Over the past 20 years, huge swathes of seas around the globe have changed color, displaying a subtle greening towards the tropics that researchers say points to the effect of climate change on life in the world's oceans.

In the new research published on Wednesday, scientists said they had detected shifts in colors across more than half of the world's oceans – an expanse bigger than Earth's total land area.

The authors of the study published in the Nature magazine think that is down to changes in ecosystems, particularly in tiny plankton, which are the centerpiece of the marine food web and play a crucial part in stabilizing our atmosphere.

"The reason we care about the color changes is because the color reflects the state of the ecosystem, so color changes mean ecosystem changes," said the lead author, B.B. Cael, of Britain's National Oceanography Centre.

The color of the seas, when seen from space, can paint a picture of what is going on in the upper layers of the water.

A deep blue would tell you that there is not much life, while if the water is greener, it is likely to have more activity, specifically from the photosynthesising phytoplankton, which, like plants, contain the green pigment chlorophyll.

These produce a significant amount of the oxygen we breathe, are a crucial part of the global carbon cycle and are a foundational part of the ocean food web.

Researchers are keen to develop ways of monitoring changes in ecosystems in order to track climate changes and enshrine protected areas. 

But previous studies have suggested it would take three decades of ocean chlorophyll monitoring to detect a trend because of annual variations.   

In the latest study, researchers broadened the color spectrum, looking at seven hues of ocean color monitored by the MODIS-Aqua satellite from 2002 to 2022.

These are too subtle for humans to see and would look largely blue to the naked eye. 

The authors analyzed the observational data to detect a trend above the year-to-year variability and then compared it to computer models of what would be expected with climate change.

They found that the real-world observations tallied closely with the changes predicted.  

While the researchers said more work would be needed to find out what exactly those color changes might mean, they said climate change was very likely to be the cause. 

"I've been running simulations that have been telling me for years that these changes in ocean color are going to happen," said co-author Stephanie Dutkiewicz, of MIT's Department of Earth, Atmospheric and Planetary Sciences at the Center for Global Change Science. 

"To actually see it happening for real is not surprising, but frightening. And these changes are consistent with man-induced changes to our climate."

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