Ocean color is turning green, boffins blame human-induced climate change

HQ Team

July 13, 2023: From blue to green, the color of the oceans is changing, and the main factor is human-induced climate change, according to the Massachusetts Institute of Technology researchers.

“The color of the oceans has changed,” said Stephanie Dutkiewicz, a senior research scientist in MIT’s Department of Earth, Atmospheric, and Planetary Sciences and the Center for Global Change Science and study co-author.

“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,” she said.

Change in 56% of world’s oceans

The global trend of ocean color change has happened over the last 20 years. That cannot be explained by natural, year-to-year variability alone, according to the study. 

These color shifts, though subtle to the human eye, have occurred over 56% of the world’s oceans — an expanse that is larger than the total land area on Earth.

Researchers found that tropical ocean regions near the equator have become steadily greener over time. The shift in ocean color indicated that ecosystems within the surface ocean must also be changing, as the color of the ocean is a literal reflection of the organisms and materials in its waters.

“I’ve been running simulations that have been telling me for years that these changes in ocean color are going to happen,” said Stephanie Dutkiewicz.

“To actually see it happening in real is not surprising, but frightening. And these changes are consistent with man-induced changes to our climate,” she said.

Human activities

B. B. Cael at the National Oceanography Center in Southampton, UK, and lead author, said the changes gave additional evidence of how human activities were affecting life on Earth over a huge spatial extent.

“It’s another way that humans are affecting the biosphere.”

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.

Greener waters indicate the presence of ecosystems, mainly phytoplankton — plant-like microbes that are abundant in the upper ocean and that contain the green pigment chlorophyll. 

The pigment helps plankton harvest sunlight, which they use to capture carbon dioxide from the atmosphere and convert it into sugars.

Phytoplankton is the foundation of the marine food web that sustains progressively more complex organisms, including krill, fish, seabirds, and marine mammals.

Powerful muscle

Phytoplankton is also a powerful muscle in the ocean’s ability to capture and store carbon dioxide.

Scientists were therefore keen to monitor phytoplankton across the surface oceans and to see how these essential communities might respond to climate change.

To do so, scientists have tracked changes in chlorophyll, based on the ratio of how much blue versus green light is reflected from the ocean surface, which can be monitored from space.

If scientists were tracking chlorophyll alone, it would take at least 30 years of continuous monitoring to detect any trend that was driven specifically by climate change.

The reason was that the larger, natural variations in chlorophyll from year to year would overwhelm any anthropogenic influence on chlorophyll concentrations. 

It would therefore take several decades to pick out a meaningful, climate-change-driven signal amid the normal noise.

Looking at whole spectrum

“So I thought, doesn’t it make sense to look for a trend in all these other colors, rather than in chlorophyll alone?” said Cael. “It’s worth looking at the whole spectrum, rather than just trying to estimate one number from bits of the spectrum.”

In the present study, Cael and the team analyzed measurements of ocean color taken by the Moderate Resolution Imaging Spectroradiometer aboard the Aqua satellite, which has been monitoring ocean color for 21 years. 

MODIS takes measurements in seven visible wavelengths, including the two colors researchers traditionally use to estimate chlorophyll.

The differences in color that the satellite picks up are too subtle for human eyes to differentiate. 

Much of the ocean appears blue to our eye, whereas the true color may contain a mix of subtler wavelengths, from blue to green and even red.

Cael carried out a statistical analysis using all seven ocean colors measured by the satellite from 2002 to 2022 together.

Seven colors

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. 

He then zoomed out to see how these annual variations in ocean color changed over a long stretch of two decades. This analysis turned up a clear trend, above the normal year-to-year variability.

To see whether this trend is related to climate change, he then looked to the 2019 Dutkiewicz’s model. This model simulated the Earth’s oceans under two scenarios: one with the addition of greenhouse gases, and the other without it.

The greenhouse-gas model predicted that a significant trend should show up within 20 years and that this trend should cause changes in ocean color 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,” Cael said. “This is consistent with anthropogenic climate change.”