December 2, 2023

A new study analyzing 20 years of data has found that the color of our oceans has shifted, pointing to major changes in marine ecosystems, likely due to human-caused climate change.

Satellites have been measuring the color of Earth’s oceans for decades, gathering data on the color of light reflected off the water’s surface. One satellite in particular, the Aqua satellite, and its Moderate Resolution Imaging Spectroradiometer (MODIS) sensor, recently completed 20 years of high-quality collection of global ocean color data. A new study led by MIT and the UK’s National Oceanography Center examined the data and found that changes in ocean color over the past 20 years cannot be explained by natural year-to-year variations alone.

“For years, I’ve been running simulations that have consistently told me that these changes in ocean color are going to happen,” said Stephanie Dutkiewicz, one of the study’s co-authors. “It’s not surprising to see this actually happening, it’s frightening. These changes are consistent with human-induced climate change.”

The colors of the ocean reflect the life that lives beneath its upper layers. The ocean surface extends from zero to a depth of about 330 feet (100 m), spanning the entire ocean. This is where the ocean meets the atmosphere and absorbs carbon dioxide from the air.

In general, dark blue water contains little life, while greener water indicates the presence of ecosystems, mostly phytoplankton, tiny plant-like microorganisms that contain the green pigment chlorophyll. Phytoplankton form the base of aquatic food webs, providing food for everything from tiny zooplankton to shellfish, which are then eaten by larger fish and marine mammals. Without phytoplankton, many marine food webs would collapse, greatly affecting marine life and humans who depend on fish for food.

However, in addition to providing nutrients, phytoplankton produce oxygen through photosynthesis, which is released into the ocean and atmosphere, and absorb carbon dioxide. Therefore, monitoring phytoplankton in the ocean surface is a good way to assess how they are responding to climate change. Measuring chlorophyll in phytoplankton has traditionally been a good indicator of long-term trends in ocean color. Yet the study found that more than 30 years of satellite data would be needed to detect trends in chlorophyll driven by climate change.

In 2019, Dutkiewicz and her colleagues developed a new model showing that the natural variation in ocean color is much smaller than that seen in chlorophyll, meaning that ocean color, in addition to these two, should be easier to detect to changes driven by climate change. Traditionally used to estimate chlorophyll. It also needs 20 years of data, not 30 years.

“So I thought, wouldn’t it make sense to look for trends in all the other colors, rather than just in chlorophyll?” said BB Cael, lead author of the study. “It’s worth looking at the whole spectrum, rather than just trying to estimate a number from individual bits of the spectrum.”

MODIS measures in multiple bands within the visible spectrum. By analyzing data from 2002 to 2022 using seven ocean colors, researchers can see how the colors of different regions changed in a given year, providing an overview of natural variation. Then, by looking at how these changes changed over a 20-year period, their analysis showed clear trends above normal year-to-year changes.

This real-world data was compared to a 2019 model developed by Dutkiewicz that simulated Earth’s oceans under two scenarios: one with greenhouse gases added and one without. Greenhouse gas models predict a significant trend within 20 years, leading to changes in ocean color in about half of the world’s surface ocean. The model predictions and real-world data were nearly identical, with more than 56 percent of the world’s oceans showing color changes. Tropical oceans around the equator, in particular, have become greener over time.

“This shows that the trends we observe are not random variations in the Earth system,” Kyle said. “This is consistent with anthropogenic climate change. It provides more evidence of how human activity affects life on Earth at enormous spatial scales.”

The findings suggest that measuring ocean color beyond chlorophyll may provide scientists with a faster and more sensitive way to detect how climate change is affecting marine ecosystems, the researchers said.

“The color of our oceans has changed,” Dutkiewicz said. “And we can’t say how. But what we can say is that the change in color reflects a change in the plankton community, which will affect everything that feeds on plankton. It will also change the amount of carbon absorbed by the ocean because different types plankton have different capabilities. So we want people to take this seriously. It’s not just models predicting these changes are going to happen. We can see it happening now, the oceans are changing.”

The researchers say ongoing research is needed, combining data from multiple satellites, to reveal trends in their study that indicate how ocean surface ecology is changing.

The study was published in the journal nature.

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