Tag: tropical Indian Ocean

Rain may increase ocean carbon uptake more than we thought, a November 11, 2026, article by Rebecca Fowler from Columbia University’s Climate School (State of the Planet and Lamont- Doherty Earth Observatory).

It reports on a new Nature Geoscience study (led by Carson Witte of Lamont-Doherty, with coauthors Christopher Zappa of Lamont and Wade McGillis of the University of Notre Dame).

The study finds that rain boosts global ocean carbon dioxide (CO₂) uptake by about 13.6% relative to wind-driven gas exchange alone. Global carbon budgets may therefore underestimate the ocean’s carbon absorption by roughly 5– 15%, because they do not fully account for rain’s effects.

Rain acts in three main ways:

• Dilution of surface seawater (the largest factor, ~8% of the relative increase): Fresh rainwater lowers salinity in a shallow surface layer (typically 1- 3 m deep). This alters carbonate chemistry and favors greater CO₂ absorption from the atmosphere. The freshened “lens” can persist for hours unless mixed downward by wind.
• Wet deposition (~3.8%): Raindrops absorb CO₂ as they fall and deliver it directly into the ocean.
• Turbulence: Raindrop impacts enhance gas exchange at the air- sea interface. Alone this contributes little, but it interacts with and amplifies the dilution effect.

The researchers used a model (RIM-3) that tracks how rainfall changes sea-surface salinity over time (validated against satellite salinity data).

They applied it to rainfall and wind data from the year 2000 for ocean areas between 60°S and 60°N. A simpler method on the same data gave a lower estimate (~8.4%), close to the upper end of a 2024 study that put the rain boost at 5- 7% (or +0.14- 0.19 PgC yr⁻¹).

The newer work suggests simpler approaches understate the dilution contribution. Rain’s cooling effect (not fully included in the main 13.6% figure) could add up to ~2 more percentage points.

Key implications and caveats

• The relative effect is strongest in places like the tropical Indian Ocean (where wind- driven uptake is smaller). Across the tropics, rain can offset some ocean CO₂ outgassing.
• Most existing ocean CO₂ measurements (e.g., ~95% in the SOCAT database) are taken at ~5 m depth, below the main rain- influenced layer, so they capture only about 30% of rain’s combined surface effects.
• The 5– 15% underestimation range is a rough guide reflecting earlier work plus uncertainties (especially in quantifying dilution). Future shifts in rainfall patterns, winds, ocean CO₂, temperature, or salinity could change rain’s net contribution, though dilution itself should consistently favor greater uptake.
• The authors note the need for better near- surface salinity and rainfall observations (especially in equatorial regions) and direct air- sea CO₂ flux measurements during and after rain events to refine these estimates.

This builds on earlier work (including Biosphere 2 experiments and regional studies) showing rain’s influence on gas exchange. The ocean already absorbs roughly a quarter of human CO₂ emissions; properly accounting for rain improves the accuracy of global carbon budgets.

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

Rain affects air- sea CO₂ flux through three mechanisms:

1. Wet deposition (direct delivery of CO₂ dissolved in raindrops).
2. Turbulence enhancement from raindrop impacts.
3. Changes in temperature and salinity of the ocean’s top microlayer (primarily freshening and dilution).

Using the most realistic available model of rain- induced dilution (RIM- 3) applied to high- resolution satellite datasets, the study finds a total rain-induced increase in ocean carbon uptake of 13.6% relative to wind- driven exchange alone.

Dilution accounts for about two- thirds of the additional uptake via freshening-induced repartitioning of the carbonate system. This roughly doubles the previous upper- bound estimate (from a 2024 study).

Current observational coverage of surface- ocean carbon captures only ~30% of the dilution effect, and global carbon budget estimates are likely underestimating the ocean carbon sink by 10 ± 5% by omitting rain effects.

Key findings and methods

• Model and data: RIM- 3 (Rain Impact Model 3) tracks the time evolution of shallow (1- 3 m), buoyant fresh layers that can persist for hours after rainfall. It was validated against Aquarius/SMAP satellite sea -surface salinity anomalies and forced with high- resolution precipitation (NOAA CMORPH), winds (ERA5), and other datasets for the year 2000 (primarily 60°S- 60°N).
• Comparison with a simpler instantaneous dilution model (S20) and the 2024 Parc et al. analysis shows RIM- 3 roughly doubles the dilution contribution, making it the dominant rain effect.
• Wet deposition contributes ~3.8%. Turbulence alone is small but amplifies the dilution effect when co- located.
• Regional importance is highest in the tropics (especially the tropical Indian Ocean and Intertropical Convergence Zone), where high rainfall, low winds, and weak CO₂ gradients make rain effects relatively large. Rain generally shifts net flux toward greater ocean uptake (except in a small eastern tropical Pacific region).
• Rain- induced cooling is discussed as an additional (secondary) contributor that could further increase uptake but was not fully included in the main 13.6% figure.

Existing surface- ocean CO₂ measurements (mostly taken at ≥5 m depth) miss much of the rain- affected surface layer, so budgets based on them under-represent the true sink.

The authors emphasize the need for better observations of near- surface salinity, temperature, and CO₂ during and after rain events, plus improved modeling of how these fresh layers evolve and mix.

This is the study referenced in the Columbia Climate School press release (“Rain May Increase Ocean Carbon Uptake More Than We Thought”).

Lead authors include researchers from Lamont- Doherty Earth Observatory (Columbia) and collaborators (including Wade McGillis).

The paper is open access; the full PDF is available via the Nature link.

Journal: Nature Geoscience, volume 19, pages 1333–1338 (2026)

DOI: 10.1038/s41561-026-02112-z (open access)

Provided: Columbia University

Published: 23 September 2026

Authors: Carson R. Witte,
Christopher J. Zappa &
Wade R. McGillis

Abstract

Rain impacts the flux of carbon dioxide (CO₂) across the air–sea interface by three mechanisms: wet deposition, enhancement of surface gas exchange through turbulence and changes in the temperature and salinity of the ocean’s top microlayer. Understanding of these effects has progressed to enable improved estimations for the impact of rain on global ocean carbon uptake, but substantial epistemic uncertainty in estimates of rain-induced salinity dilution limits further constraint on the ocean carbon sink. Here we present a global analysis of rain effects on CO₂ exchange employing the most realistic available model of rain-induced dilution for high-resolution satellite datasets, yielding a total rain-induced increase in ocean uptake of 13.6% relative to a wind-only effect. Dilution is found to be responsible for about two-thirds of the total additional uptake due to freshening-induced repartitioning of the carbonate system. This doubles the previously reported upper bound on rain-enhanced ocean carbon uptake, highlighting the importance of saltwater dilution by rain and the corresponding need to improve quantitative understanding of the surface ocean during and after rain. We estimate that observational coverage of carbon in the surface ocean implicitly captures 30% of the dilution effect and find that current global carbon budget estimates are probably underestimating the ocean carbon sink by 10 ± 5% due to their exclusion of rain effects.