Earth's deep blue oceans are fading and changing color — and scientists aren't sure why

• Earth's oceans are getting greener, scientists have found using satellite imagery.
• More than 56% of the Earth's oceans have changed color over the past 20 years.
• It's not clear exactly what's driving this, though it's likely linked to the changing climate.

Earth's deep blue oceans have been fading and changing in color. Scientists aren't exactly sure why, but they think humans are to blame.

Two decades of satellite images have revealed that more than 56% of the world's oceans have seen significant changes in color.

The change is most dramatic in tropical oceans, which have become greener over the past two decades, per a study published in July in the journal Nature.

Scientists fear these color changes could be harbingers of yet another global crisis in ocean food chains as the planet warms.

"This gives additional evidence of how human activities are affecting life on Earth over a huge spatial extent," B.B. Cael a researcher at the National Oceanography Center in Southampton, UK, and an author on the study, said.

Greener oceans mean more life — and that's not necessarily good

The color of the oceans can tell us a lot about their health.

"The reason we care about this is not because we care about the color, but because the color is a reflection of the changes in the state of the ecosystem," said Cael, per The Guardian.

The ocean's color depends on what's in the upper layer of the water column. A deep blue sea doesn't have not much life in that upper layer.

When an ocean turns green, however, that usually means the ocean is teeming with phytoplankton, tiny organisms that can use sunlight to capture carbon and turn it into sugars, just like plants do.

In the latest study, scientists used imagery from NASA's Aqua satellite, which has been monitoring ocean color changes for more than two decades.

They found that while some oceans are getting marginally redder or bluer, most of the oceans that have seen a change in color are getting greener, which could mean more phytoplankton.

That's not necessarily a good thing.

Though these plankton are crucial to marine ecosystems — they form the basis of a complex food chain that feeds everything from sea birds to whales — ocean ecosystems are very finely tuned, and any change to the ecosystem can ripple across the food chain.

"All changes are causing an imbalance in the natural organization of ecosystems. Such imbalance will only get worse over time if our oceans keep heating," said Stephanie Dutkiewicz, a researcher at MIT's Department of Earth, Atmospheric and Planetary Sciences and the Center for Global Change Science who worked on the study, to CNN.

Scientists are racing to understand why our oceans are fading

For now, these changes are small and likely not big enough to cause a major overhaul of the food systems, said Cael, per The Guardian. Still, these results are "sobering," Dutkiewicz, told CNN.

"I've been running simulations that have been telling me for years that these changes in ocean color are going to happen," she said in a press release. "To actually see it happening for real is not surprising, but frightening," she said.

Scientists will now try to understand exactly what is causing the changes in color to better predict how our oceans could evolve as the planet gets warmer.

Most likely, it will come down to "multiple factors changing in parallel," said Michael J Behrenfeld, a researcher of ocean productivity at Oregon State University, who was not involved in the research, The Guardian reported.

It's possible, for instance, that microplastics in the water could contribute to the changes in color.

Asked if these changes in color might ever become bright enough to be visible to the naked eye, Dutkiewicz told CNN it's difficult to know for sure.

"If a big tipping point was reached in some places: maybe," she said.