The deep, blue sea is becoming deeper — in color, that is. Climate change, along with human development, has reduced how much light can filter through the water. Reduced light can significantly disrupt the marine food chain as well as lead to the large-scale worsening of climate change.
Zoning issues
Ocean darkening occurs when “changes in the optical properties of the oceans reduce the depth to which sufficient light penetrates to facilitate biological processes guided by sunlight and moonlight,” said a 2025 study published in the journal Global Change Biology. The color shift can make the water look more opaque. The part of the ocean that sunlight is able to penetrate is called the photic zone and it is “home to 90% of marine species,” said the World Economic Forum. Organisms like phytoplankton also “convert sunlight and CO2 into energy, producing nearly half the planet’s oxygen and absorbing vast amounts of carbon emissions” in the photic zone.
Rather than just some patches of darkening, the phenomenon has affected “large, connected regions,” Tim Smyth, a marine scientist at Plymouth Marine Laboratory and co-author of the study, said to New Scientist. “Roughly one‑fifth of the world’s oceans have darkened in some way.” Already, the depth of the photic zone has reduced by more than 10% “across 9% of the global ocean,” said the study.
In coastal areas, darkening is “closely linked to changes in the rivers that flow into the sea,” Smyth said. “Shifts in land use affect what becomes dissolved or suspended in the water, which, in turn, alters the optical quality of the water entering the ocean.” In addition, “fertilizers used in industrial agriculture are washed into rivers, stimulating phytoplankton growth,” which reduces “how deeply light can penetrate the water column.”
However, darkening is not limited to the coast. The open ocean has also increased in opacity, which “may be linked to shifts in phytoplankton blooms driven by climate change.” There have been “rising ocean temperatures, more frequent marine heatwaves and changes in salinity in some regions.” Such changes “influence large‑scale ocean circulation patterns.”
Light direction
Dark oceans are bad news and the consequences have already begun to appear. As the photic zone shrinks, “many marine species are forced to move closer to the surface in order to survive,” said Diario AS. This “pushes large numbers of organisms into a much smaller space, increasing competition for food, raising biological stress and leaving them far more exposed to predators, including human fishing vessels.”
Along with disrupting the marine food chain, ocean darkening hinders the ocean’s ability to perform photosynthesis, weakening the “ocean’s role as a carbon sink, its natural capacity to capture and store the carbon dioxide that warms the planet.” If the ocean is helping to sequester carbon from the atmosphere, climate change will worsen at a faster rate.
Some of the main culprits of ocean darkening are “sediment runoff from agriculture, deforestation and development,” especially in coastal regions, said the World Economic Forum. Improved land management can play a large role in reducing the level of darkening. This includes reducing fertilizer use as well as encouraging conservation efforts. In the open ocean, the problem is much more difficult to tackle as “even if global emissions dropped to net zero tomorrow, the ocean would take decades, if not centuries, to respond,” said Smyth. The good news is that the ocean “still has a remarkable capacity to heal itself. Give marine ecosystems a little room to recover and they often respond with surprising speed.”
Less light spells trouble for humans and animals