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The oceans are getting greener because of climate change

Ask a child to draw a picture of the ocean, and they will reach for a blue crayon. But new research suggests climate change is in fact making oceans greener in colour.

Over the past two decades, vast swathes of the seas have subtly changed hue, as phytoplankton communities respond to the impacts of rising global temperatures.

BB Cael at the National Oceanography Centre in Southampton, UK, and his colleagues used data gathered by the MODIS instrument on NASA’s Aqua satellite to monitor changes in the radiance coming from the water’s surface, a key metric for analysing shifts in ocean colour, from 2002 to 2022.

The study found that “significant” changes in colour have occurred over 56 per cent of the world’s ocean in this time, with an overall trend towards more green.

By comparing the results with computer simulations of the climate, Cael and his colleagues are able to point the finger at climate change as the cause.

Tammi Richardson at the University of South Carolina, who wasn’t involved in the study, said the findings “confirm suspicions” about how oceans are changing in response to climate change. “It’s giving us much more solid evidence that the ocean is becoming greener, beyond the few data points that we’ve had historically,” she says.

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The most likely cause for the colour changes is a strengthening of ocean stratification, which describes the natural division between less dense warm waters at the surface and cooler, denser water below.

“As the ocean warms, the upper ocean becomes more stratified,” says Cael. “That means it is harder for nutrients to make it up to where the light is, because there’s less exchange of water between the top and the bottom.” The phytoplankton communities, which dwell in the top waters and rely on this upwelling of nutrients, are changing in response, Cael says.

However, it isn’t yet clear how exactly phytoplankton populations are changing. It may be that smaller-celled phytoplankton are becoming more dominant, occupying the oceans at a higher concentration. “It’s not trivial to disentangle changes in the light to [determine] what’s in the water that is causing those changes,” says Cael.

Read more:

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The changes are unlikely to ever become dramatic enough for green crayons to become the default choice for a child working on an ocean portrait. But even subtle shifts in ocean colour could herald dramatic impacts for the marine food chain and the ocean’s ability to store carbon – although, again, it isn’t yet clear how this will manifest.

“There are multiple different ways the ecosystem could change that might give the same light signature,” says Cael. “So what we really need is more fine-grained observations of the light.” NASA’s PACE satellite, which launches in January 2024, should provide more information, he says.

Journal reference:

Nature DOI: 10.1038/s41586-023-06321-z

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