More than half of the deep blue sea is turning green, but scientists aren’t sure why.

In the past two decades, 56 percent of Earth’s oceans have become greener — an area larger than the planet’s total land mass, according to a new study by a team of scientists from the United States and the United Kingdom.

“The reason we care about this is not because we care about the colour, but because the colour is a reflection of the changes in the state of the ecosystem,” said lead author of the study B. B. Cael, an ocean and climate scientist at the National Oceanography Centre in Southampton, UK, as The Guardian reported.

There are a host of things that can lead to changes in the ocean’s color, reported the journal Nature. One example is when deep-sea nutrients rise up to feed on phytoplankton blooms containing green-tinted chlorophyll.

Scientists are able to approximate the levels of chlorophyll, as well as how many organisms like algae and phytoplankton there are, by observing sunlight wavelengths reflected off the surface of the ocean.

In surface waters, the amount of chlorophyll can vary greatly each year, so it can be difficult to pick up differences between natural changes and those brought on by climate change.

Theoretically, warmer ocean waters due to climate change should lead to differences in biological productivity, but scientists believe it could take as long as four decades to be able to pinpoint any clear shifts.

“These are not ultra, massive ecosystem-destroying changes, they may be subtle,” Cael said, as The Guardian reported. “But this gives us an additional piece of evidence that human activity is likely affecting large parts of the global biosphere in a way that we haven’t been able to understand.”

The study, “Global climate-change trends detected in indicators of ocean ecology,” was published in Nature.

The research team looked at data from NASA’s Aqua satellite sensor Moderate Resolution Imaging Spectroradiometer (MODIS) to try and spot trends in seven distinct ocean light wavelengths, reported Nature.

“I’ve thought for a long time that we could do better by looking at the full colour spectrum,” Cael said.

By analyzing 20 years of MODIS data, the researchers noticed long-term differences in the color of the ocean. Most of the changes were in waters between the tropical and subtropical latitudes 40 degrees South and 40 degrees North. The waters in these regions don’t usually have significant color changes during the year due to their lack of extreme seasons, so Cael said the team was able to pick up on smaller long-term shifts.

“On the whole, low-latitude oceans have become greener in the past 20 years,” the study said.

In order to find out if the changes could be caused by climate change, the scientists used the results of a simulation model that played out the possible responses of marine ecosystems to increasing levels of atmospheric greenhouse gases. They found that the changes in the model matched those of their observations.

Cael said the cause of the increased greenish hue of the ocean is likely not the warming ocean because the parts of the ocean that had changed color didn’t match those where temperatures have increased. Cael went on to say that the distribution of nutrients could have affected the shift, as stratification of upper ocean layers occurs as surface waters warm, making it more difficult for nutrients to rise. Fewer nutrients mean smaller phytoplankton can survive better, which alters the ecosystem and could affect the overall color of the water.

However, the scientists aren’t exactly sure why the ocean is changing color.

NASA’s next big mission to observe the color of the ocean will be the Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) satellite — set to launch in early 2024 — which will be able to measure the ocean’s color in more wavelengths than any satellite has before.

“All of this definitely confirms the need for global hyperspectral missions such as PACE,” said Ivona Cetinić, an oceanographer at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who works on PACE, according to Nature. The satellite “should allow us to understand the ecological implications of the observed trends in ocean ecosystem structure in years to come.”

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