Climate change may be changing colour of Earth’s oceans: Study

PTI, Jul 25, 2023, 9:42 AM IST

Image for representation

Climate change has likely changed the colour of the Earth’s oceans significantly over the past two decades, according to a study.

The research, published recently in the journal Nature, detected changes in ocean colour over the past 20 years that 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, the researchers said.

The team of researchers from the Massachusetts Institute of Technology (MIT), US, the National Oceanography Center in the UK and others found that tropical ocean regions near the equator have, in particular, become steadily greener over time.

According to the researchers, 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.

It is not certain how exactly marine ecosystems are changing to reflect the shifting colour, the researchers said, adding, however, human-induced climate change is likely the driver.

”I have been running simulations that have been telling me for years that these changes in ocean colour are going to happen,” said study co-author Stephanie Dutkiewicz, senior research scientist in MIT.

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

The ocean’s colour is a visual product of whatever lies within its upper layers. Generally, waters that are deep blue reflect very little life, whereas greener waters indicate the presence of ecosystems, and mainly phytoplankton — plant-like microbes that are abundant in upper ocean and that contain the green pigment chlorophyll.

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 researchers carried out a statistical analysis using all seven ocean colours measured by the satellite from 2002 to 2022 together.

They first looked at how much the seven colours changed from region to region during a given year, which gave them an idea of their natural variations.

The team 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.