View of Miami skyline including Bird Key, a private island on Biscayne Bay that is now for sale, upsetting environmentalists because developers could build on the island, displacing all the birds, on Thursday, May 23, 2024. pportal@miamiherald.com

For thousands of years, writers have come up with increasingly creative ways to describe the “wine-dark sea.” But a new study suggests that modern poets may be faced with a slightly different palette.

A study published Thursday in the journal Science found that the ocean is changing color as it warms.

By analyzing satellite data from 2003 to 2022, researchers from Duke University and the Georgia Institute of Technology noticed that waters near the equator were getting bluer, while areas near the poles were turning greener.

Lead author Haipeng Zhou calls it “this greener greens or bluer blues phenomenon.” The culprit, the paper suggests, is the teeny tiny plant-like creatures that form the building blocks of the marine food web — phytoplankton.

A graphic showing how concentrations of chlorophyll have shifted in different parts of the globe, leading to “bluer blues” near the equator and “greener greens” near the poles. Haipeng Zhao

Phytoplankton are filled with a green pigment called chlorophyll that allows them to absorb energy from sunlight through photosynthesis. As waters near the equator warm, they’re less hospitable to the microscopic critters, so the water appears bluer.

At the poles, the colder water is far more nutrient-dense, so there’s lots more phytoplankton to go around, giving the waters a rich, green tinge.

It’s not a new phenomenon. There’s a reason the Caribbean is known as the land of dazzling turquoise waters, while Arctic waters are a dark teal contrast to the ivory ice floes around them.

But Zhou, who began the research at Duke University and completed it as a postdoctoral researcher at Georgia Tech, found that as the ocean warms, this phenomenon is getting stronger.

Their study only looked at the open ocean, not coastal waters. Near the coast, there are plenty of complicating factors that make it hard to clearly point to phytoplankton as the main cause of color changes, like dirt and sand floating in the water, shifting winds, pollution or even seagrass die off.

To understand the concentrations of phytoplankton, the research team relied on a National Oceanic and Atmospheric Administration database made up of samples scooped from various research cruises all over the world. This database, plus satellite imagery, helped show the color shift in different regions of the ocean.

The changing levels of phytoplankton could be bad news for fish in the tropics, or for communities that rely on those fish. But it could also be a boon for their northern and southern cousins.

“We all know that phytoplankton is the bottom of the food chain. Any impact on phytoplankton will have impacts on its predators,” Zhou said.

Phytoplankton need sun and nutrients to flourish, but when the ocean warms, the individual layers that make up the sea grow more stratified, so it’s harder for phytoplankton to float up and down the water column to access the same light and nutrients they’re used to, he said.

So while scientists can clearly say that warmer waters lead to fewer phytoplankton, and they know that climate change is one of the main reasons the oceans are heating up, it’s not clear if climate change is the reason for the color shift.

“We need longer records, 30 years, 40 years, to make us more confident whether it is linked to climate change or global variability,” Zhou said.

The study only looked at about 20 years of data, which Zhou said is enough to confidently say that something has changed, but not long enough to know what caused that change. Other factors can and do affect water temperature, like the shift in trade winds over the Atlantic that led to a coral-killing marine heat wave in 2023.

“The study period was too short to rule out the influence of recurring climate phenomena such as El Niño,” wrote co-author Susan Lozier, Dean of the College of Sciences at Georgia Tech, in a statement.

However, Zhou added, more research may very well find that this color-shifting trend continues into the future as human-caused climate change continues to heat up the oceans.

“The temperature of the water is rising. While there’s no evidence showing that this progress will slow down, it’s very likely we’ll have warmer waters in the future, which means we’ll have a continuous impact on the ocean ecosystem.”

Correction: An initial version of this story incorrectly listed the start of the period analyzed on satellite as 2009. It was 2003.

This story was originally published June 19, 2025 at 2:00 PM.