The balance of phytoplankton populations in the sea is shifting due to human-induced climate change, profoundly altering ocean ecosystems.

Contrary to the idyllic image of sparkling turquoise waters, recent research indicates that significant portions of the world's oceans are becoming greener, particularly near the equator, while others are growing bluer as global temperatures rise.

Although these color shifts are not visible to the naked eye, satellite studies have been able to map them.

B.B. Cael, a scientist at the National Oceanography Centre in Southampton, UK, explains that these changes are imperceptible to humans but might be visible to creatures like mantis shrimp or butterflies.

The European State of the Climate report, published in April 2024 by the European Union's Copernicus Climate Service, highlights the extent of these changes. In April 2023, chlorophyll levels, which give phytoplankton their green hue, were 200-500% higher than average in the Norwegian Sea and Atlantic Ocean north of the UK, but 60-80% lower west of the Iberian Peninsula. In June 2023, the Mediterranean Sea saw chlorophyll levels 50-100% higher than average, with the average based on data from 1998-2020.

These color changes suggest warming ocean temperatures beyond natural yearly variations. BBC News analysis of sea surface temperatures from Copernicus data revealed record ocean warming over the past year.

Cael, the lead author of a recent study in Nature, collaborated with scientists from MIT to analyze two decades of NASA satellite data. They found that 56% of the world's ocean area has changed color, an area larger than all the world's landmass.

Phytoplankton, microscopic photosynthesizing organisms that form the marine food web's base, play a significant role in these changes. They contain chlorophyll and contribute to carbon dioxide transfer from the atmosphere to the ocean. The ocean's color often reflects the upper layers' contents, with blue waters indicating less life and green waters signifying more phytoplankton.

By analyzing sunlight wavelengths reflected off the ocean's surface, scientists estimate chlorophyll levels. Different phytoplankton have various pigments for photosynthesis, absorbing light at different wavelengths, explains Cael.

Using the Modis instrument on NASA's Aqua satellite, which measures seven visible wavelengths, Cael created a model for simulations. This "virtual Earth" model compared scenarios with and without climate change, revealing that 56% of the world's oceans had changed color, especially in tropical regions near the Equator due to increasing phytoplankton.

These changes are global, affecting all major ocean basins, confirming a theory from a 2019 study by Stephanie Dutkiewicz of MIT, which predicted future changes in ocean color using computer models. Cael's study, incorporating satellite data, confirmed that these changes are likely due to human-induced climate change.

The impact of these changes on the ocean will be dramatic, with predictions that phytoplankton will shift north at about 35 km per decade as temperatures rise. This will affect the distribution of zooplankton and fish, leading to decreased species richness in the tropics and increased richness in temperate and subpolar waters.

While the ocean's colors won't change overnight, these shifts indicate a significant trend linked to warming temperatures. "The change in color reflects the change in the ecosystem," says Cael, highlighting the broader ecological implications.