There are lots of ways climate change is altering the world’s oceans: the waters are becoming warmer, higher and more acidic, which in turn is having a devastating impact on marine life. As Brady Dennis and Chris Mooney report for the Washington Post, a new study has found that climate change may also change the oceans’ color, making them greener in some regions and bluer in others.
Sea water can, depending on where you are, appear in a range of different blue and green shades. When sunlight hits the sea, most of the wavelengths are absorbed by water molecules, but short blue and violet wavelengths are reflected back, which is why ocean water sometimes look blue, according to NASA. If organisms are present in the water, however, they can absorb and reflect light too, which in turn changes how we see the ocean’s color. Green hues stem from the presence of phytoplankton, microscopic algae that contain the pigment chlorophyll, which primarily absorbs blue portions of sunlight and reflects green light. So when there is a large number of phytoplankton floating through a given area of the sea, the waters appear green.
The thousands of plankton species that exist across the globe are specially adapted to either cold or warm water, as Sarah Gibbens of National Geographic notes. So shifts in ocean temperature can have a marked impact on where—or if—certain algae are able to survive. When ocean surfaces become warmer, these waters don’t mix as frequently with deeper waters that are rich in nutrients that phytoplankton need in order to thrive. Previous research has shown that some of the organisms die in the face of these changes, while others respond by moving to cooler patches of the ocean.
For the new study, published in Nature Communications, a team of researchers developed a model that simulates how various algae species will grow and interact as global temperatures rise—and how these changes will in turn impact ocean color. The models were based on data from satellites that measure the light reflected from Earth. The researchers ran their model through to the year 2100, accounting for changes in global temperatures of up to three degrees Celsius (5.4 degrees Fahrenheit). And they found that by the end of the 21st century, more than 50 percent of the world’s oceans may change in color.
The team predicts that blue patches of sea, like the ones in the subtropics, will become even bluer due to a reduced number of phytoplankton. And green oceans, like the ones near the poles, will get greener as more algae flourishes there. These changes in color will be all but imperceptible to the human eye, according to the researchers, but satellites will be able to detect the difference. Ocean color color could therefore be an important indicator of changes that are happening within the marine ecosystem. Phytoplankton are the lifeblood of the marine food web, feeding everything from microscopic organisms to huge whales; their absence in certain areas would deplete an important food source for ocean life.
“What was special about the model is it suggests the subtle shifts in color are an early warning sign,” Stephanie Dutkiewicz, lead author of the study and principal research scientist at MIT’s Department of Earth, Atmospheric, and Planetary Sciences, tells Gibbens. “Phytoplankton are the base of the marine food web. Everything in the ocean requires phytoplankton to exist. The impact will be felt all the way up the food chain.”