No matter how sunlight and storms seem to change the color of the ocean, those varying shades of blue and green are mainly a play of light on our eyes.
The actual color mainly depends on the composition of the water, which largely has to do with the tiny organisms that inhabit it. And that rarely changes very much.
Now, in a sign that climate change could be driving a profound change in marine ecosystems, scientists have, for the first time, detected significant changes in the color of oceans across the planet.
In a new scientific paper, researchers at MIT, the University of Maine, and elsewhere pieced together satellite data from the past two decades to detect the shift, a worrying development they said could be a harbinger of major changes in the marine food chain.
“This is the first time we can say there’s definitely a signal of change, and it’s likely due to human-caused climate change,” said Stephanie Dutkiewicz, a senior research scientist in MIT’s Department of Earth, Atmospheric, and Planetary Sciences, who was one of the paper’s authors.
She called the observations “frightening,” a possible sign, she worries, that delicate marine ecosystems “are being pushed out of their natural balance.”
The changing colors, in some cases so subtle that they’re not discernible to human eyes, likely indicate changes in ecosystems near the oceans’ surfaces.
The authors of the paper said it’s likely that what the satellite detected were changes in the abundance of tiny organisms known as phytoplankton, which make up the foundation of the marine food web.
Dutkiewicz said deeper blue, or cobalt-colored, waters typically reflect that relatively few organisms inhabit those regions; more emerald-colored waters likely indicate the presence of chlorophyll, a primary component of phytoplankton. The green pigment of chlorophyll enables those microscopic organisms to absorb sunlight, which they use to capture carbon dioxide from the atmosphere and to grow. Phytoplankton tend to thrive in warmer water, and as oceans absorb more carbon dioxide, it’s expected that the tiny organisms will become more prevalent.
The authors said the most pervasive of those color changes were observed in equatorial waters.
There could be other causes of the changes detected by the satellites, the scientists said, and it’s yet unclear — without more data and analysis — what the longer-term implications might be. They noted that possibilities include changes in oceans’ ability to store carbon, the viability of regional ecosystems, and shifts in fisheries.
“This work adds more evidence to show that human activity is causing changes in the ocean’s ecosystem,” said Carlos E. Del Castillo, chief of NASA’s Ocean Ecology Laboratory in Maryland. “Clearly, any changes in the ecology of the ocean over such short periods of time are very concerning.”
While the study found that the oceans, in large part, are becoming greener, the data from the satellite suggested that some parts of the world’s oceans may be getting bluer, suggesting lower levels of phytoplankton in some areas.
Barney Balch, a senior research scientist at the Bigelow Laboratory for Ocean Sciences in Maine, said the study corroborates more than a quarter century of his surveys of the Gulf of Maine, which have shown that the waters off New England are also changing color. But the Gulf of Maine has instead been yellowing, Balch said, as a rise in the frequency of heavy storms has increased the amount of runoff from the region’s sediment-laden rivers and other waters.
That increased amount of silt has likely reduced the amount of light in the Gulf of Maine, which, combined with changes in ocean currents, may be making it harder for phytoplankton to thrive. Indeed, Balch’s research has shown a decline in phytoplankton productivity in the Gulf of Maine by about 65 percent between 2001 and 2018, despite the fact that it is considered one of the fastest-warming bodies of water on the planet.
“Large-scale satellite measurements that distill global-scale observations, like the changing color of the sea, provide fundamental insights that our global ocean garden is also changing, something we would be wise to take careful note of,” Balch said.
The authors of the paper relied on data that a NASA satellite called Aqua collected between 2002 and 2022. When they analyzed the data from seven wavelengths of light reflected off the ocean, they found significant changes in color in 56 percent of the world’s oceans. Most of those changes were observed in tropical and subtropical waters, which in the past have tended to remain more stable in color than waters closer to the poles, where there are greater temperature changes.
While there could be other causes for the changing colors, the authors of the paper said their results match how sophisticated computer models predict the way marine ecosystems would be affected by rising carbon emissions.
“What’s most concerning about these findings is the scale,” said Emmanuel Boss, a professor of oceanography at the University of Maine in Orono, who co-authored the paper. “This is the first time we’re seeing a noticeable, large-scale change in the base of the marine food chain.”
Byran Franz, assistant chief for science research at the Ocean Ecology Laboratory, called the paper’s findings “credible” but said it will take time and additional analysis to understand the causes and potential implications. It’s possible changes in the distribution of species, unrelated to climate change, could be playing a role in the shifting colors.
“We should be concerned that we don’t know what is causing this long-term change in color,” he said. “For that, we need more detailed measurements.”
Cecile Rousseaux, another research scientist at NASA, said she has high hopes for a new satellite scheduled to launch in January that will measure ocean color in many more wavelengths than the Aqua satellite and offer more detailed, potentially more revealing observations.
The new satellite, she said, “should allow us to understand the ecological implications of the observed trends in ocean ecosystem structures.”
Satellites are not the only way to observe changes in ocean color. Many who spend most of their days at sea have observed similar changes.
Among them is Michael Packard, a commercial lobster diver from Wellfleet, who has spent years searching for the valuable crustaceans in the waters off Provincetown.
The changes in the ocean’s color have become more noticeable to him and fellow fishermen in recent years.
“It’s been the talk around the dock,” he said. “The water often doesn’t have that neutral green color anymore. It’s almost lime green — and that’s definitely concerning.”