Study: The color of the ocean is changing as a result of climate change

Scientists at MIT, the National Oceanographic Center in the UK and elsewhere report that the color of the oceans has changed dramatically over the past 20 years, and that this global trend may be a consequence of the change. anthropogenic climate change.

In a study appearing today in Nature, The team writes that they have detected changes in ocean color over the past two decades that cannot be explained by natural annual variation alone. These color changes, though barely noticeable to the human eye, occur in 56% of the world’s oceans — an area larger than the total land area on Earth.

In particular, the researchers found that tropical oceans near the equator have gradually become bluer over time. The change in ocean color suggests that the ecosystems inside the surface ocean must also change, as the ocean’s color is a literal reflection of the organisms and matter in its waters. It.

At this point, the researchers cannot say exactly how the marine ecosystem is changing to reflect the color shift. But they’re pretty sure of one thing: Human-caused climate change may be to blame.

“I have been running simulations that have told me for years that these changes in the color of the ocean will occur. for the Science of Global Change. “Really witnessing that happen is not surprising, but terrifying. And these changes are consistent with anthropogenic changes to our climate.”

Lead author BB Cael PhD ’19 of the National Oceanographic Center in Southampton adds: «This provides further evidence of how human activities are affecting life on Earth in a range of ways. vast microspace. United Kingdom “It is another way that humans are affecting the biosphere.”

Study co-authors include Stephanie Henson of the National Oceanographic Center, Kelsey Bisson at Oregon State University, and Emmanuel Boss of the University of Maine.

On the noise

The color of the ocean is a visual product of whatever lies within its upper layers. In general, waters that are dark blue reflect very little life, while bluer waters indicate the presence of ecosystems and are mainly phytoplankton — plant-like bacteria that are abundant in the ocean. upstream oceans and contains green pigmented chlorophyll. This pigment helps plankton harvest sunlight, which they use to capture carbon dioxide from the atmosphere and convert it into sugar.

Phytoplankton is the foundation of the marine food web that sustains more and more complex organisms, down to krill, fish, seabirds and marine mammals. Phytoplankton is also a powerful muscle in the ocean’s ability to capture and store carbon dioxide. Therefore, scientists are keen to track phytoplankton on the ocean’s surface and to see how these essential communities can respond to climate change. To do so, the scientists monitored changes in chlorophyll, based on the ratio of how much blue to green light is reflected from the ocean’s surface, which can be monitored from space. time.

But about a decade ago, Henson, a co-author of the current study, published a paper, along with others, that showed that, if scientists only tracked chlorophyll, it would take at least 30 years of continuous monitoring to detect any trends. specifically driven by climate change. The team argues that the reason is that the large, natural variability of chlorophyll from year to year outweighs any human influence on chlorophyll concentrations. Therefore, it will take several decades to pick out a meaningful, climate change-oriented signal amid the usual noise.

In 2019, Dutkiewicz and her colleagues published a separate paper, showing through a new model that the natural variation of other ocean colors is much smaller than that of matter. chlorophyll. As a result, any signal of a change due to climate change will be easier to detect than normal, smaller variations of other ocean colors. They predict that such changes will be apparent within 20, rather than 30 years of follow-up.

“So, I thought, shouldn’t we look for trends in all these other colors, rather than just in chlorophyll?” Cael said. «It’s worth looking at the whole spectrum, rather than just trying to estimate a number from bits of the spectrum.»

The power of seven

In the current study, Cael and team analyzed color measurements of the ocean taken by the Moderate Resolution Imaging Spectrometer (MODIS) on the Aqua satellite, which has tracked the color of the ocean. ocean color for 21 years. MODIS takes measurements in seven visible wavelengths, including two colors that researchers commonly use to estimate chlorophyll.

The difference in color captured by the satellite is too small for the human eye to discern. Much of the ocean appears blue to our eyes, while true colors can contain a more subtle mixture of wavelengths, from blue to green and even red.

Cael performed the statistical analysis using all seven colors of the ocean measured by satellite between 2002 and 2022 together. First, he looked at how the seven colors changed from region to region in a given year, which gave him an idea of ​​their natural variations. He then zoomed out to see how the annual variations in the ocean’s color changed over a longer period of two decades. This analysis shows a clear trend, above the normal annual variation.

To see if this trend was related to climate change, he then looked at Dutkiewicz’s model from 2019. This model simulates Earth’s oceans under two scenarios: one One scenario has greenhouse gases added and the other scenario has no greenhouse gases. Greenhouse gas modeling predicts that a significant trend will emerge within 20 years and that this trend will cause changes in ocean color in about 50% of the world’s ocean surface – almost exactly what Cael found in his analysis of real-world satellite data. .

“This shows that the trends we observe are not a random variation in the Earth system,” says Cael. “This is consistent with human-caused climate change.”

The team’s results suggest that monitoring ocean color beyond chlorophyll could give scientists a clearer, faster way to detect climate change-induced changes to the ecosystem. marine ecology.

“The colors of the oceans have changed,” says Dutkiewicz. “And we cannot say how. But we can say that the changes in color reflect changes in the plankton community, which will impact everything that eats plankton. It will also change the amount of carbon the ocean will absorb, because different types of plankton have different abilities to do that. So we hope everyone takes this seriously. It’s not just models that predict these changes will happen. Now we can see that happening and the ocean is changing.”

This research was supported in part by NASA.

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