Tropical waters have turned green in colour, courtesy climate change: Study

The green hue comes from chlorophyll, a pigment that helps microscopic plant-like phytoplankton make food; change in ocean colour indicates alterations in phytoplankton communities
IStock photo for representation.
IStock photo for representation.

Climate change has altered the colour of 56 per cent of the world’s oceans, according to a new study.

The waters in the tropics have turned green, the study published in Nature stated. The southern Indian Ocean, in particular, has seen a significant colour change.

Green-coloured water indicates life, especially phytoplankton, which are microscopic plant-like organisms. Blue, in contrast, indicates little life. The colour also determines the amount of carbon dioxide absorbed by the ocean. Currently, oceans absorbed 25 per cent of all carbon dioxide (CO2) emissions.

“Changes in the ocean colour indicate alternations to the phytoplankton communities — since phytoplankton are essential for most life in the ocean as the base of the marine food web (in the same way as plants are essential for life on the earth),” study co-author Stephanie Dutkiewicz, a senior research scientist at the Massachusetts Institute of Technology, told Down To Earth. Also, phytoplankton absorbs CO2.

In 2019, Dutkiewicz and her colleagues published a study which predicted that more than 50 per cent of the world’s oceans will shift in colour due to climate change by 2100.

In the new study, the researchers looked at long-term trends. They analysed data generated from the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite, which has been monitoring ocean colour for two decades between 2002 and 2022.

MODIS takes measurements in seven visible wavelengths (Light of different wavelengths produces different perceptions of colour).

Human eyes, the researchers explained, are not sensitive enough to differentiate subtle colour changes. The oceans appear blue, but the true colour may contain a mix of subtler wavelengths, from blue to green and even red.

The oceans’ hue changes naturally from one year to another. So the researchers observed how the seven colours changed from region to region during a given year.

Next, they checked how annual variations in ocean colour have changed over the two decades. This analysis showed that climate change was driving the change.

The team then used to model to simulate two scenarios: one with the addition of greenhouse gases and the other without them.

The scenario that considered the addition of greenhouse gas emissions showed that colour could change in about 50 per cent of the world’s surface oceans, which is comparable to satellite observations. These observations estimated that 56 per cent of the saltwater bodies are turning green or blue.

“I’ve been running simulations that have been telling me for years that these changes in ocean colour are going to happen,” Dutkiewicz said in a statement.

“To actually see it happening for real is not surprising, but frightening. And these changes are consistent with human-induced changes to our climate,” she added.

The green hue comes from chlorophyll, a pigment that helps phytoplankton make food. A change in colour due to an increase or decline in the population will impact organisms that feed on plankton.

“It will also change how much the ocean will take up carbon because different types of plankton have different abilities to do that. So, we hope people take this seriously,” Dutkiewicz explained.

Though the southern Indian Ocean is seeing a significant change, the waters near India are not following the same trend.

Dutkiewicz, however, explained that this doesn’t mean things aren’t changing. “It very likely means that the natural variability in that region [near India] is very large and so the signal of the trend is still masked,” she added.

The researchers also noted that these findings could help experts and policymakers designate marine protected areas where human activity is restricted.

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