The color of deep blue oceans, shallow turquoise waters, and emerald green coasts is quickly changing as the planet warms, according to new research published in the journal Nature.
Analyzing 20 years of satellite data, the study's authors found that over half the world's ocean, 56 percent, experienced a shift in color. The cause? Changes in the density and distribution of plankton. These tiny organisms contain chlorophyll, the bright green pigment that helps plants make food from sunlight.
The recent study supports a similar prediction made by a Nature Communications study published in 2019 that modeled how phytoplankton will change as oceans continue to warm.
And while the new study used satellites to detect subtle changes in color, the prior research predicted significant changes by 2100, if the world keeps warming at its current pace.
Under a “business-as-usual” scenario in which greenhouse gas emissions continue unabated, the bluest subtropical zones of the ocean will become bluer, and greener regions along the equator and poles will become greener, that study found.
More than just an oddity, the changing color is a warning sign, say the 2019 study authors, of drastic global changes that will take place in a world warmed by climate change.
How the ocean gets its color
Sunlight penetrates over 600 feet below the surface of the ocean. Everything deeper is enshrined in darkness. Above that, most water molecules are capable of absorbing all colors except blue, which is why blue is reflected out.
Organic matter that blankets the surface of the ocean, like phytoplankton, changes this color. As the ocean warms, currents become more irregular, and the layers in the water become more stratified, meaning warm regions don't mix as easily with cold regions.
There are thousands of phytoplankton species, uniquely adapted to warm or cold water. As oceans continue warming, some species may die off, some will thrive, and others will migrate to different regions.
But just looking at chlorophyll alone, however, won't tell scientists how a warming climate is altering phytoplankton. Naturally occurring events like El Niños and La Niñas can influence how much phytoplankton is concentrated in a given area
Stephanie Dutkiewicz, an author on both papers and marine ecologist at the Massachusetts Institute of Technology, said in 2019 that models used to predict future changes in color factor phytoplankton life cycles and movements into naturally occurring ocean patterns.
The 2023 study revealed that many of these predicted changes have already occurred. Using light-measuring devices aboard NASA satellites, scientists observed that over half of the world covered by ocean already showed a measurable shift in blue and green wavelengths, an approximation for the amount of chlorophyll in a given region.
What do these changing colors mean?
It's too early to say for sure what effect these changing colors will have on the environment, but scientists think more ecosystems could be dominated by smaller-sized plankton in the future, according to a press release from the National Oceanography Center in the U.K., which supported the 2023 study.
The ocean has absorbed about a third of the world's carbon emissions, and marine life like kelp, seagrass, and algae play a critical role in helping pull that carbon out of the atmosphere.
But smaller algae could reduce that climate change-fighting power.
“Phytoplankton are the base of the marine food web. Everything in the ocean requires phytoplankton to exist," Dutkiewicz told National Geographic in 2019. "The impact will be felt all the way up the food chain."