Climate change is modifying colour of ocean: it is becoming greener

Even the colour of the sea is changing due to global warming, and this is nothing but a consequence of deeper ecological transformations. In the last twenty years, the colour of the ocean has undergone significant changes, as revealed by a study published in Nature. This alteration is a direct consequence of human-induced climate change, even though it is not perceptible to the naked eye.

Scientists from the Massachusetts Institute of Technology (MIT) and the National Oceanography Center of the United Kingdom conducted the study, which highlights a notable change observed in 56% of the Earth's oceans. This area encompasses a greater expanse than the entire landmass of our planet.

The article highlights a significant change in tropical ocean regions near the equator, showing an increase in greenery. This transformation suggests a simultaneous alteration in surface ecosystems, as the colour of the ocean directly represents the conditions of organisms and materials residing in its waters.

The researchers still can't say exactly how marine ecosystems are changing, but they're sure it's caused by climate change.

"I have been conducting simulations for several years, and the results consistently indicate that these alterations in the ocean's colour will occur. Therefore, witnessing their manifestation is no longer surprising; rather, it evokes a sense of dread," expressed Stephanie Dutkiewicz, a research scientist at MIT and co-author of the study.

changes in plankton

The colour of the ocean can reveal the contents of its upper layers: when the waters appear deep blue, it suggests a scarcity of marine life, whereas greener waters hint at the thriving presence of ecosystems, primarily due to the abundant phytoplankton - plant-like microbes that dominate the upper regions of the ocean.

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

That's why scientists have spent decades monitoring phytoplankton on the ocean's surface and studying how these essential communities respond to climate change, monitoring from space.

"The colour of the oceans has changed, because it reflects changes in plankton communities, which will affect everything that feeds on plankton."

"Those changes will also change how much carbon 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," Dutkiewicz said.

The prediction came true

"The significance of this lies not in our concern for the colour itself, but rather in the colour's role as a reflection of ecosystem changes," explained BB Cael, a scientist at the National Oceanography Centre in Southampton and author of the study.

Cael and his team analyzed measurements of ocean colour made by the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Aqua satellite, which has been monitoring ocean colour for 21 years and makes measurements at seven visible wavelengths.

And it is that, although much of the ocean appears blue to our eyes, the real colour can contain a mixture of more subtle wavelengths, from blue to green and even red.

Cael did a statistical analysis using together the seven ocean colours measured by the satellite between 2002 and 2022. He first looked at how much the seven colours changed from one region to another over a year and then extended the analysis to two decades.

The analysis revealed a clear trend, above the normal annual variability.

To see if the trend is related to climate change, he turned to Dutkiewicz's 2019 model, which simulates Earth's oceans under 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 colour of the oceans in about 50 per cent of the world's surface oceans (exactly what Cael found in his analysis of real-world satellite data).

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