(NewsNation) — Over the last two decades, the color of Earth’s oceans have been changing. Scientists say it is likely driven by human-induced climate change, though an exact reason has not been found.

Researchers at the Massachusetts Institute of Technology wrote in a recent study that tropical ocean regions near the equator have become greener over time.

According to the study, published in the British scientific journal Nature, this shift in the color of the water indicates ecosystems in the ocean’s surface are also fluctuating since the ocean’s shade reflects the organisms that reside there.

To track these color changes, a news release from MIT said, those involved with the study analyzed measurements of ocean color taken by the Moderate Resolution Imaging Spectroradiometer, or MODIS, aboard the Aqua satellite. It has been monitoring ocean color for 21 years.

Tropical ocean regions near the equator have become steadily greener over time, researchers said. The shift in ocean color shows ecosystems within the surface ocean must also be changing, as the color of the ocean is a reflection of the organisms and materials that reside in it.

“I’ve been running simulations that have been telling me for years that these changes in ocean color are going to happen,” study co-author Stephanie Dutkiewicz, a senior research scientist in MIT’s Department of Earth, Atmospheric and Planetary Sciences and the Center for Global Change Science, said in a statement. “To actually see it happening for real is not surprising, but frightening, and these changes are consistent with man-induced changes to our climate.”

Other authors included lead author B. B. Cael, of the National Oceanography Center in Southampton, United Kingdom; Stephanie Henson of the National Oceanography Center; Kelsey Bisson at Oregon State University and Emmanuel Boss of the University of Maine. NASA partially funded this project.