Climate Change Will Shift the Ocean's Color by the End of the 21st Century

We take for granted that the sky is blue, leaves are green, and the ocean is kind of blue-green, but scientists warn that some of those things won’t stay the same for long. As the Earth’s climate warms, scientists say, the color of the water in the world’s oceans will change over time — and it could happen within the next century.

New research from scientists at Massachusetts Institute of Technology and the National Oceanography Centre Southampton in the UK shows that nearly two-thirds of the world’s oceans could look significantly different by 2100 as climate change continues to wreak havoc on the Earth, and that color change will come with major consequences.

In a paper published Monday in the journal Nature Communications, the team reports they can use the color of ocean water as a “signature” of rising water temperatures.

In the next 80 years, they write, the color will change enough to be detectable by satellites, though probably not to the naked eye: The warm, blue parts of the ocean will become bluer, while the cold, green parts of the ocean will turn greener. Using satellite imaging, the team found a way to interpret what color light the water is reflecting, even when the differences are very small. As different parts of the ocean change color over the next few decades, the scientists will be able to use the shifting hues to tell how warm the ocean is in those regions.

The color of the ocean is a result of the way water absorbs and scatters light, which in turn is influenced by minerals dissolved in the water and the presence of the tiny, green, photosynthetic organisms known as phytoplankton. As the oceans warm, the team predicts, the warm regions with less phytoplankton will likely support even less life — becoming bluer — while warmer temperatures in cold regions of the ocean will foster larger populations of plankton — turning it green.

Scientists commonly use satellite data to estimate levels of chlorophyll-a, a green chemical used in photosynthesis, to measure levels of phytoplankton. Where there’s a lot of chlorophyll-a, there’s a lot of phytoplankton, which in turn is correlated with the temperature of the water in that region.

“Chlorophyll is changing, but you can’t really see it because of its incredible natural variability,” Stephanie Dutkiewicz, Ph.D., a planetary science researcher at MIT and the first author of the paper, says. “But you can see a significant, climate-related shift in some of these wavebands, in the signal being sent out to the satellites. So that’s where we should be looking in satellite measurements, for a real signal of change.”

The team, however, improves on this color-detecting method with a metric called remote sensing reflectance (RSS), which estimates how much light hitting the water reflects back up. This measure, importantly, is even more accurate than measuring chlorophyll color changes, and it doesn’t fluctuate season-to-season as much as phytoplankton does. RSS, they write, may be the single most reliable indicator of how quickly our oceans are warming due to climate change.

“The change is not a good thing, since it will definitely impact the rest of the food web,” Dutkiewicz told CNN. “Phytoplankton are at the base, and if the base changes, it endangers everything else along the food web, going far enough to the polar bears or tuna or just about anything that you want to eat or love to see in pictures.”

Abstract: Monitoring changes in marine phytoplankton is important as they form the foundation of the marine food web and are crucial in the carbon cycle. Often Chlorophyll-a (Chl-a) is used to track changes in phytoplankton, since there are global, regular satellite-derived estimates. However, satellite sensors do not measure Chl-a directly. Instead, Chl-a is estimated from remote sensing reflectance (RRS): the ratio of upwelling radiance to the downwelling irradiance at the ocean’s surface. Using a model, we show that RRS in the blue-green spectrum is likely to have a stronger and earlier climate-change-driven signal than Chl-a. This is because RRS has lower natural variability and integrates not only changes to in-water Chl-a, but also alterations in other optically important constituents. Phytoplankton community structure, which strongly affects ocean optics, is likely to show one of the clearest and most rapid signatures of changes to the base of the marine ecosystem.