- The ocean is losing its greenness, a new study has found: Global chlorophyll concentration, a proxy for phytoplankton biomass, declined over the past two decades, especially in coastal areas.
- Phytoplankton are the base of the marine food web, supporting fisheries and broader ecosystems, so their decline could have far-reaching implications, experts say.
- The phytoplankton decline could hurt coastal communities that live off the sea, and affect the ocean’s ability to act as a carbon sink, the authors say.
The consequences of global warming, caused mainly by burning fossil fuels, are varied and many. Now scientists have documented yet another one: The ocean is losing its “greenness.”
The ocean’s chlorophyll concentration, a proxy for phytoplankton biomass, declined over the past two decades, especially in coastal areas, a new study has found. Phytoplankton are plant-like organisms that are the base of the marine food web, supporting fisheries and broader ecosystems.
The findings in the study, published Oct. 17 in the journal Science Advances, have far-reaching implications, according to the authors, most of them based at Tsinghua University in Beijing.
“These changes will profoundly affect the magnitude and distribution of marine ecosystem functioning,” they wrote in the study’s conclusion.
Curtis Deutsch, a professor of geosciences at Princeton University in the U.S., who wasn’t involved in the study, echoed their concern.
“It’s not good,” Deutsch told Mongabay, speaking of the finding of reduced ocean greenness.
“[It] almost certainly means that there’s less production of new organic matter, of algae in the ocean,” he added, with the qualifier that the trend of phytoplankton loss was relatively slow and he didn’t expect a “sudden collapse.”
Deutsch said the findings weren’t shocking, as scientists have theorized that a loss of greenness could be occurring. That’s because rising sea surface temperatures increase stratification — that is, they increase the difference in the density of surface water and deep water — which disrupts the upwelling of the nutrients that phytoplankton feed on.
And yet the empirical data on ocean greenness has been inconclusive, with mixed findings in previous research; a 2023 study in the journal Nature, which worked with fewer sources of data than the new study, found an overall increase in ocean greenness at low latitudes. Still, Deutsch said the new study shows a pretty robust signal.
Chlorophyll A is the green pigment in phytoplankton that allows the organisms to convert sunlight into energy. But direct, on-site measurements of chlorophyll concentration aren’t anywhere near comprehensive spatially. To complement them, the research team also used satellite observations and model-based environmental data, including, for example, on salinity and sea surface pressure. They put these data into a model they developed that uses artificial intelligence.
The study addressed only low- to mid-latitude waters because high-latitude waters, nearer the poles, are less productive and the satellite data for them aren’t as good, according to Di Long, a professor at Tsinghua’s department of hydraulic engineering and the study’s supervising author.
At the low and middle latitudes, chlorophyll concentrations fell at an average rate of about 0.00035 milligrams per cubic meter per year between 2001 and 2023, with coastal regions averaging a loss rate of about double that. Many regions did experience an increase in chlorophyll concentrations, but the overall trend was clear.
“The low- to mid-latitude oceans are becoming progressively less green,” the authors wrote.
The authors also found a “substantial reduction” in the overall frequency of high-chlorophyll events, better known as algal blooms. Other studies have in fact found an increase in harmful blooms, mostly in certain regional or local waters. Such events can be caused by agricultural runoff or other human pollution; some can generate toxins and lead to dead zones. Yet not all blooms are necessarily bad; they can be a natural part of marine ecosystems, leading to a feeding frenzy further up the food chain.
Deutsch expressed less confidence about the study’s algal bloom findings than the long-term global chlorophyll averages, given that blooms are rare events that are statistically harder to track. Long agreed that the blooms’ rareness does make tracking them with satellite data “challenging,” but said his team’s method was “validated against existing studies and field data.”
The study also established a causal link between sea surface temperature and both chlorophyll concentrations and algal blooms; blooms were found to be especially sensitive to temperature changes, Long said.
The study’s findings don’t bode well for people who rely on the ocean’s productivity, especially coastal communities that live off the sea, Long said.
“It could have a very large impact [on] the fishery industry,” he told Mongabay.
Long pointed to the west coast of Africa as a place of concern, where the loss of greenness is happening at above-average rates. Deutsch also singled out that area and said it was concerning.
“Regions like that are going to be far more vulnerable for all kinds of reasons,” Deutsch said.
In addition to disrupting food webs, the loss of greenness also could reduce the ocean’s capacity to act as a carbon sink, according to the study authors. To date, the ocean has absorbed at least a quarter of human-caused carbon dioxide emissions, acting as a buffer against climate change. One reason the ocean has this capacity is that phytoplankton turns carbon into organic matter, some of which sinks into the deep sea, away from the atmosphere.
While the study authors said their findings show that the ocean’s ability to act as a carbon sponge is declining, Deutsch said more research will be required to be sure, because the progression from decline of phytoplankton to reduced ocean carbon storage has “steps in between.”
Long said the possible effect on the carbon sink should be taken seriously.
“I think it can inform the researchers and policymakers in this area to really consider the cascading factor of the global warming and adopt more reasonable and more informed policies [on] carbon emission reduction,” he said.
Banner image: An algal bloom in the Baltic Sea on July 20, 2019. Image by European Space Agency via Flickr (CC BY-SA 2.0).
Record North Atlantic heat sees phytoplankton decline, fish shift to Arctic
Citations:
Hong, Z., Long, D., Shan, K., Zhang, J., Woolway, R. I., Liu, M., … Fang, H. (2025). Declining ocean greenness and phytoplankton blooms in low to mid-latitudes under a warming climate. Science Advances, 11(42). doi:10.1126/sciadv.adx4857
Cael, B. B., Bisson, K., Boss, E., Dutkiewicz, S., & Henson, S. (2022). Global climate-change trends detected in indicators of ocean ecology. Nature, 619, 551-554. doi:10.1038/s41586-023-06321-z
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