Oceans turning greener thanks to climate change

The sea is becoming greener due to changes in plankton populations, analysis of Nasa images finds

EB Content Studio and Agencies

NASA satellites show more than half of the Earth’s oceans are green due to climate change disturbing marine ecosystems.

Strange changes to the colour of the ocean have spurred investigations from scientists.

Satellite data shows that over the last 20 years, colour shifts from blue to green have occurred over 56 per cent of the world’s oceans. The changes are particularly evident in tropical regions near the equator.

Researchers say that this subtle greening of our oceans points to the effect that climate change is having on life under the water.

What these shifts are and the exact cause are unconfirmed, but BB Cael says it’s likely to be linked to the creatures at the base of most food chains – phytoplankton. These organisms also play a vital role in producing much of the oxygen we breathe and stabilising our atmosphere.

“The effects of climate change are already being felt in the surface marine microbial ecosystem,” the study notes.

“The reason we care about this is not because we care about the colour, but because the colour is a reflection of the changes in the state of the ecosystem,” said BB Cael, a scientist at the National Oceanography Centre in Southampton and author of the study published in Nature.

Signs of a bigger problem

change in the colour of the ocean could reflect a change in the state of its ecosystems, according to the study’s authors. Deep blue indicates less life while greener hues indicate more activity from phytoplankton.

It paints a picture of what is going on in the surface layers of the water.

But the colour of the ocean can change from year to year with chlorophyll levels at the surface varying wildly. It makes it difficult to differentiate whether the shift from blue to green is being affected by climate change.

Scientists thought it could take up to 40 years of monitoring the colour of the ocean before they spotted any trends. Different satellites also measure colour changes in different ways too. It means that the data from each one often can’t be combined.

Prior research focused on changes in the greenness of the ocean – from the verdant chlorophyll in its plankton – to learn about trends in the changing climate. But Cael’s team pored over 20 years of observations by Nasa’s Modis-Aqua satellite, an exhaustive data repository, and looked for patterns of change in the ocean’s hue through a fuller colour spectrum including red and blue.

Plankton of different sizes scatter light differently, and plankton with different pigments absorb light differently. Examining changes in colour can give scientists a clearer picture of changes in plankton populations around the globe. Phytoplankton is crucial to ocean ecosystems because it is at the base of most of its food chains.

When comparing these changes in colour with those hypothesised from a computer model simulating what the oceans would look like if human-caused global heating had never taken place, the change was clear.

“We do have changes in the colour that are significantly emerging in almost all of the ocean of the tropics or subtropics,” said Cael.

The changes have been detected over 56% of the world’s oceans – an area greater than all of the land on Earth.

In most areas there’s a clear “greening effect”, Cael said, but he added that there are also places where red or blue colourings are rising or falling.

“These are not ultra, massive ecosystem-destroying changes, they may be subtle,” said Cael. “But this gives us an additional piece of evidence that human activity is likely affecting large parts of the global biosphere in a way that we haven’t been able to understand.”

To delve deeper into the changing colours in the ocean, a NASA mission called Pace (plankton, aerosol, cloud, ocean ecosystem)  is scheduled to launch in January 2024. It will monitor plankton, aerosol, clouds and the ocean ecosystem and also measure hundreds of colours in the ocean instead of a handful, progressing studies like these further.

“Making more meaningful inferences about what the changes actually are ecologically is definitely a big next step,” said Cael.

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