Warming climate changed colour of over 56% world’s ocean: Study

NEW YORK:  Human-induced climate change has changed the colour of over 56 per cent of the world’s oceans in the last 20 years, according to an alarming study.

In particular, the tropical ocean regions near the equator have become steadily greener over time, reveal the study published in the journal Nature. The shift in ocean colour indicates that ecosystems within the surface ocean must also be changing, as the colour of the ocean is a literal reflection of the organisms and materials in its waters.

Scientists including from the Massachusetts Institute of Technology (MIT) in the US and the National Oceanography Center in the UK detected the changes in ocean colour over the past two decades which according to them cannot be explained by natural, year-to-year variability alone. 

These colour shifts, though subtle to the human eye, have occurred over 56 per cent of the world’s oceans — an expanse that is larger than the total land area on Earth.

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

“This gives additional evidence of how human activities are affecting life on Earth over a huge spatial extent. It’s another way that humans are affecting the biosphere,” added lead author BB Cael, of the National Oceanography Center in Southampton, UK.

To understand the effect of climate change in oceans, scientists earlier monitored only phytoplankton — plant-like microbes that are abundant in the upper ocean and that contain the green pigment chlorophyll, and are foundation of the marine food web as well as enable oceans to capture and store carbon dioxide.

But they realised it would take at least 30 years of continuous monitoring to detect any trend that was driven specifically by climate change.

In the current study, Cael and the team analysed measurements of ocean colour taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite, which has been monitoring ocean colour for 21 years. MODIS takes measurements in seven visible wavelengths, including the two colours researchers traditionally use to estimate chlorophyll.

The differences in colour that the satellite picks up are too subtle for human eyes to differentiate. Much of the ocean appears blue to our eye, whereas the true colour may contain a mix of subtler wavelengths, from blue to green and even red.

Cael carried out a statistical analysis using all seven ocean colours measured by the satellite from 2002 to 2022 together. He first looked at how much the seven colours changed from region to region during a given year, which gave him an idea of their natural variations.

He then zoomed out to see how these annual variations in ocean colour changed over a longer stretch of two decades. This analysis turned up a clear trend, above the normal year-to-year variability.