• Posted July 17, 2023

Ocean Color Is Changing, and Climate Change is Likely Behind It

The color of the oceans has changed over the past two decades, which has scientists concerned about the impact of climate change.

“I've been running simulations that have been telling me for years that these changes in ocean color are going to happen,” said study co-author Stephanie Dutkiewicz, senior research scientist in Massachusetts Institute of Technology's Department of Earth, Atmospheric and Planetary Sciences.

“To actually see it happening for real is not surprising, but frightening. And these changes are consistent with man-induced changes to our climate," Dutkiewicz added in a school news release.

Though subtle to the human eye, these color changes have happened over 56% of the world's oceans -- an expanse that is larger than the total land area on Earth, the researchers said.

Tropical ocean regions near the equator have become steadily greener over time. This suggests that ecosystems within the surface ocean must also be changing, the authors said.

“This gives additional evidence of how human activities are affecting life on Earth over a huge spatial extent,” said study lead author B.B. Cael of the National Oceanography Centre in Southampton, England.

While deep blue water reflects very little life, greener waters indicate ecosystems, including plant-like microbes known as phytoplankton that contain the green pigment chlorophyll. The pigment helps plankton harvest sunlight, which they use to capture carbon dioxide from the atmosphere.

Phytoplankton are also the foundation of the marine food web and an important part of ocean's ability to capture and store carbon dioxide.

For the current study, researchers analyzed measurements of ocean color taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA's Aqua satellite. MODIS takes measurements in seven visible wavelengths.

In a statistical analysis using all seven ocean colors measured by the satellite from 2002 to 2022 together, Cael looked at how much the seven colors changed from region to region during a given year. He then zoomed out to see how these annual variations in ocean color changed over two decades.

Cael then looked at Dutkiewicz's model from 2019, which simulated the Earth's oceans with the addition of greenhouse gases and without. The greenhouse-gas model predicted almost exactly what Cael found in his analysis of real-world satellite data.

“This suggests that the trends we observe are not a random variation in the Earth system,” Cael said in the release. “This is consistent with [human-led] climate change."

“The color of the oceans has changed,” Dutkiewicz said. “And we can't say how. But we can say that changes in color reflect changes in plankton communities, that will impact everything that feeds on plankton. It will also change how much the ocean will take up carbon, because different types of plankton have different abilities to do that. So, we hope people take this seriously. It's not only models that are predicting these changes will happen. We can now see it happening, and the ocean is changing.”

This research was supported in part by NASA. The study findings were published July 12 in the journal Nature.

More information

NASA has more on climate change.

SOURCE: Massachusetts Institute of Technology, news release, July 12, 2023