Why Are Earth's Oceans Getting Darker And Should We Be Worried?

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4 days ago

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A new study has revealed that over one-fifth of the global ocean has lost sunlight in just two decades, a quiet shift with potentially profound consequences
It's not something you can see with the naked eye, but it's happening at a planetary scale. Earth's oceans are getting darker—and fast. In just 20 years, more than 21 per cent of the global ocean has lost measurable sunlight, according to a sweeping study led by the University of Plymouth in collaboration with the Plymouth Marine Laboratory, based on satellite data from 2003 to 2022.
The findings are based on two decades of satellite data from NASA's Ocean Colour Web, which tracks changes in how sunlight penetrates ocean surfaces globally.
Covering over 75 million square kilometres, this reduction in ocean light is compressing the photic zone—the sunlit layer of the sea where marine life thrives and photosynthesis occurs.
But what's behind this shift? And why should we care?
What's Causing Ocean Darkening?
Scientists point to multiple, overlapping reasons:
Runoff and sedimentation: In coastal regions, fertilisers, sewage, and industrial waste flow into the sea. This triggers algal blooms and increases turbidity, both of which block light from penetrating the water column.
Climate-driven shifts in plankton: Rising sea surface temperatures are changing the distribution, size, and composition of phytoplankton—microscopic organisms that not only form the base of the food chain but also reflect or absorb sunlight depending on their type.
Rainfall and erosion: Heavier monsoon patterns and land-use change are sending more sediment into the oceans, especially in the tropics and around the Indian subcontinent.
Loss of water clarity: In the open ocean, changes in biological productivity—likely climate-induced—are altering how much light the sea absorbs.
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While some regions have experienced slight brightening, particularly in clearer mid-ocean zones, the overall global trend points decisively toward darkening.
Why This Is A Red Flag
The compression of the photic zone is not just a biological concern—it's a planetary one.
Disrupts marine food webs: Phytoplankton need sunlight to grow. Less light means fewer plankton, and that ripples up the chain to fish, marine mammals, and seabirds.
Weakens ocean's climate role: Phytoplankton are critical to the carbon cycle, absorbing CO₂ and helping cool the planet. Less sunlight means reduced photosynthetic activity and a shrinking oceanic carbon sink.
Pushes species into shallow zones: With the sunlit layer shrinking, marine life is being squeezed into shallower, warmer waters—making them more vulnerable to heat stress, overfishing, and habitat loss. This compression may also affect coastal economies dependent on fishing, as marine species shift or decline in response to changing light conditions.
'Our results provide evidence that such changes cause widespread darkening that reduces the amount of ocean available for animals that rely on the sun and the moon for their survival and reproduction," said Dr Thomas Davies, Associate Professor of Marine Conservation at the University of Plymouth, and co-lead author of the study Darkening of the Global Ocean, published in Global Change Biology in May 2025.
'We also rely on the ocean and its photic zones for the air we breathe, the fish we eat, our ability to fight climate change, and for the general health and wellbeing of the planet. Taking all of that into account, our findings represent genuine cause for concern."
A Call To Action
Experts stress that reversing or even slowing ocean darkening will require action on multiple fronts: curbing land-based pollution, adapting coastal infrastructure, and most urgently, addressing climate change at a global scale.
The study also highlights the importance of expanding marine observation networks, which are currently underfunded and patchy across vast parts of the ocean.
For now, one thing is clear: the world's largest ecosystem is losing its light. And that should never be taken lightly.
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