The colour of the ocean is changing, and scientists blame global warming

NEW YORK – More than half of the world’s ocean has changed colours in the past 20 years, a phenomenon that is likely driven by climate change, according to a study published on Wednesday in the journal Nature. 

The study, which analyses decades’ worth of satellite data, found that 56 per cent of the global ocean – a territory larger than the total land area on Earth – experienced colour change between 2002 and 2022. While the researchers did not identify an overall pattern, tropical ocean regions near the Equator seem to have become steadily greener over time.

The study’s authors say that natural, year-to-year variability alone cannot explain those changes.

The ocean’s colour is the result of what exists in its upper layers. In general, the more phytoplankton – microbes containing a type of green pigment called chlorophyll – that live in the aquatic environment, the greener the water is; otherwise, it is bluer.

The ocean also boasts light-absorbing organic matter that can change water’s colour from blue to yellow and brown, depending on its concentration level. For that reason, a shift in colour, though sometimes subtle to the human eye, signals changes in the organisms and substances in ocean waters. 

“The statistical technique used in the analysis of the trend tells us that colour is changing, but doesn’t really say how it is changing,” said senior research scientist Stephanie Dutkiewicz at the Massachusetts Institute of Technology and a co-author of the study. “Though some low-latitude regions do suggest a greener shift, that is by no means the only direction of the shift.”

It is not clear exactly how marine ecosystems are changing to lead to the differences in colour, the researchers say. The colour shift could be caused by changes in plankton communities that are critical to the marine food chain.

Biodiversity is not the only thing at stake: The shift could also affect how much carbon dioxide the ocean takes up, since different types of plankton have different abilities to absorb it.

The changes are consistent with global heating from the burning of fossil fuels, say the scientists. “This gives additional evidence of how human activities are affecting life on Earth over a huge spatial extent,” lead author B.B. Cael of the UK’s National Oceanography Centre said in a statement. “It’s another way that humans are affecting the biosphere.”

To gain a better understanding of how marine ecosystems might have changed over time, Dr Cael and his team analysed data collected by a sensor aboard the Aqua satellite, which has been monitoring ocean colour for 21 years and taking measurements in seven visible wavelengths.

Much of the ocean appears blue to human eyes, but the true colour may contain a mix of wavelengths, from blue to green and even red. The satellite-based sensor can recognise ocean colours that are too subtle for human eyes to differentiate. 

Dr Cael carried out a statistical analysis using all seven ocean colours measured by the satellite from 2002 to 2022. He first examined how much the colours changed from region to region during a given year, which helped set the baseline of their natural variations. Then he zoomed out to look at these annual variations over a longer period of two decades. Those changes outstripped normal variability.

The analysis of real-world satellite data is largely in line with modelling Dr Dutkiewicz did in 2019, which simulated the Earth’s oceans under two scenarios: one with the addition of greenhouse gas, and another without it. The greenhouse-gas model predicted that colour shifts could occur in about 50 per cent of the world’s surface oceans within 20 years.

Climate change threatens global oceans and the life that depends on them in myriad ways. For instance, polar bears could largely disappear by the end of the century if global warming continues, a 2020 study warned, citing alarming losses of Arctic sea ice.

Another study found that half of the world’s coral reefs have already been killed by warmer waters and ocean acidification. BLOOMBERG