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Over half the ocean has changed color in just two decades. That could be a very big deal.

DAILY SCIENCE

Over half the ocean has changed color in just two decades. That could be a very big deal.

While the exact reasons behind the color shift are a big unknown, they signal changes in the plankton that determine how much carbon the ocean takes up.
July 19, 2023

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Add this to the myriad ways global warming is changing the ocean: We are making it more green.

Artists won’t need to retouch seascape paintings with a little extra green on their brushes. The shift is too subtle to detect with the naked eye. But it’s an indicator that something is shifting with the organisms living in the watery regions covering more than two thirds of the globe. That could mean a great deal.

“I’ve been running simulations that have been telling me for years that these changes in ocean color are going to happen,” says Massachusetts Institute of Technology scientist Stephanie Dutkiewicz. “To actually see it happening for real is not surprising, but frightening.”

The oceans are so vast and deep that it’s difficult for people to observe planet-wide changes to sea-based ecosystems. For some time, ocean color measured from satellites has been seen as one possible way to detect such changes.

The particular hues of a stretch of ocean are determined by what’s living there. Seas that are more blue are like a desert, with relatively little life. Greener oceans, by contrast, boast more organisms such as the tiny plants known as phytoplankton.

 

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But attempts to detect changes tied to long-term forces such as climate change have been stymied by the natural year-to-year fluctuations in ocean ecosystems.  Much of the research has focused on the colors of light reflected by chlorophyll, the green-tinted molecules help plants absorb sunlight and drive photosynthesis. In 2010, however, scientists warned that it would take as much as 40 years of satellite observations before any meaningful trend tied to climate change could emerge from this statistical static.  

Recently a group of scientists got the idea that they might find an answer more quickly by expanding the color palette. “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, a former MIT student now at the United Kingdom’s National Oceanography Center in Southampton. “It’s worth looking at the whole spectrum, rather than just trying to estimate one number from bits of the spectrum.”

He and other scientists, including Dutkiewicz, turned to Aqua, a NASA satellite launched in 2002 and equipped with a sensor that detects different wavelengths of light reflected into space from the Earth’s surface. Rather than just the two wavelengths typically used to track chlorophyll, they could look at seven, ranging from blues to reds.

Unlike chlorophyll, some of these colors didn’t fluctuate a lot from year to year, reducing the chaotic ups and downs that can obscure bigger trends. When they looked at 20 years of data, it revealed a notable shift in color across 56% of the ocean surface, mostly in waters close to the equator. That water has, in general, turned a bit more green, the scientists reported July 12 in Nature.

The results mirror earlier findings from a sophisticated computer model that Dutkiewicz and colleagues built to estimate how climate change might induce color shifts. A simulation of ocean physics, chemistry and biology projected detectable shifts across nearly half the ocean over a 20-year span, concentrated in lower latitudes.

The overlap between the model and the satellite observations “suggests that the trends we observe are not a random variation in the Earth system,” said Cael. “This is consistent with anthropogenic climate change.”

What’s less clear are the exact dynamics driving the changes, or what it might mean for these ecosystems. There was not an obvious link between where ocean color changed and where sea surface temperatures have risen. It could instead be due to changes in the strength of boundaries separating water near the surface and deeper ocean layers, or the depth where deep and shallow water meet and mix, the authors wrote.

Changes in the interplay between water at different depths can influence how critical nutrients and other life-shaping molecules circulate through the ocean. For instance, it has been implicated in declining amounts of oxygen in the ocean and the growth of low-oxygen zones.

While the exact reasons behind the color shift remain a big unknown, “we can say that changes in color reflect changes in plankton communities, that will impact everything that feeds on plankton,” said Dutkiewicz. “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.”

Cael, et. al. “Global climate-change trends detected in indicators of ocean ecology.” Nature. July 12, 2023.

Images: ©NASA

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