The world's oceans are losing their greenery due to global warming, according to a study that suggests our planet's ability to absorb carbon dioxide may be weakening.
The changing palette of the seas is caused by the decline phytoplanktontiny sea creatures that account for almost half of the biosphere's productivity.
The findings, which also have alarming implications for oxygen levels and food chains, are based on a pioneering study of daily chlorophyll concentrations in low- and mid-latitude oceans from 2001 to 2023.
Chlorophyll 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. This is one of the fundamental blocks of life on Earth.
Using deep learning algorithms, the authors of the new paper collected data from satellites and observing ships to estimate the changing hue of the oceans. They found a significant decline in greenness—about 0.35 micrograms per cubic meter each year—over the more than twenty-year study period. The trend was twice as high in coastal regions and more than four times higher in estuaries.
They attribute this to a decline in ocean ecological functioning, finding an annual decline in carbon capture capacity of 0.088%, equivalent to 32 million tonnes. “The decline in the ability of surface phytoplankton to sequester carbon has profound implications for the carbon cycle,” said one of the authors, Di Long of Tsinghua University in Beijing.
The new paper says this change was likely caused by rising temperatures associated with climate change.
Warming of the upper ocean near the surface has increased the temperature difference with the colder depths, which is thought to block the vertical transport of nutrients on which phytoplankton depend.
This supports theories about the influence of global heating on ocean stratification. Co-author Michael Mann from the University of Pennsylvania said: “This is the first study to convincingly demonstrate that, yes, we can confidently conclude that we are seeing a decrease in ocean greenness, indicating a decline in marine productivity, which poses another threat to humanity associated with the burning of fossil fuels and human-caused planetary warming.”
The new study contradicts several previous studies that suggested an increase in algae blooms in the oceans.
The latest article states that previous studies have been less comprehensive. It acknowledges that the situation varies from region to region and may be influenced by runoff from agriculture and other human activities, but concludes that the broader picture of “significant declines” in phytoplankton is clear at low and mid-latitudes.
“These changes will have profound impacts on the extent and distribution of marine ecosystem functioning,” the report says.
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The authors say policymakers should analyze the marine ecological environment in coastal areas and formulate countermeasures, including better management of agricultural fertilizers, wastewater discharges, deforestation and water pollution.
But the bigger challenge will be tackling the climate crisis as the world's largest natural carbon sink declines. The Paris climate agreement, already behind schedule, may not be enough.
“Based on the findings, we have concerns about future reductions in global emissions. The decline in the ocean's ability to sequester carbon means we may face greater emissions reduction pressure than expected,” Di Long said.
