Vast Areas of Oceans Are Darkening, Threatening Marine Life, Scientists Warn
Large sections of the world’s oceans have become significantly darker over the past two decades, according to new research that raises concerns about the health of marine ecosystems and the planet’s climate balance.
Using satellite data and advanced modelling, scientists found that more than 20% of the global ocean darkened between 2003 and 2022. This reduction in light penetration shrinks the vital “photic zone” — the sunlit upper layer of the ocean where most marine life exists.
The phenomenon has affected approximately 75 million square kilometres of ocean — an area equivalent in size to Europe, Africa, China, and North America combined. This upper layer, home to 90% of marine species, is crucial for life that depends on sunlight or moonlight to feed, grow, and reproduce.
Dr Thomas Davies, a marine conservation biologist at the University of Plymouth, described the findings as a “genuine cause for concern” due to their potentially serious consequences for marine biodiversity, global fisheries, and oceanic carbon and nutrient cycles.
The photic zone is where phytoplankton — microscopic organisms that drive nearly all marine food chains — carry out photosynthesis. These organisms not only support marine ecosystems but also produce close to half of the world’s oxygen. Many fish, marine mammals, and other species rely on these sunlit waters for feeding and reproduction.
The study, published in Global Change Biology, analysed 20 years of satellite data and used algorithms to assess changes in the depth of the photic zone. Researchers found that 21% of the ocean had darkened, with 9% experiencing a reduction in photic zone depth by 50 metres and 2.6% by as much as 100 metres.
Darkening is typically caused by reduced light penetration, often near coasts where sediments and nutrients wash into the ocean or where cold, nutrient-rich water rises from the depths. However, the causes of deep-sea darkening are less understood. Scientists believe rising global temperatures and shifting ocean currents may be contributing.
“The areas where there are major changes in ocean circulation, or ocean warming driven by climate change, seem to be darkening, such as the Southern Ocean and up through the Gulf Stream past Greenland,” Davies said.
Interestingly, not all parts of the ocean are getting darker. The study found that 10% of the ocean — around 37 million square kilometres — became brighter over the same period. One example is off Ireland’s west coast, where the ocean surface has brightened, even as adjacent areas further offshore have darkened.
“Marine organisms use light for a whole variety of purposes. They use it for hunting, for mating, for timing reproductive events. They use it for basically every single part of their biology,” said Davies. “With ocean darkening, they have to move up the water column, and there is less space, they’re all being squished up towards the surface.”
Professor Oliver Zielinski, director of the Leibniz Institute for Baltic Sea Research, called the overall trend “worrying,” stressing that the darkening of such vast ocean areas could have long-term impacts on marine ecosystems and global climate stability.
“Such changes can disrupt marine food webs, alter species distributions, and weaken the ocean’s capacity to support biodiversity and regulate climate,” he said. “Coastal seas, being closest to human activity, are particularly vulnerable, and their resilience is crucial for both ecological health and human wellbeing.”
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