Why the world’s oceans change color

Esa A plankton bloom off the coast of Ireland (Credit: Esa)Eza

The balance of marine phytoplankton populations is shifting due to human-induced climate change, profoundly altering the oceans.

When you picture the ocean, you might think of sparkling turquoise water – but recent research suggests that parts of our world’s oceans could in fact become greener. And the biggest culprit could be climate change.

And while some waters are becoming increasingly greenespecially at low latitudes near the equator, others become increasingly bluer as global temperatures rise.

Although this color shift is not visible to the naked eye, satellite studies can map the change.

“The color itself is not something that is easily described with human language, or that you can even see very well,” says BB Cael, a scientist at the National Oceanography Center in Southampton, Britain. Instead, this could be something a mantis shrimp or a butterfly might see, he adds.

The recent one European State of the Climate Report, published in April 2024 by the European Union’s Copernicus Climate Service, revealed how much change is happening in the ocean. It found that chlorophyll – a photosynthetic pigment found in phytoplankton and plants that gives them their green hue – was 200-500% higher than average in the Norwegian Sea and the Atlantic Ocean north of Britain in April 2023, while 60-80% was. lower in the ocean west of the Iberian Peninsula. In June 2023, the chlorophyll content in the Mediterranean Sea was 50-100% higher than average. In both cases, the average comes from measurements between 1998 and 2020.

It comes as analysis by BBC News of sea surface temperatures collected by Copernicus has revealed that the world’s oceans are experiencing record warming. It showed that temperature records were broken every day in the world’s oceans last year.

Copernicus Climate Change Service/ECMWF Research has shown huge concentrations of chlorophyll in Europe's warming waters by 2023 (Credit: Copernicus Climate Change Service/ECMWF)Copernicus Climate Change Service/ECMWF

Research has shown huge concentrations of chlorophyll in Europe’s warming waters by 2023 (Credit: Copernicus Climate Change Service/ECMWF)

Cael is the lead author of a recent study published in Naturewhich mapped twenty years of data using NASA satellites.

Together with scientists from the Massachusetts Institute of Technology (MIT), he discovered that more than half of the world’s ocean area – 56% – changed color. To put this into perspective, this area is larger than the entire landmass of the world.

The role of phytoplankton

Although scientists are unsure of the exact causes of these changes, they believe the mass and distribution of phytoplankton play a major role.

Phytoplankton are microscopic, photosynthesizing organisms that form the basis of the marine food web and sustain other organisms further down the food chain, from krill to whales. They contain chlorophyll, the same green pigment that plants use to obtain energy from sunlight during photosynthesis. Phytoplankton also play a major role in transferring carbon dioxide from the atmosphere to the ocean.

Getty Images The greener waters are caused by the blooming of microscopic plants rich in chlorophyll (Credit: Getty Images)Getty Images

The greener water is caused by the blooming of microscopic plants rich in chlorophyll (Credit: Getty Images)

Most of the time, the color of the ocean is a result of what is in the upper layers. In the open ocean this is the phytoplankton ecosystem. Waters that are deep blue contain less life, while greener waters indicate the presence of more phytoplankton.

By studying the wavelengths of sunlight reflected from the ocean’s surface, scientists can estimate how much chlorophyll is present. “Different phytoplankton have different combinations of pigments used for photosynthesis. These pigments absorb light at different wavelengths,” says Cael.

“A glass of water with red food coloring looks red because it contains something that absorbs non-red wavelengths. Phytoplankton, because they are particles in the water, scatter light,” he adds.

‘Virtual Earth’

Modis, an instrument on NASA’s Aqua satellite, makes measurements in seven visible wavelengths, which is a fuller color spectrum than captured in previous studies based on computer models.

Cael used this to create a model to perform simulations. “We have a virtual Earth where we can have two different versions of history. In one case we don’t have climate change, in the other we do,” he says.

Modis takes measurements in seven visible wavelengths, which is a fuller color spectrum than captured in previous studies.

Cael used this to create a model to perform simulations. “We have a virtual Earth where we can have two different versions of history. In one case we don’t have climate change, in the other we do,” he says.

“We can look at how those two virtual Earths diverge over time and what we see there is something similar to what we see in the real ocean,” Cael adds. It was this experiment that discovered that the colors in 56% of the world’s oceans had changed. In particular, the tropical ocean regions near the equator have become steadily greener over time, due to added chlorophyll from increasing phytoplankton.

‘We’re seeing changes in all major ocean basins – it’s not just limited to the Pacific, Atlantic or Indian Oceans [oceans]. These are truly global scale changes that we are seeing,” says Cael.

Esa A plankton bloom larger than that of Greece, stretching across the Barents Sea, off the tip of northern Europe (Credit: Esa)Eza

A plankton bloom bigger than Greece’s and stretching across the Barents Sea, off the tip of northern Europe (Credit: Esa)

In 2019, Dutkiewicz used computer models to predict future changes in the color of the ocean. However, it was difficult to determine whether this was due to climate change or normal oceanic patterns, such as those observed during El Niño and La Niña.

“The natural variability is very large. So whether it is increasing or decreasing due to climate change was quite difficult to say,” says Dutkiewicz.

Cael’s study, which added satellite data, expanded the scope beyond chlorophyll and looked at different wavelengths, including red and blue, as light bounces off the particles and sediment. Dutkiewicz, who also worked on Cael’s research, says the recent paper confirmed her statistical predictions.

“The satellite measurements in the real world match what is seen in the model,” she says. “So it can be inferred that the changes we see in the real world are most likely due to human-induced changes in climate.”

The impact these changes will have on the ocean is likely to be dramatic. Some researchers have predicted that phytoplankton will too is shifting northward at a rate of about 35 km (21 mi) per decade while the tempteruares continue to heat up. This will also lead to changes in the distribution of zooplankton – the small animals that feed on phytoplankton – with species diversity expected to decrease in the tropics and increase dramatically in temperate and subpolar waters, leading to further degradation of biodiversity. effects in the interconnected food chains and fish who depend on these beings.

Emerald green bays and deep blue open stretches of sea will not suddenly change color. But the changes show a trend that could increase as temperatures rise.

“We don’t really care about the color itself,” says Cael. “What matters is that the color change reflects the change in the ecosystem.”

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