The oceans are becoming greener, as satellite images show. This is likely due to changes in plankton communities. This could affect marine life and the amount of carbon the oceans can store.

Ask the children to draw a picture of the ocean and they will take a blue crayon. But now it seems that the oceans are due to climate change getting greener and greener. In the last twenty years, much of the sea has changed color slightly. This is because phytoplankton responds to the consequences of global warming.

Oceanographer BB Cael from the National Oceanography Center in Southampton, United Kingdom, and colleagues studied data collected by the MODIS instrument on NASA’s Aqua satellite. They looked at what kind of radiation was emitted from the water surface between 2002 and 2022.

During this period, 56 percent of the world’s oceans were found to have experienced a notable color change. Overall, the oceans have become greener. Cael and his colleagues compared the results with computer climate models, which showed that climate change is the cause of the color change.

Low nutrient layer

According to the marine biologist Tammy Richardson from the University of South Carolina, who was not involved in the study, the results confirm previous suspicions about how the oceans respond to climate change. “It gives us much stronger evidence that the ocean is getting greener than what little data we had in the past,” she says.

The color changes are probably due to a strengthening of ocean stratification or ocean stratification. That’s the natural separation between the warmer, less dense water at the surface and the cooler, denser water below.

“As the ocean warms, there is less water exchange between the top and the bottom,” says Cael. “This makes it difficult for nutrients to reach the top and light part.” As a result, the phytoplankton communities that live in the upper waters and depend on this nutrient upwelling are changing.

It is still unclear exactly how phytoplankton populations are changing. Phytoplankton with smaller cells can become more dominant, so that this type occurs in increasing concentrations. “It’s not easy to unravel the changes in light and determine what is in the water that causes them,” Cael says.

food chain

The changes probably won’t be so dramatic that children will soon choose green crayons for their ocean drawings. But even subtle changes in ocean color can have drastic consequences for the marine food chain. This also has an impact on the ocean’s ability to store carbon. It’s just not yet clear exactly how these consequences will manifest.

‘There are several ways the ecosystem can change. They all produce the same light signature,” says Cael. “We need more detailed observations of light.” He says NASA’s PACE satellite, launching in January 2024, will provide more information.