The world’s oceans have darkened by over a fifth in the past two decades, with potential severe consequences for marine ecosystems and global fisheries. Researchers analyzed satellite data and numerical modeling to reveal that 75m sq km of ocean has become darker, equivalent to the combined land area of Europe, Africa, China, and North America.
The darkening is affecting the photic zones, where sunlight penetrates and allows life to thrive. Microscopic plant-like organisms called phytoplankton photosynthesize in these areas, producing nearly half the planet’s oxygen and supporting marine food webs. However, as the ocean darkens, it reduces the area where light can reach, forcing marine life to move up the water column.
The study found that 21% of the global ocean has become darker over the past 20 years, with photic zones being 50 meters shallower in 9%, and 100 meters shallower in 2.6%. The drivers for this darkening are unclear, but global heating and changes in ocean currents are thought to be involved.
Despite the overall trend, about 10% of the ocean has become lighter over the past 20 years, possibly due to increased nutrient and sediment input from rainfall and upwellings along coastlines. However, this localized brightening is not enough to offset the global darkening.
Experts warn that the effects of ocean darkening will have severe implications for marine ecosystems, global fisheries, and climate regulation. “Marine organisms use light for every single part of their biology,” said Dr. Thomas Davies. “With ocean darkening, they’re being squished up towards the surface, reducing space and potentially disrupting food webs.”
Source: https://www.theguardian.com/environment/2025/may/27/planets-darkening-oceans-pose-threat-marine-life-scientists-say