The ocean soaked up an average of 2.8 billion metric tons of carbon a year from 1990 to 2019, about 35 percent of fossil fuel emissions, and that uptake kept climbing, according to a study led by Scripps Institution of Oceanography at UC San Diego. Over the same 30 years, the net amount absorbed by forests, soils and other land ecosystems did not grow at all.
The paper, “Atmospheric O2 reveals a strengthening ocean carbon sink and a stagnant net land sink since 1990,” was published in Science on November 12, 2026. Scripps project scientist Eric Morgan is lead author, with co-authors at the University of Exeter in England, the Max Planck Institute for Biogeochemistry in Germany, Japan’s National Institute for Environmental Studies and the US National Oceanic and Atmospheric Administration (NOAA).
How oxygen in air flasks separates land from sea
The method rests on a neat bit of chemistry. Burning coal, oil and gas adds carbon dioxide to the air and removes oxygen. Plants on land pull in carbon dioxide and give off oxygen, so the land sink leaves an oxygen fingerprint. The ocean takes up carbon dioxide mainly through chemical reactions in seawater that do not touch oxygen at all.
So if you know how much fossil fuel was burned and you track both gases precisely enough, the oxygen record tells you how the carbon split between land and sea. The Scripps O2 Program, founded by geoscientist Ralph Keeling in 1989, collects air in glass flasks at 10 sites strung along the Pacific from the high Arctic to the South Pole and ships them back to San Diego for measurement.
That record is a companion to the Keeling Curve, the carbon dioxide series that Ralph Keeling’s father, Charles David Keeling, began at Mauna Loa, Hawaii, in 1958.
What 30 years of measurements showed
- Ocean: about 2.8 billion metric tons of carbon a year, roughly 35 percent of fossil emissions, growing by about 0.63 billion metric tons per decade.
- Land: about 1 billion metric tons a year, roughly 13 percent of fossil emissions, with big swings and no clear long-term rise. It peaked in the decade centered on 2009 and has mostly fallen since.
Combined, the two sinks took up about 48 percent of fossil emissions in the study period, which matches the long-standing rule of thumb that nature has absorbed about half.
“It was very surprising to see how the net land sink has not kept pace with rising CO2 levels,” Morgan said in the Scripps release. “Our data underscore that the land sink may be pretty fragile.”
That clashes with many land computer models, which expect plants to grow more as carbon dioxide and nitrogen rise. For those models to fit, emissions from deforestation would have had to climb, but current estimates show them falling. On the ocean side, the University of Exeter says the sink strengthened substantially more than current model simulations show.
Limits of the record and why the ocean may not keep up
Neither sink can be observed directly, and the oxygen method depends on accurate fossil fuel figures and measurements precise to parts per million. The study covers net land uptake, so it cannot say which forests or regions gained or lost. A stronger ocean sink also has a cost: dissolved carbon dioxide lowers seawater pH, a process called ocean acidification. Warmer water holds less carbon dioxide, and warm surface layers mix less into the deep ocean, where carbon can stay locked away for hundreds of years.
For more on Earth’s atmosphere, read our explainer on noctilucent clouds, which form about 50 miles up, or browse the Science section.
“It’s not easy to keep these kinds of measurements going year after year, but the payoff for improving our understanding of the carbon cycle is huge,” said Keeling, a co-author. His flask network, now in its 37th year, keeps sampling.
Further reading
- Wikipedia: Keeling Curve
- UC San Diego: New study shows ocean taking up growing share of carbon emissions
- University of Exeter: Ocean carbon sink strengthens while land carbon sink stalls
Image: frank mckenna (frankiefoto), CC0 via Wikimedia Commons
This article was produced with AI assistance and reviewed by our Editorial Desk. How we work.
