To track the changes in ocean color, scientists analyzed measurements of ocean color taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite, which has been monitoring ocean color for 21 years. Credit: NASA and Joshua Stevens, using Landsat data from the U.S. Geological Survey and MODIS data from LANCE/EOSDIS Rapid Response.
The ocean’s color has changed significantly in the past 20 years, a consequence MIT scientists attribute to human-induced climate change in a new study. While the alterations are subtle to the human eye, they are widespread, occurring in over 56% of the world’s oceans.
The ocean’s color is a visual product of whatever lies within its upper layers. Generally, waters that are deep blue reflect very little life, Meanwhile, greener waters indicate the presence of ecosystems, mainly phytoplankton, which contain the green pigment chlorophyll.
For years, scientists have tracked changes in chlorophyll to get a beat on ocean health. This can be accomplished fairly easily based on the ratio of how much blue versus green light is reflected from the ocean surface—an analysis that can even be monitored from space.
However, about 10 years ago, scientists began to question if the large, natural variations in chlorophyll from year to year would overwhelm any anthropogenic evidence of climate change—or lack thereof. So, they began looking into alternative monitoring methods.
In 2019, Stephanie Dutkiewicz, a co-author of the current study and a senior scientist in MIT’s Department of Earth, Atmospheric and Planetary Sciences and the Center for Global Change Science, found one. Through the use of a new model, Dutkiewicz and colleagues showed that the natural variation of ocean colors beside green is much smaller and therefore easier to detect and monitor.
While must of the ocean appears blue and green to our eyes, the true color is a mix of subtler wavelengths—seven colors in all.
“So I thought, doesn’t it make sense to look for a trend in all these other colors, rather than in chlorophyll alone?” said B.B. Cael, lead author of the current study and researcher at the National Oceanography Center in the UK. “It’s worth looking at the whole spectrum, rather than just trying to estimate one number from bits of the spectrum.”
For the study, published in Nature, Cael and the team analyzed measurements of ocean color taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite, which has been monitoring ocean color for 21 years. MODIS takes measurements in seven visible wavelengths.
Cael then carried out a statistical analysis using all seven ocean colors from 2002 to 2022. He first looked at how much the seven colors changed from region to region during a given year, which gave him an idea of their natural variations. Then, he zoomed out to see how these annual variations in ocean color changed over a longer stretch of two decades—and a troubling trend became clear.
According to the study results, tropical ocean regions near the equator have become steadily greener over time. The shift in ocean color indicates that ecosystems within the surface ocean must also be changing. Additionally, the researchers found that the color shifts have occurred in over 56% of the world’s oceans—an expanse that is larger than the total land area on Earth.
To verify the origins of the color shifts, Cael turned to Dutkiewicz’s 2019 model, which simulated the Earth’s oceans under two scenarios: one with the addition of greenhouse gases, and the other without.
The greenhouse gas model predicted that, in 20 years, changes to the color of the ocean would occur in about 50% of the world’s surface oceans—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,” said Cael. “This is consistent with anthropogenic 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 Dutkiewicz. “To actually see it happening for real is not surprising, but frightening. These changes are consistent with man-induced changes to our climate.”