**Large Swathes of Earth’s Oceans Undergo Alarming Darkening, Scientists Warn**
Vast regions of the world’s oceans are undergoing a troubling transformation, according to a newly published scientific study. The findings highlight that more than one fifth of the planet’s ocean surface has grown noticeably darker over the last 20 years—a shift with potentially far-reaching consequences for marine biodiversity and human wellbeing.
The research, carried out by experts at the University of Plymouth and Plymouth Marine Laboratory, utilised comprehensive analyses of satellite data from NASA alongside advanced numerical models. This approach enabled the team to systematically track annual changes in the penetration of sunlight through the ocean’s surface, offering an unprecedented view of shifting light conditions beneath the waves.
Astonishingly, the study reveals that in the period from 2003 to 2022, a region surpassing 75 million square kilometres—home to an estimated 90% of all marine life—has experienced significant darkening. This vast zone, critical to sustaining the health and productivity of marine ecosystems, is now less illuminated by sunlight than at the start of the century.
Dr Thomas Davies, an associate professor specialising in marine conservation at the University of Plymouth, shed light on the importance of these changes. “Previous research had detected modifications in the surface colour of the oceans, likely reflecting alterations in plankton populations. Our findings go further, showing that these surface shifts are linked to widespread reductions in light reaching deeper ocean layers—the so-called photic zone—which is crucial for countless marine species.”
Access to sufficient sunlight in the ocean’s upper layers is vital not only for plankton, which form the foundation of the food chain, but also for a vast array of animals that synchronise their life cycles to daily and seasonal light patterns. According to Dr Davies, the reduced penetration of light could limit the available habitat for these creatures, posing a threat to marine food webs—and, by extension, to humans who depend on the oceans for food and the air they breathe.
The study also found that over 9% of the world’s oceans now have a photic zone—which marks the depth to which sunlight can penetrate—that is 50 metres shallower than it was two decades ago. In even more profoundly affected areas, comprising more than 2.6% of the ocean, the loss extends beyond 100 metres. As a counterpoint, about 10% of ocean waters have actually become brighter, though scientists are still probing the complex implications of this disparity.
Professor Tim Smyth, who heads marine biogeochemistry at Plymouth Marine Laboratory, commented on the intricate dynamics of the ocean. He noted, “Ocean light levels fluctuate dramatically within any given day, impacting the behaviour of organisms. If large portions of the sea are now darker and the sunlit zone is shrinking, animals reliant on light will be forced closer to the surface, intensifying competition for already limited resources.”
This clustering nearer the ocean’s surface could disturb established ecological balances, as species adapt or are forced out, potentially instigating broad shifts throughout entire marine ecosystems. The findings suggest that the issue is particularly acute in certain regions, such as the Gulf Stream and around polar waters, already under stress from rapid climate change.
Globally, the shrinkage of the sunlit ocean is not just a concern for wildlife. The photic zone plays a vital role in generating much of the oxygen humanity breathes, in sequestering atmospheric carbon dioxide, and in supporting fisheries that millions rely on for food. This ongoing darkening, therefore, represents not merely an environmental problem, but one that could touch countless aspects of human health and society.
The scientific article, now available in the journal *Global Change Biology*, serves as a stark warning about the interconnectedness of climate and marine systems. As researchers continue to monitor these changes, the results underline the urgent need for continued scientific attention and robust policy responses to safeguard the oceans on which all life depends.