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Oceans Are Losing Their Green as Global Heating Weakens Earth’s Carbon Shield

The world’s oceans are turning less green as global temperatures rise — a signal that the planet’s ability to absorb carbon dioxide may be faltering, according to new research.

The colour shift is being driven by a steady decline in phytoplankton, the microscopic organisms that form the base of the marine food web and power nearly half of Earth's biological productivity.

A comprehensive 22-year study, analysing daily chlorophyll data from satellites and ocean monitoring vessels, found that between 2001 and 2023, ocean greenness decreased by around 0.35 micrograms per cubic metre each year. The decline was twice as pronounced in coastal waters and four times greater near river estuaries.

Researchers linked the fading hues to a loss of ecological function, estimating that the oceans' carbon sequestration capacity has dropped by 0.088% annually — roughly 32 million tonnes less CO₂ absorbed every year. "The weakening of surface phytoplankton's carbon capture capacity poses serious implications for the global carbon cycle," said Di Long of Tsinghua University, a co-author of the study.

The findings point to ocean stratification as a major culprit. As surface waters warm, the temperature gap with the colder depths widens, hindering the upward movement of nutrients essential for phytoplankton growth.

"This is the first robust evidence showing that ocean greenness is declining, indicating reduced marine productivity — another clear warning of the consequences of fossil fuel-driven global heating," said Michael Mann from the University of Pennsylvania.

The study overturns earlier claims that algal blooms were expanding, suggesting those assessments were too limited in scope. While local factors such as agricultural runoff and pollution may alter conditions regionally, the researchers concluded that a "significant global decline" in phytoplankton is evident across tropical and mid-latitude seas.

Scientists warn the loss of marine greenness could disrupt oceanic food chains and oxygen production, while weakening one of Earth's key natural carbon sinks. The authors urge governments to strengthen coastal management and curb agricultural and industrial pollution — but stress that the ultimate challenge lies in addressing global warming itself.

"With the oceans losing strength as a carbon absorber, the task of cutting emissions becomes even harder," said Di Long. "This finding adds urgency to the need for stronger, faster action beyond what the Paris Agreement envisions."