current estimates do not fully account for rain’s effects on the ocean surface.

A new study published in Nature Geoscience estimates that rain increases global ocean carbon uptake by 13.6% compared with calculations based on wind-driven exchange alone. Rain makes the surface water less salty, changing its chemistry in ways that favor greater CO2 uptake.

“When we do global carbon accounting, we need to account for rain,” said Carson Witte, a postdoctoral research scientist at Lamont-Doherty Earth Observatory, part of the Columbia Climate School. “It’s a big enough effect that getting it wrong biases our ocean carbon budgeting.”

Rain helps the ocean absorb more CO2 in several ways. It dilutes the surface water, carries dissolved CO2 into the ocean, a process called wet deposition, and creates turbulence that enhances gas exchange between the ocean and atmosphere. In the new analysis, dilution accounts for the largest share of the increase, about 8% relative to wind-driven exchange alone. Wet deposition adds 3.8%. Turbulence contributes little on its own, but it boosts dilution when the two processes occur together.

To arrive at the 13.6% estimate, the researchers used a model called RIM-3, which simulates how rainfall changes sea-surface salinity over time, accounting for recent rainfall and the evolution of the freshened surface layer. The model has been validated with satellite salinity observations. They applied it to rainfall and wind data from the year 2000 for ocean regions between 60°S and 60°N.

For comparison, the researchers ran the same data through a simpler method that produced a smaller estimate of 8.4%, close to the 7.7% upper estimate from a 2024 study. The comparison suggests that simpler approaches may underestimate rain’s contribution to ocean carbon uptake by failing to fully capture how rain dilutes surface water.

The study’s main 13.6% calculation also excludes rain’s cooling effect, which could further increase ocean carbon uptake. In a separate estimate, the researchers calculated that cooling could add up to about 2 percentage points, which they consider a likely upper bound.

Rain has the largest relative effect on ocean carbon exchange in the tropical Indian Ocean, where wind-driven CO2 uptake is comparatively small. Across the tropics, rain offsets some of the 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.

Witte said existing measurements of CO2 in the surface ocean often miss the fresher layers rain creates, making it hard to determine how much rain increases CO2 uptake. These layers are typically 1 to 3 meters deep and can last for hours near the surface unless winds mix them downward. Nearly 95% of measurements in SOCAT, a leading ocean carbon database, are taken at 5 meters—below the main zone of rain’s influence. The researchers estimate that current ocean carbon observations capture about 30% of rain’s combined effects on CO2 exchange at the ocean surface, including dilution and turbulence.

The authors describe the 5% to 15% range as a general guide, not a precise correction. The range reflects findings from the 2024 research and other uncertainties, particularly how rain dilutes surface water. Researchers cannot yet quantify that dilution effect with statistical confidence. The 13.6% figure compares the new model’s results with wind-driven exchange alone, while the 5% to 15% estimate accounts for the rain effects already captured in existing ocean carbon measurements.

Coauthors Christopher Zappa, a Lamont research professor and project lead, and University of Notre Dame’s Wade McGillis, who was at Lamont at the time, began investigating how rain affects the exchange of gases between the ocean and atmosphere in 2003. Experiments at Biosphere 2 in Arizona in 2003 and 2004 provided early evidence that rain-induced dilution could increase ocean carbon uptake. Subsequent research by Zappa and McGillis demonstrated rain’s influence on CO2 exchange in the western equatorial Pacific, laying the groundwork for the new global analysis.

Future changes in rainfall, wind, ocean CO2 levels, ocean temperature and salinity could increase or decrease rain’s overall contribution to ocean carbon uptake, depending on how these changes overlap geographically, said Zappa. The effect of dilution itself is less ambiguous: “The dilution due to rain itself will always work to increase the uptake by the ocean,” he said.

Quantifying rain’s contribution requires a better understanding of how rain and wind interact to mix surface waters and how the stratified layers left by precipitation evolve. “We still have much to learn about the mixing and evolution of the near-surface stratified layers that remain following precipitation events as well as the longer-term ventilation of the surface waters,” Zappa said.

More measurements of near-surface salinity and rainfall, especially in equatorial oceans, along with direct measurements of air-sea CO2 exchange during and after rain, could help fill these knowledge gaps.