Climate change is changing the color of the oceans

July 12 () –

The color of the ocean has changed significantly in the last 20 yearsand the global trend is likely a consequence of human-induced climate change.

The team, made up of researchers from the Massachusetts Institute of Technology (MIT) in the United States, the National Oceanography Center in the United Kingdom and other research centers, claims to have detected changes in the color of the oceans over the past two decades that they cannot be explained solely by interannual natural variability.

These color changes, though subtle to the human eye, have occurred in 56% of the world’s oceans, an area greater than the Earth’s total land area, the study reveals. published in the journal Nature.

Specifically, they found that tropical ocean regions near the equator have become increasingly greener over time. The changing color of the oceans indicates that the ecosystems of the ocean surface must also be changing, since the color of the ocean is a literal reflection of the organisms and materials in its waters.

At the moment, the researchers can’t say exactly how marine ecosystems are changing to reflect the color change. But of one thing they are sure: human-induced climate change is probably to blame.

“I’ve been running simulations for years that tell me these ocean color changes are going to happen,” says Stephanie Dutkiewicz, study co-author and principal investigator in MIT’s Department of Earth, Atmospheric, and Planetary Sciences and the Center for Earth Sciences. Global Change Science — Seeing it actually happen is not surprising, but terrifying. And these changes are consistent with human-induced changes in our climate.”

“This provides further evidence of how human activities are affecting life on Earth across a huge expanse of space,” added lead author BB Cael, from the National Oceanography Center in Southampton, UK. It’s another way that humans are affecting the biosphere.”

The color of the ocean is a visual product of what is in its upper layers. In general, deep blue waters reflect very little life, while greener waters indicate the presence of ecosystems, primarily phytoplankton — plant-like microbes that thrive in the upper layers of the ocean and contain the Chlorophyll green pigment. This pigment helps plankton to capture sunlight, which it uses to capture carbon dioxide from the atmosphere and convert it into sugars.

Phytoplankton is the foundation of the marine food web that supports progressively more complex organisms, up to krill, fish, and marine birds and mammals. It is also a powerful muscle in the ocean’s ability to capture and store carbon dioxide.

For this reason, scientists are very interested in monitoring phytoplankton on the surface of the oceans and seeing how these essential communities may respond to climate change. To do this, scientists have followed changes in chlorophyll, based on the relationship between the amount of blue and green light that reflects off the ocean surface, which can be monitored from space.

But about a decade ago, Stephanie Henson of the National Oceanography Center and co-author of the current study co-published a paper showing that if scientists were to follow chlorophyll alone, it would take at least 30 years of follow-up. continued to detect any trends that were specifically due to climate change.

The reason, according to the team, is that the large natural variations in chlorophyll from year to year would overwhelm any anthropogenic influence on chlorophyll concentrations. Thus, it would take several decades to detect a significant signal driven by climate change amid normal noise.

In 2019, Dutkiewicz and colleagues published a separate paper, showing through a new model that the natural variation in other ocean colors is much smaller compared to that of chlorophyll. Therefore, any signs of changes driven by climate change should be easier to detect above the normal, smaller variations in other ocean colors. They predicted that these changes would be apparent after 20 years of follow-up, instead of 30.

“So I thought: doesn’t it make sense to look for a trend in all these other colors, instead of just chlorophyll?” recalls Cael, quoted by Eureka Alert–. It’s worth looking at the whole spectrum, rather than trying to estimate a number from chunks of the spectrum.”

In the current study, Cael and his team analyzed measurements of ocean color made by the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite, which has been monitoring ocean color for 21 years. MODIS makes measurements at seven visible wavelengths, including the two colors that researchers traditionally use to estimate chlorophyll.

The color differences captured by the satellite are too subtle for the human eye to distinguish. Much of the ocean appears blue to our eyes, while the actual color may contain a mix of more subtle wavelengthsfrom blue to green and even to red.

Cael performed a statistical analysis using together the seven ocean colors measured by the satellite between 2002 and 2022. He first looked at how much the seven colors changed from one region to another during a given year, which gave him an idea of ​​its natural variations.

He then extended the analysis to see how these yearly variations in ocean color changed over two decades. This analysis revealed a clear trend, above the normal annual variability.

To see if this trend is related to climate change, he turned to Dutkiewicz’s 2019 model. This model simulated Earth’s oceans in two scenarios: one with the addition of greenhouse gases and one without.

The greenhouse gas model predicted that a significant trend should appear in 20 years and that this trend should cause changes in the color of the oceans in about 50 percent 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 are observing are not random variation in the Earth system,” Cael says. This is consistent with anthropogenic climate change.”

The team’s results demonstrate that tracking ocean colors beyond chlorophyll could give scientists a clearer and faster way to detect changes in marine ecosystems driven by climate change.

“The color of the oceans has changed,” Dutkiewicz said. “And we can’t say how. But we can say that the color changes reflect changes in plankton communities, which will affect everything that feeds on plankton. It will also change.” the amount of carbon that the ocean will absorb, because different types of plankton have different abilities to do that. So we hope people take it seriously. It’s not just models that predict these changes. Now we can see it, and the ocean is changing.”he concludes.