Why the oceans are changing colour and what it means

For centuries, the poets and romantics have described the vast ocean as ethereal and striking blue. But now, satellites are revealing that Earth’s oceans are subtly but significantly shifting in colour.

A 2023 study led by B. B. Cael from the UK’s National Oceanography Centre, published in Nature, found that over half of the world's ocean surface — 56% — has changed colour in the last 20 years. The likely driver? Human-induced climate change.

Cael and fellow researchers from MIT and NASA used two decades of satellite data to monitor the ocean’s colour spectrum, particularly through NASA's Aqua satellite and its MODIS instrument. This tool captures light in seven visible wavelengths, offering a deeper look at subtle shifts in the ocean's hue than earlier models ever could.
The study’s results were striking: tropical regions near the Equator are turning greener, while other zones, especially in low-productivity waters, are growing bluer. These shifts are tied closely to the health and distribution of phytoplankton — microscopic, plant-like organisms that not only fuel marine ecosystems but also influence the ocean’s colour.


Phytoplankton contain chlorophyll, which gives them a green tint and plays a key role in photosynthesis. When their populations grow or move, they scatter and absorb sunlight differently, changing how the ocean reflects light. This is why greener waters often suggest a higher concentration of life, while deeper blue indicates fewer organisms.


The recent European State of the Climate report highlighted dramatic changes in chlorophyll levels: increases of 200–500% in the Norwegian Sea and North Atlantic in April 2023, and significant drops west of the Iberian Peninsula. In the Mediterranean, chlorophyll spiked 50–100% above average by June. These patterns suggest that warming seas are disrupting the balance of marine ecosystems, not just temporarily, but potentially long-term.

What makes this even more concerning is that these colour changes are invisible to the naked eye. “The colour itself is not something that you can even see that well,” Cael notes. Yet the shift is detectable through satellite data, and it’s not confined to one region. “We're seeing changes in all the major ocean basins.”

This isn’t the first time scientists have tried to predict such transformations. In 2019, MIT oceanographer Stephanie Dutkiewicz used models to project that ocean colours would eventually shift due to warming. But natural variability — like El Niño and La Niña — made it difficult to confirm. Cael’s study, however, provides real-world evidence that confirms those predictions.