The colour of over 56 per cent of the world's oceans, larger than Earth's total land expanse, has changed significantly over the last two decades and human-caused climate change is likely the driver, according to researchers.
These colour changes, subtle to the human eye, cannot be explained by natural, year-to-year variability alone, the researchers from the Massachusetts Institute of Technology (MIT), US, and other institutes write in their paper published in the journal Nature.
Ocean colour, a literal reflection of the life and materials in its waters, in regions near the equator was found to have steadily turned greener over time, indicating changes in the ecosystems within the surface oceans.
The green colour of the ocean waters comes from the green pigment chlorophyll present in phytoplankton, the plant-like microbes abundant in upper ocean. Scientists are, therefore, keen to monitor phytoplankton to see their response to climate change.
However, the authors of this study showed through previous studies that it would take 30 years of tracking chlorophyll changes before climate-change-driven trends would show, because natural, annual variations in chlorophyll would overwhelm those influenced by human activities.
In a 2019 paper, study co-author Stephanie Dutkiewicz and her colleagues showed that monitoring other ocean colours, whose annual variations are much smaller than those of chlorophyll, would convey more clear signals of climate-change-driven changes and that they might even be apparent in 20 years, rather than 30.
"It's worth looking at the whole spectrum, rather than just trying to estimate one number from bits of the spectrum," said lead author B. B. Cael of the National Oceanography Center.
Cael and team then statistically analysed all the seven ocean colours recorded by satellite observations from 2002 to 2022 together. He initially studied the colours' natural variations by seeing how they changed regionally in a given year.
He then observed how these annual variations changed over two decades.
To understand climate change's contribution to all these changes, he used Dutkiewicz's 2019 model to simulate the Earth's oceans under two scenarios - one with greenhouse gases and the other without them.
The greenhouse-gas model predicted changes to the colour of about 50 per cent of the world's surface oceans in under 20 years - close to Cael's conclusions from his real-world satellite data analysis.
"This suggests that the trends we observe are not a random variation in the Earth system," said Cael.
"This is consistent with anthropogenic climate change."
"I've been running simulations that have been telling me for years that these changes in ocean colour are going to happen," said Dutkiewicz, senior research scientist in MIT's Department of Earth, Atmospheric and Planetary Sciences.
"To actually see it happening for real is not surprising, but frightening. And these changes are consistent with man-induced changes to our climate," she said.
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