The oceans are changing color. The balance of phytoplankton populations in the sea is shifting as a result of human-induced climate change, altering the oceans in profound ways.

       When you picture the ocean, you might imagine sparkling turquoise waters, but recent research suggests swathes of our world’s oceans may in fact be turning greener, and the leading culprit might be climate change.  In addition, while some waters are becoming more green, particularly in low latitudes near the equator, others are growing more blue as global temperatures rise. Although this shift in colour isn’t visible to the naked eye, satellite studies are able to map the change. 

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

       “The colour itself is not something that’s easy to describe with human language, or that you can even see that well,” says B. B. Cael, a scientist at the National Oceanography Centre in Southampton in the UK. Alongside scientists from Massachusetts Institute of Technology (MIT), he 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.

       Although scientists aren’t sure about the exact causes of these changes, they believe the mass and spread of phytoplankton plays a huge part. 

       Phytoplankton are microscopic, photosynthesising organisms which form the base of the marine food web, sustaining other organisms further up the food chain from krill to whales. They contain chlorophyll, the same green pigment that plants use to harvest energy from sunlight when photosynthesising. Phytoplankton also play a large part in transferring carbon dioxide from the atmosphere to the ocean. 

       By studying the wavelengths of sunlight reflected off the ocean’s surface, scientists can estimate how much chlorophyll is present. “Different phytoplankton have different combinations of pigments used to photosynthesise. These pigments absorb light at different wavelengths,” says Cael. To map the spectrum of colour changes, Cael drew from 20 years of data from an instrument onboard Nasa’s Aqua satellite, known as the Modis (Moderate Resolution Imaging Spectroradiometer) data.

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

       “We’re seeing changes in all the major ocean basins – it’s not confined to just the Pacific or Atlantic or Indian [oceans]. These are really global scale changes that we’re seeing,” says Cael. This confirms a theory in an earlier paper from Stephanie Dutkiewicz, an oceans scientist in MIT and the Center for Global Change Science. 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 those observed during El Niño and La Niña.

       Cael’s study, which added 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 statistical predictions. 

       The impact these changes will 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 warm. This will lead to changes in the redistribution of zooplankton – the tiny animals that feed on the phytoplankton, leading to further knock on effects in the interconnected food webs and fish that rely upon these creatures.

       “It’s not really the colour itself that we care about,” says Cael. “What matters is the change in colour reflects the change in the ecosystem.” 

       I am writing about this because it is just another example of how we are changing the environment without having any idea of what we are doing. Fortunately, a few scientists are developing techniques that will allow us to understand a little better what we are doing. However, whether we are willing to then do something about it is a whole different question.

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