Our oceans are changing colour. Scientists say the reason is grim

The Earth’s oceans are changing colour – and it is almost certainly due to climate change.

The seas, once a brilliant blue, have been steadily turning more green over the past two decades, with scientists detecting ‘significant’ changes in the colour they say cannot be explained by natural, year-to-year variations alone.

The colour shifts, though subtle to the human eye, have occurred over 56% of the world’s oceans – an expanse larger than the total land area on Earth, according to the findings published in the journal Nature.

In particular, the research team found that tropical ocean regions near the equator have become steadily greener over time.

They say the shift in ocean colour indicates that ecosystems within the surface of the ocean must also be changing, as the colour of the ocean is a literal reflection of the organisms and materials in its waters.

The researchers cannot say how exactly marine ecosystems are changing to reflect the shifting colour, but the team, including scientists from the UK’s National Oceanography Centre, are fairly certain of one thing.

Human-induced climate change is likely the driver.

‘I’ve been running simulations that have been telling me for years that these changes in ocean colour are going to happen,’ said co-author Dr Stephanie Dutkiewicz, senior research scientist at Massachusetts Institute of Technology (MIT) in the US.

‘To actually see it happening for real is not surprising, but frightening.

‘And these changes are consistent with man-induced changes to our climate.’

Lead author Dr B B Cael, of the National Oceanography Centre in Southampton, said: ‘This gives additional evidence of how human activities are affecting life on Earth over a huge spatial extent.

‘It’s another way that humans are affecting the biosphere.’

The team explained that the ocean’s colour is a visual product of whatever lies within its upper layers.

Waters that are deep blue reflect very little life, whereas greener waters indicate the presence of ecosystems, and mainly phytoplankton – plant-like microbes that are abundant in the upper ocean and that contain the green pigment chlorophyll.

The pigment helps plankton harvest sunlight, which they use to capture carbon dioxide from the atmosphere and convert it into sugars.

Dr Dutkiewicz said phytoplankton are the foundation of the marine food web that sustains progressively more complex organisms, on up to krill, fish, and seabirds and marine mammals.

She said phytoplankton are also a powerful ‘muscle’ in the ocean’s ability to capture and store carbon dioxide. Scientists are keen to monitor phytoplankton across the surface oceans and to see how the essential communities might respond to climate change.

In the current study, Dr Cael and the team analysed measurements of ocean colour taken by the MRI Spectroradiometer (MODIS) aboard the Aqua satellite, which has been monitoring ocean colour for 21 years.

The differences in colour that the satellite picks up are too subtle for human eyes to differentiate. Much of the ocean appears blue to our eye, whereas the true colour may contain a mix of subtler wavelengths, from blue to green and even red.

Dr Cael analysed all seven ocean colours measured by the satellite from 2002 to 2022 together, and the analysis turned up a clear trend, above the normal year-to-year variability.

To see whether the trend is related to climate change, he then looked at previously modelling that simulated the Earth’s oceans under two scenarios: one with the addition of greenhouse gases, and the other without it.

The greenhouse-gas model predicted that a significant trend should show up within 20 years and that the trend should cause changes to ocean colour in about 50 per cent of the world’s surface oceans – almost exactly what Dr Cael found in his analysis of real-world satellite data.

‘This suggests that the trends we observe are not a random variation in the Earth system,’ he said. ‘This is consistent with anthropogenic climate change.’

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