More than half of the world’s oceans have changed significantly in color over the past 20 years, with climate change as the likely cause, new research suggests. Oceans around the equator have shifted to a greener hue, a trend that cannot be explained by natural, year-to-year variability alone.
The ocean’s color changes based on what’s found in its upper layers, according to a statement from MIT. Bluer oceans tend to have little life, while greener oceans have more phytoplankton—marine algae that photosynthesize. Phytoplankton are the base of the marine food web, serving as fuel for zooplankton and fish, which in turn are eaten by larger fish, seabirds and marine mammals.
But phytoplankton are also critical for combating the climate crisis. Researchers estimate the oceans absorb about 30 percent of the carbon dioxide produced by humans, largely thanks to the photosynthesis of these algae.
Different kinds of plankton reflect and absorb light in different ways, meaning that a shifting ocean color equates to a changing ecosystem, Cael tells Vice’s Becky Ferreira. Such changes could have a ripple effect on the entire food web and potentially even affect the ocean’s ability to store carbon, per CNN’s Jack Guy.
Cael and his colleagues examined data from the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA’s Aqua satellite, which has been monitoring ocean color for 21 years. The team looked at measurements from seven visible wavelengths and found that color shifts had occurred between 2002 to 2022 in 56 percent of the oceans, primarily around the tropics and subtropics. They published their findings Wednesday in the journal Nature.
To determine whether the trend was related to climate change, the team turned to a model created by study co-author Stephanie Dutkiewicz in 2019, which simulated how the Earth’s oceans would respond under two scenarios: one with added greenhouse gases and one without. The results predicted in the greenhouse gas model aligned almost exactly with what the researchers found from real-world data—within 20 years, about half of the oceans significantly shifted in color, per the MIT statement.
“I’ve been running simulations that have been telling me for years that these changes in ocean color are going to happen,” Dutkiewicz tells MIT. “To actually see it happening for real is not surprising, but frightening. And these changes are consistent with man-induced changes to our climate.”
Though the team says climate change is to blame for the greener waters, the process within the oceans that’s causing this shift is still a mystery. Sea surface temperatures have increased, but the ocean areas that changed color were not the same as the specific regions that warmed at the surface, Cael tells Nature News. Instead, scientists suggest the trend comes from nutrient distribution—rising temperatures reduce the mixing between different layers of water and limit upwelling of nutrients. This, in turn, might alter which types of plankton can survive best.
The new findings “confirm suspicions” about how oceans are responding to climate change, Tammi Richardson, a phytoplankton researcher at the University of South Carolina who wasn’t involved in the study, tells New Scientist’s Madeleine Cuff. “It’s giving us much more solid evidence that the ocean is becoming greener, beyond the few data points that we’ve had historically.”
While the MODIS satellite could pick up on the oceans’ shifting hue, scientists still aren’t sure whether these changes will ever be visible to the human eye.
“If a big tipping point was reached in some places: maybe,” Dutkiewicz tells CNN. “Though you’d have to study the colors for a while to be able to pick up on the changes.”