Scientists say Earth’s oceans are slowly getting darker, according to new findings that reveal a concerning trend affecting the largest bodies of water that cover our planet.
According to the new findings, nearly one-fifth of the global ocean has darkened within the past two decades. This phenomenon is being driven by changes in the optical properties of ocean water, which spans more than 75 million square miles of the Earth’s surface.
This progressive darkening results in less light reaching beneath the waves into areas known as photic zones, where nearly 90% of all ocean life currently resides. The new findings have aroused concern among scientists over how ocean darkening may affect organisms that normally rely on light from the sun and moon.
The research, carried out by scientists from the University of Plymouth and Plymouth Marine Laboratory, comprises more than a decade of studies on the impact of light on Earth’s oceans.
The new research, published in Global Change Biology, combines satellite data and advanced numerical models to study changes in the depth of photic zones around the world. It revealed that large portions of open ocean and shallower coastal waters—an estimated 21% of all global ocean water—had darkened between 2003 and 2022.
Dark Revelations About Earth’s Oceans
The new research also identified that more than 9% of the ocean, comprising a region roughly the size of the African continent, has seen a reduction in photic zone depths by more than 50 meters. Additionally, 2.6% of the global ocean’s photic zone has been reduced by twice that much, affecting depths up to 100 meters.
Despite the concerning darkening observed in many ocean regions, the recent research also revealed a surprising outlier: nearly 10% of Earth’s ocean water in other regions also appears to have become lighter during the last two decades.
Currently, scientists are unsure of the long-term effects of these changes, although they could impact large numbers of marine species and their ecosystems.
What Drives Earth’s Darkening Oceans?
The research seems to point to a combination of factors behind the changes observed over the last two decades. Nutrients entering the ocean, along with organic materials and sediments that enter through coastal waterways, are all likely factors, many of them driven by factors like agricultural runoff. Increased rainfall is also believed to be a contributing factor in the movement of these materials into the ocean.
Further from the coasts, algal blooms and changes in sea surface temperatures have also reduced the amount of light that can penetrate the ocean water.
Dr Thomas Davies, an Associate Professor of Marine Conservation at the University of Plymouth, says that past research has shown that the surface of the ocean has undergone color changes over the last two decades, which scientists link to another complicating factor: changes in ocean plankton communities.
According to Davies, “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.”
Davies and his colleagues say that in addition to impacting marine life forms, the darkening of oceanic photic zones could also impact humans in several ways.
“We also rely on the ocean and its photic zones for the air we breathe, the fish we eat, our ability to fight climate change, and for the general health and well-being of the planet,” Davies said in a statement. “Taking all of that into account, our findings represent genuine cause for concern.”
Monitoring Changes in Earth’s Oceans
The discovery was made in part thanks to data from NASA’s Ocean Color Web, which relies on satellite data that allowed the research team to detect changes on the ocean surface occurring on a pixel-for-pixel scale, with each pixel representing a region of the ocean spanning roughly 9 kilometers.
The data also employs an algorithm that measures the photic qualities of seawater, which allowed researchers to compare changes occurring in various photic regions around the world.
Finally, this data was combined with models that gauged the influence of sunlight and moonlight on marine organisms, which revealed that changes to photic depth were smaller at night than in the daytime.
Of all regions the team observed, the greatest amount of photic zone depth change in the open ocean seems to be occurring in three locations: near the top of the Gulf Stream, and around Earth’s icy polar regions. All of these locations are areas where the planet is experiencing other significant changes that are believed to be related to Earth’s changing climate.
Broader Impacts of the Ocean Getting Darker
That isn’t to say that ocean darkening isn’t occurring in other areas too, many of which are closer to humans. The team’s study also revealed darkening occurring in regions like the Baltic Sea, where increased annual rainfall is likely carrying more nutrients and sediments into the ocean.
These nutrients and other organic components stimulate plankton growth, which the team says is a significant contributor to overall reductions in light penetration into our ocean environments.
“The ocean is far more dynamic than it is often given credit for,” said Professor Tim Smyth, Plymouth Marine Laboratory’s Head of Science for Marine Biogeochemistry and Observations.
“If the photic zone is reducing by around 50m in large swathes of the ocean, animals that need light will be forced closer to the surface where they will have to compete for food and the other resources they need,” Smyth said.
“That could bring about fundamental changes in the entire marine ecosystem,” he added.
Smyth and Davies’ new study, “Darkening of the Global Ocean,” appeared in Global Change Biology on May 27, 2025.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. He can be reached by email at micah@thedebrief.org. Follow his work at micahhanks.com and on X: @MicahHanks.