The world's oceans are changing colour due to climate change, a study suggests

The ocean's water has changed colour over the past 20 years due to climate change. Huge swathes of the world's oceans have turned from blue to green, as the bodies of water display the effect of climate change on life in the world's water. 

Research published in the journal Nature on 13 July said scientists had detected shifts in colours across more than half of the world's oceans - an expanse bigger than Earth's land mass. 

Why are the Earth's oceans greener? 

The authors of the paper think the hue shift is due to changes in ecosystems, and in particular, tiny plankton. There is a possibility that the shift may be to do with something with how nutrients are distributed in the ocean. 

As surface waters warm, the upper layers of the ocean become more stratified, which makes it harder for nutrients to rise to the surface. When there are fewer nutrients, smaller phytoplankton are better at surviving than larger ones. Changes in nutrient levels could lead to changes in the ecosystem - reflected in changes in the water's overall colour.

“We are affecting the ecosystem in a way that we haven’t seen before. 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 at the University of Southampton. 

The ocean can change colour for many different reasons, but by studying the wavelengths of sunlight reflected off the ocean's surface, scientists can estimate how much chlorophyll there is and thus how many living organisms such as phytoplankton and algae are present. 

However, full observations may take up to 40 years, as the amount of chlorophyll in surface waters differ from year to year. 

Study co-author Dr Stephanie Dutkiewicz, of the Massachusetts Institute of Technology, US, said: “I’ve been running simulations that have been telling me for years that these changes in ocean colour are going to happen. To see it happening for real is not surprising, but frightening.”

How was the change noticed? 

Cael's team analysed data from MODIS, a NASA senor board on Aqua satellite - which was launched in 2002 and is still orbiting the Earth.

With over two decades of MODIS data, the changes to the ocean colour became noticeable, and they saw shifts in 56% of the world's ocean surface - mostly in water near the equator.

NASA will be launching an advanced satellite mission in January 2024 called Pace (plankton, aerosol, cloud, ocean ecosystem) to 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.