A new study from the University of Plymouth has found that over one-fifth of the global ocean has been losing sunlight over the past two decades, a shift scientists warn could have serious consequences for marine ecosystems and the planet.
The research, published in Global Change Biology, examined how sunlight has been declining in the upper layers of the ocean between 2003 and 2022. The study found that approximately 21% of the global ocean now receives much less light than it did just two decades ago.
More alarmingly, researchers noted that over 9% of the ocean, an area roughly the size of Africa, has experienced a loss in sunlight reaching depths of at least 50 meters (164 feet). An additional 2.6% of the ocean has lost over 100 meters (328 feet) of light penetration, making those waters even darker.
Scientists link darkening to plankton and climate shifts
Dr. Thomas Davies, associate professor of marine conservation at the University of Plymouth, said the findings show a fundamental shift in how oceans function.
Darkening is where less light enters the seas. It disrupts marine food webs & weakens the ocean’s capacity to support biodiversity & regulate climate,”
“The areas where there are major changes in ocean circulation, or ocean warming driven by #ClimateChange, seem to be darkening” pic.twitter.com/MPSFTDib3A— Dr Jill Belch (@JillBelch) May 27, 2025
“There has been research showing how the surface of the ocean has changed colour over the last 20 years, potentially as a result of changes in plankton communities,” Davies said. “But 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.”
Loss of sunlight could disrupt oxygen, food supplies
While scientists are still studying this phenomenon’s full impact, they believe the changes could influence everything from fish populations to global oxygen levels. Sunlight supports tiny ocean plants that form the base of the marine food chain and help produce the oxygen we breathe. These plants also absorb carbon dioxide, making them crucial in the fight against climate change.
The darkening of the ocean could affect the air we breathe, the fish we eat, and the planet’s ability to regulate climate, Davies said. “Our findings represent genuine cause for concern.”
Professor Tim Smyth, head of marine biogeochemistry at the Plymouth Marine Laboratory, warned that reduced sunlight could force light-dependent marine animals to move closer to the surface. That shift could create new challenges.
Animals crowding into shallower waters may face more competition for food and space, Smyth said. “That could bring about fundamental changes in the entire marine ecosystem.”
Researchers call for global attention to the trend
Researchers cited rising sea temperatures, changes in algae growth, and artificial light from ships and coastal cities as possible reasons behind the darkening trend.
The study adds to growing evidence that Earth’s oceans are undergoing rapid changes. Scientists are calling for increased monitoring and global attention to better understand the long-term effects of a darkening ocean on biodiversity and climate.
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