The physical story begins at a scale almost too small to imagine. The exchange of carbon dioxide between air and water is controlled by a concentration gradient across the mass boundary layer, a film of water only microns thick at the very top of the sea. Temperature and salinity variations within this layer and in the upper metres beneath it act on the gas in two opposing but reinforcing ways. At the interface itself, cooler, fresher water holds more dissolved carbon dioxide, raising solubility and pulling gas out of the air. At the base of the boundary layer, the same variations reshuffle the carbonate system—the chemical equilibrium among dissolved carbon dioxide, bicarbonate and carbonate ions—in a direction that further steepens the gradient. Scientists have already incorporated the temperature side of this story, the so-called cool-skin and diurnal warm-layer corrections, into global carbon budgets. Salinity, however, has remained the neglected sibling, because the haline skin is normally far too thin to drive meaningful carbonate repartitioning. Rain is the exception that proves the rule.

Rain alters air–sea carbon exchange through three distinct mechanisms. First, raindrops hammering the surface generate turbulence that enhances gas transfer, pushing carbon dioxide in whichever direction the local gradient points. Second, raindrops themselves carry dissolved carbon dioxide, delivering a small but measurable direct input known as wet deposition. Third, and most elusive, rain freshens and cools the upper ocean, diluting the salt content of a buoyant layer one to three metres deep that can persist for hours after the storm has passed. That freshening lowers the fugacity of carbon dioxide at the surface and repartitions the carbonate system below, strengthening the ocean’s pull on atmospheric carbon. While the first two mechanisms have been satisfactorily parameterized, the third has been crippled by a paucity of observations: nearly all routine oceanographic measurements come from ships, Argo floats and moorings sampling at five metres or deeper, entirely below the zone where rain’s influence is strongest.

The scale mismatch is the crux of the problem. Satellite instruments can detect the salinity anomalies left by rain, but rain is extraordinarily patchy on sub-kilometre scales, so even the finest satellite pixels—eight kilometres across—smear the signal. Earlier global analyses, notably a 2024 study by Parc and colleagues, attempted to capture the dilution effect using a one-dimensional prognostic model and an instantaneous empirical correction for satellite salinity, arriving at a combined rain enhancement of the ocean carbon sink of 5.3 to 7.7 percent, with wet deposition contributing more than half. The new study takes a different route. The authors selected the Rain Impact Model 3, or RIM-3, a physically grounded scheme that tracks the time evolution of freshwater lenses over the preceding twenty-four hours of precipitation history, and which has been validated against salinity retrievals from the Aquarius and SMAP satellites. Crucially, RIM-3 acknowledges what instantaneous models cannot: fresh layers linger long after the rain stops.

The results are striking. Running RIM-3 against high-resolution satellite rainfall data for the year 2000, the team found that dilution alone roughly doubles the effect estimated by the earlier work, making it the dominant term in the rain-driven carbon budget. In total, rain boosts global ocean carbon uptake by 13.6 percent relative to a wind-only calculation, against the wind-driven flux of 1.468 petagrams of carbon per year. Wet deposition contributes about 3.8 percent, consistent with both the previous study and an earlier independent estimate, while rain-induced turbulence adds a smaller, interacting share—the turbulence and dilution effects reinforce one another because they occur in the same places at the same times. When the authors ran the same analysis with the older instantaneous parameterization, the dilution effect came out at less than half the RIM-3 value, underscoring how sensitive the answer is to how faithfully a model represents the persistence of freshwater at the sea surface.

Regionally, the story is even more dramatic. Rain matters most where precipitation is heavy, winds are light and the carbon dioxide gradient between air and sea is weak—conditions that converge spectacularly in the Intertropical Convergence Zone of the tropical Pacific and, above all, in the tropical Indian Ocean. In the western equatorial Pacific, previous work has shown that accounting for rain can flip a region from a weak source of carbon dioxide to a strong sink over the course of a year. The new maps reveal zonal peaks in rain-driven flux centred on the tropical convergence belts and midlatitude storm tracks, with the southern midlatitude peak amplified by the breadth of the Indian Ocean. Outside a small wind-dominated patch of the eastern equatorial Pacific, rain always pushes the net flux toward greater uptake—either strengthening absorption where the ocean already takes up carbon or suppressing outgassing where it releases it. The authors even suggest that regions where rain effects are strongest could be promising candidates for ocean alkalinity enhancement, a proposed carbon removal technology.

Rain also cools the surface, a mechanism the main analysis does not explicitly include. Combining known temperature sensitivities with an observational relationship between rain rate and skin temperature yields a rough additional uptake of 0.027 petagrams of carbon per year, about 1.9 percent of the wind-driven flux. The authors judge this a likely upper bound, since the rain–temperature relationship probably saturates at high rain rates, though persistence of cooler temperatures after rainfall could push the other way. Either way, cooling appears to be a real but secondary player, less important than dilution and deposition, and the total rain effect on the ocean sink may ultimately reach around fifteen percent.

The implications for global carbon accounting are uncomfortable. Current Global Carbon Budget estimates do not explicitly account for rain effects, and the surface ocean measurements that feed them—about ninety-five percent of which are taken at five metres depth—capture only part of the story. Using the physics of RIM-3, the authors calculate that a hypothetical global network of continuous five-metre carbon dioxide measurements would implicitly capture about sixty-two percent of the rain-induced interfacial flux. But real observations are sparse in space and time relative to the episodic nature of rainfall, and an analysis of the SOCATv2025 database shows that today’s observing system captures only about thirty percent of the interfacial rain influence. Worryingly, the enormous growth in observational coverage in recent decades has not translated into better coverage of rain-affected conditions, and five-metre measurements capture especially little of the effect in the equatorial oceans where rain matters most.

Taken together, the authors conclude that current global carbon budget estimates are probably underestimating the ocean carbon sink by 10 ± 5 percent. That figure echoes earlier work showing that correcting for cool-skin and warm-layer effects brought flux estimates into line with independent measurements of ocean carbon inventory, and it suggests rain deserves the same rigorous treatment. The team is careful to note that the point is not to crown RIM-3 as the definitive model but to expose how poorly constrained rain dilution remains, and what that gap costs our understanding of the planet’s largest carbon sink. What is needed, they argue, is a coordinated observational campaign: widespread, systematic measurements of very near-surface salinity profiles and rain rates, with particular emphasis on the equatorial oceans, alongside process studies that directly measure carbon dioxide fluxes, salinity and carbon dioxide anomalies within the boundary layer during and after rain. Until then, a substantial slice of the ocean’s climate service may be hiding, quite literally, beneath a layer of fresh water thinner than a swimming pool is deep.

Subject of Research: The effect of rain-induced salinity dilution on global air–sea carbon dioxide flux and the ocean carbon sink

Article Title: Global carbon dioxide flux into the surface ocean intensified by rain-induced dilution

Article References: Global carbon dioxide flux into the surface ocean intensified by rain-induced dilution. (n.d.). https://doi.org/10.1038/s41561-026-02112-z

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02112-z

Keywords: ocean carbon sink, air-sea CO2 flux, rain-induced dilution, sea surface salinity, carbonate chemistry, wet deposition, gas transfer velocity, satellite remote sensing, global carbon budget, Intertropical Convergence Zone, mass boundary layer, Nature Geoscience