Global Oceans Darken at Alarming Rate, Threatening Marine Ecosystems and Biodiversity

 A substantial portion of the world’s oceans has grown significantly darker over the past two decades, a development that could pose serious consequences for marine ecosystems, biodiversity, and the planet’s climate regulation, according to a newly published study.

The research, led by scientists from the University of Plymouth and published in Global Change Biology, reveals that between 2003 and 2022, more than 21% of the global ocean experienced a measurable darkening, reducing the depth and reach of sunlight in the water column. This change affects an area of approximately 75 million square kilometers—roughly equivalent to the combined landmass of Europe, Africa, China, and North America.

Dr. Thomas Davies, a marine conservationist and the study’s lead author, described the findings as “a genuine cause for concern,” noting that darkening oceans could severely disrupt the photic zone—the upper layer of the ocean that supports most marine life and drives critical biological processes.

“Marine ecosystems, global fisheries, and the vital exchange of carbon and nutrients in the ocean all rely on light penetrating through the water,” Davies said. “The reduction of this light has profound implications across the food web.”

The photic zone, typically extending to a depth of around 200 meters, is where microscopic phytoplankton carry out photosynthesis, forming the foundation of marine food chains and producing nearly half of the Earth's oxygen. This sunlit region is also where a vast array of marine species feed, reproduce, and carry out essential biological functions.

By analyzing two decades of satellite data with a specialized algorithm that measures light penetration in seawater, the researchers found that in nearly 10% of the global ocean, the photic zone has become at least 50 meters shallower, and in 2.6% of cases, it has receded by more than 100 meters.

The causes of ocean darkening vary. Along coastlines, it is often linked to nutrient and sediment run-off from land, or cold-water upwellings that bring deep, nutrient-rich waters to the surface. However, in offshore regions, the drivers are less clear. The study suggests climate change and shifts in ocean circulation may be contributing factors. Key areas experiencing darkening include the Southern Ocean and the North Atlantic, particularly along the Gulf Stream and near Greenland.

While the trend is predominantly one of darkening, the researchers also found that approximately 10% of the ocean has brightened during the same period. Off the coast of Ireland, for instance, large sections of ocean have become lighter, although neighboring regions have dimmed.

Dr. Davies emphasized the ecological importance of ocean light. “Light plays a central role in marine biology—from hunting and mating to the timing of reproduction. With decreasing light levels, marine organisms are being forced into narrower, shallower bands of water. This compression reduces habitable space and increases competition.”

Prof. Oliver Zielinski, director of the Leibniz Institute for Baltic Sea Research, echoed these concerns. “The darkening of vast ocean areas is a worrying trend,” he said. “Such changes have the potential to destabilize marine food webs, shift species distributions, and weaken the ocean’s role in regulating Earth’s climate. Coastal regions, which are more directly affected by human activities, are particularly vulnerable.”

The findings underscore the urgent need to better understand and monitor light conditions in the world’s oceans, as scientists warn that further darkening may intensify the cascading effects of climate change, biodiversity loss, and ecological disruption.