Why oceans are changing colour

As a result of human-induced climate change,  oceans are changing in profound ways and the usual turquoise waters are turning greener. According to recent research,  reason is the shifting balance of phytoplankton populations in the water. In low latitudes near the equator, others are becoming bluer as global temperatures rise.

“The colour itself is not something that’s easy to describe with human language or that you can even see that well, but 20 years of Nasa satellite studies are mapping the change and it is even possible that  a mantis shrimp or a butterfly could see it” – B Cael, lead author and scientist at the National Oceanography Centre in Southampton in the UK.

Cael and other scientists from Massachusetts’s Institute of Technology (MIT), found that more than half the world’s ocean area – 56% – changed colour. To put this into perspective, this area is larger than all of the world’s landmass.

The 2024 European State of the Climate report, showed the extent of oceanic change. Chlorophyll, the photosynthetic pigment in phytoplankton and plants, providing its green hue, was 200-500% higher than average in the Norwegian Sea and Atlantic Ocean north of the UK in April 2023. At the same time, it was 60-80% lower in the ocean west of the Iberian Peninsula. The Mediterranean Sea saw Chlorophyll levels 50-100% higher than average in June 2023. Scientists believe these shifts in colour far beyond natural year-to-year variations,  a sign of warming ocean temperatures. 

Typically, the ocean’s colour is a result of what lies within its upper layers. In the open ocean, this is the phytoplankton ecosystem. Deep blue waters contain less life, while greener waters signify more phytoplankton. By studying the wavelengths of sunlight reflected off the ocean’s surface, scientists can estimate how much chlorophyll is present. Different phytoplankton has different combinations of pigments used to photosynthesise. These pigments absorb light at different wavelengths. Phytoplankton, as particles in water, scatter light.  

 Virtual Earth

 Modis, an instrument of NASA’s Aqua satellite, takes measurements in seven visible wavelengths, providing a fuller colour spectrum than in previous studies based on computer models. According to Cael, a virtual earth has been created where scientists can track two different versions of history. “In one case, we don’t have climate change. In the other case, we do,” he says.

With the latest information, it is possible to see two virtual Earths diverge over time, similar to what is seen in the real ocean.  It was this experiment which discovered that the colours in 56% of the world’s oceans had changed. This is occurring on a global scale in all the major ocean basins, not only in the Pacific, Atlantic and Indian Oceans. This confirms a theory in an earlier paper by Stephanie Dutkiewicz, a senior ocean scientist with MIT and the Center for Global Change Science.

A Female scientist’s findings confirmed

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

Cael’s study, adding satellite data, widened the scope beyond chlorophyll, looking at different wavelengths, including red and blue, as light bounces off the particles and sediment. Dutkiewicz, who also worked on Cael’s study, says the recent paper confirmed her former statistical predictions. “The real-world satellite measurements are consistent with what is seen in my model,” she says. “So by inference, the changes we’re seeing in the real world are very likely due to human-induced changes to the climate.”

The impact of these changes would have on the ocean are likely to be dramatic. Some researchers have predicted that phytoplankton will shift north at a speed of around 35km (21 miles) each decade as temperatures continue to rise. This will lead to changes in the distribution of zooplankton as well. Species richness are expected to decline in the tropics and increase dramatically in temperate and sub-polar waters, leading to more knock-on effects in the interconnected food webs and fish  relying on zooplankton.

Emerald coves and deep blue open stretches of sea won’t suddenly change colour overnight. But the changes reveal a trend that may increase as temperatures warm. Scientists are not concerned about the colour per se, but the changes it signifies in the ecosystem.