NASA

Many of the world’s oceans are greener than they were 20 years ago, a new study published in Nature has found—suggesting climate change is altering sea-surface ecosystems.

A single satellite orbiting the Earth for the past two decades has been keeping tabs on the colour of the oceans by measuring the way different wavelengths of light reflect from its surface. Scientists had thought we’d need many more years of data before signs of climate change would show up in this way, but by looking at seven light wavelengths, the researchers found that more than half of the oceans worldwide have significantly altered in hue— especially in temperate and tropical areas—and that the shift can’t be explained by natural variability.

What’s causing the deviation isn’t yet clear, but it likely has something to do with the interaction between warming waters, ocean nutrients and phytoplankton—tiny aquatic plants that contain the green pigment chlorophyll. Changes in plankton communities on such a vast, global scale have implications for ocean carbon storage, food webs and fisheries, the study authors say, meaning the switch from blue to green could be an early-warning signal of wider shifts.