Climate Change Stripping Oceans of Their Green Hue
Deep-learning analysis shows phytoplankton decline weakening oceans’ carbon absorption capacity by 32 million tons annually
The world’s oceans are steadily losing their characteristic greenness as a result of global heating, according to a new study that warns the planet’s ability to absorb carbon dioxide may be weakening.
The colour shift, researchers say, stems from a sharp decline in phytoplankton — microscopic marine organisms that drive nearly half of Earth’s biological productivity. Using deep-learning algorithms, scientists compiled data from satellites and monitoring ships to track ocean hue changes between 2001 and 2023. They found a significant decline in greenness of about 0.35 micrograms per cubic metre each year, with the trend twice as high in coastal areas and over four times greater near river estuaries.
This decline corresponds with a 0.088% annual drop in the oceans’ carbon sequestration capacity — equivalent to 32 million tons. “The decline in surface phytoplankton’s carbon sequestration capacity has profound implications for the carbon cycle,” said Di Long of Tsinghua University in Beijing, one of the study’s authors.
Researchers attribute the change primarily to rising ocean temperatures linked to climate change. Warming in the upper ocean layers has intensified the temperature difference with the colder depths, hindering the vertical flow of nutrients vital for phytoplankton growth. This finding supports long-standing theories about the impact of global heating on ocean stratification.
Michael Mann of the University of Pennsylvania, a collaborating author, said: “This is the first study to robustly demonstrate that, yes, we can confidently conclude that we are seeing a decrease in the greenness of the ocean, indicating a lowering of marine productivity, constituting yet another threat to humanity associated with fossil fuel burning and human-caused planetary warming.”
The study contradicts earlier research suggesting an increase in oceanic algal blooms. Authors say previous studies were less comprehensive, though they acknowledge regional variations caused by factors such as agricultural runoff and human activity. Despite these differences, the paper concludes that the broader trend of “significant decline” in phytoplankton is evident across low- and mid-latitude oceans.
“These changes will profoundly affect the magnitude and distribution of marine ecosystem functioning,” the study warns.