Published in Science on November 12, 2026, the study by Morgan et al. estimates the two sinks’ trends using more than 30 years of atmospheric oxygen and carbon dioxide measurements from the Scripps O₂ Program.

By measuring oxygen alongside carbon dioxide, researchers can separate ocean uptake from land uptake: fossil-fuel burning consumes oxygen, plant growth releases it, and ocean carbon uptake has no corresponding oxygen release.

The ocean absorbed an average of 2.8 billion tonnes of carbon annually—about 35% of fossil-fuel emissions—and its sink strengthened by roughly 0.63 billion tonnes per decade.

Land ecosystems absorbed around one billion tonnes of carbon annually, or about 13% of fossil-fuel emissions, but the net land sink peaked around 2009 and has generally declined since.

These results diverge from many carbon-cycle models: the ocean sink grew more than simulations suggest, while land uptake failed to rise as models often project when elevated carbon dioxide and nitrogen stimulate plant growth.

The findings raise concern that the land sink may be fragile, while the shift toward greater ocean uptake and lower land uptake could prompt a reassessment of the global carbon budget.

Distinguishing the sinks’ contributions also helps researchers identify the processes behind carbon uptake and estimate how much more carbon dioxide can be emitted while limiting warming to targets such as 1.5°C or 2°C.

The oxygen-based approach has limits: only about 10 laboratories routinely make these challenging measurements, calibration is difficult, warming-related oxygen release must be accounted for, and ocean variability requires averaging results over roughly 10-year periods.

Researchers say stronger estimates will require combining atmospheric oxygen measurements with ocean carbon and oxygen observations, computer models and satellite data, supported by sustained long-term monitoring.

A study led by scientists at UC San Diego’s Scripps Institution of Oceanography finds that from 1990 to 2019, the ocean’s uptake of fossil-fuel carbon grew substantially, while the net land sink showed no notable long-term growth.

Together, natural land and ocean sinks absorb roughly half of annual human-caused emissions, making accurate estimates crucial for climate projections and decisions about emissions pathways and the remaining carbon budget.

The ocean sink may weaken in coming decades because warmer water holds less carbon dioxide and mixes less efficiently into the deep ocean, while absorbed carbon dioxide also contributes to ocean acidification.