The world’s oceans are losing their greenness owing to global heating, according to a study that suggests our planet’s capacity to absorb carbon dioxide could be weakening.
The change in the palette of the seas is caused by a decline of phytoplankton, the tiny marine creatures that are responsible for nearly half of the biosphere’s productivity.
The findings, which also have alarming implications for oxygen levels and food chains, are based on a groundbreaking study of daily chlorophyll concentrations in low- to mid-latitude oceans from 2001 to 2023.
Chlorophyl is a green pigment responsible for photosynthesis, the process by which plants, algae and phytoplankton convert sunlight, water and carbon dioxide into oxygen and glucose. It is one of the foundation blocks of life on Earth.
Using deep-learning algorithms, the authors of the new paper compiled data from satellites and monitoring ships to assess the change in the oceans’ hue. They found a significant decline of greenness – about 0.35 micrograms per cubic metre each year – over the more than two decade period of the study. The trend was twice as high in coastal regions and more than four times greater near river estuaries.
They associate this with a reduction in the ecological functioning of the ocean, finding a 0.088% annual decrease in 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 one of the authors, Di Long of Tsinghua University in Beijing.
The new paper says the change was probably caused by rising temperatures associated with climate change.
The heating of the upper strata of ocean near the surface has widened the temperature difference with the colder depths, which is thought to be blocking the vertical transport of the nutrients on which the phytoplankton depend.
This confirms theories about the impact of global heating on ocean stratification. Collaborating author Michael Mann, of the University of Pennsylvania, 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 new research contradicts several previous studies that suggested algal blooms were increasing in the oceans.
The latest paper says earlier studies were less comprehensive. It acknowledges that the situation differs from region to region and can be affected by runoffs from agriculture and other human activity, but concludes the broader picture of “significant decline” of phytoplankton is clear across the low and mid latitudes.
“These changes will profoundly affect the magnitude and distribution of marine ecosystem functioning,” it says.
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The authors said policymakers should analyse the marine ecological environment in coastal area and formulate countermeasures, including more careful management of agricultural fertiliser, sewage discharge, deforestation and water pollution.
But the bigger challenge will be tackling the climate crisis while the world’s biggest natural carbon absorber is losing strength. The Paris climate agreement, which is already behind schedule, may not be enough.
“Based on the findings, we have concerns about future global emissions reduction. The decline in ocean carbon sequestration capacity means that we may face greater emissions reduction pressure than expected,” said Di Long.
