Much like raging wildfires, scorching heat waves and drenching floods, the ocean’s changing color is yet another warning sign of human-driven climate change, according to a new study that analyzed two decades’ worth of specialized satellite observations.

Much of that change has to do with phytoplankton, the microscopic marine algae that live in the water’s upper layer. Like grass and trees, phytoplankton use a green pigment called chlorophyll to convert sunlight into food.

That pigment is often seen from space, and it’s the main indicator scientists use to study the ocean’s color. But phytoplankton are very susceptible to climate change — a process that’s throwing their populations out of whack and, thus, changing the ocean’s color, the study found.

“These ecosystems have taken millions of years to evolve together and be in balance,” said Stephanie Dutkiewicz, a researcher at the Massachusetts Institute of Technology who co-authored the study, which was published last week in Nature. “Changes in such a short amount of time are not good because they put the whole ecosystem out of balance.”

Dutkiewicz predicted those changes — and the effects they could have on marine life — in 2019. And even then, she said satellites would “be the sentinels” in determining whether the ocean’s color was shifting.

In the new study, the researchers first analyzed data from NASA’s Aqua MODIS satellite, which since 2002 has been monitoring ocean color changes, some of which are too subtle for human eyes to see. Twenty years-worth of data showed that colors had shifted in more than half of the world’s oceans, the study states. And scientists said the changes went beyond what’s expected due to natural occurrences.

Then, to find out whether that trend was related to climate change, the researchers compared those findings with the results of two models. One of them, Dutkiewicz said, simulated what would happen to the ocean’s colors if greenhouse gases weren’t heating the planet. The other model added in the presence of emissions, which resulted in a shift in color in 50 percent of the ocean — a pattern consistent with the satellite’s observations.

Dutkiewicz said it was a worrisome sign for the future of the planet.

“I knew that this could happen because I’ve been working on these models for 10 to 15 years, so it’s not surprising,” she said. “But now we can see it firsthand — we have a signal of it going on in the real world. And that’s frightening because it means it’s not just my computer saying this anymore: It’s satellite sensors saying, ‘Yes, the ocean’s color is changing and really fast.’”

The ocean displays an impressive range of colors — a result of how light waves interact with the molecules in the water, getting either scattered or absorbed, said Ivona Cetinić, an oceanographer with NASA Goddard Space Flight Center. The ocean’s composition, or the nutrients and lives it sustains, is what defines the color of its water, she said.

“We think about ocean as this big bunch of water, but it has a huge variation in ecosystems and organisms and nutrients,” Cetinić said. “There’s no other way of understanding what’s happening and observing all of that continuously than from space — and the only way we can do it is by looking at the different colors in the ocean.”

In the ocean, water can have a brownish tint if it’s loaded with dead leaves and sediments spewing from rivers, she said. In other places, the ocean can showcase a range from deep navy blue to a greener-looking shade — and that’s where phytoplankton come into play.

On the show “SpongeBob SquarePants,” plankton is a power-hungry villain. But in real life, the tiny plants are responsible for up to half of the oxygen we breathe. They help suck up much of the atmosphere’s carbon and are the backbone of the marine food web, serving as food for zooplankton, which then get eaten by fish, which fuel even bigger fish, and so on.

Water that has a larger density of phytoplankton — like that in the tropics — tends to look greener, while water with fewer phytoplankton is bluer. Now, however, the cascading effects of climate change are taking a toll on phytoplankton, Dutkiewicz said.

In some places, rising temperatures are changing ocean currents, disrupting the flow of deep-sea nutrients that surface-dwelling phytoplankton need to survive. That lack of nutrition can decrease phytoplankton populations — turning the water bluer. In other areas, Dutkiewicz said the water has taken on a greener tint as phytoplankton populations increase — a boom that can be “too much” for the ecosystem to sustain.

Warmer water and changes in its acidity — a product of more carbon dioxide dissolving into the ocean — can also transform the species of phytoplankton that live in different areas. That not only changes the water’s color but also affects food chains, Dutkiewicz said.

“If one phytoplankton was the size of a tennis ball, the largest would be the size of Manhattan, meaning they are going to support very different food webs,” Dutkiewicz said. “Climate change can cause a shift equivalent to, say, having palm trees grow in the middle of a tundra. And then you know, what do the elks do since they’re not used to palm trees?”

For now, at least, the changes in the ocean’s colors are hard to perceive with the naked eye — “it’s not like you’re going to go to the beach one day and it’s a different color,” Dutkiewicz said.

“But just because you can’t see it, it doesn’t mean it’s not happening and that it doesn’t have the capability of impacting us in different ways,” Dutkiewicz added.

She said it should be a wake-up call — “though we’ve gotten quite a few of those over the years.”

“It’s almost like we keep pressing the snooze button on an alarm that’s telling us we have to act now,” she added.