More than 20% of the global ocean has undergone significant darkening over the last two decades, dramatically reducing the depth of photic zones – the sunlit layers essential for most marine life.
Driven by factors like nutrient runoff and shifting climate patterns, this change could disrupt delicate ecosystems, force light-dependent creatures into tighter spaces, and even challenge our reliance on oceans for oxygen, food, and climate regulation.
Ocean Darkening: A Growing Global Phenomenon
New research reveals that more than 20 percent of the world’s oceans, covering over 75 million square kilometers, have been getting darker over the past two decades. This large-scale shift is raising concerns among scientists about what it could mean for marine life and the planet’s health.
This phenomenon, known as ocean darkening, happens when changes in the water’s clarity reduce the depth of the photic zone, the sunlit layer of the ocean where most marine life lives. This zone is crucial for everything from plankton to fish, and even affects how the ocean helps regulate Earth’s climate.
Satellite Data Reveals Startling Trends
To uncover these changes, scientists used nearly 20 years of satellite data combined with advanced computer modeling. Their analysis, published on May 27 in Global Change Biology, showed that between 2003 and 2022, large parts of both coastal areas and the open ocean became darker, meaning sunlight now travels less deeply into the water.
Even more striking, over 9 percent of the ocean, an area larger than the continent of Africa, saw the photic zone shrink by more than 50 meters. In some regions, that depth was reduced by over 100 meters. However, not all areas are following the same pattern. About 10 percent of the ocean has actually become lighter during the same period.
Marine Life at Risk
While the precise implications of the changes are not wholly clear, the researchers say it could affect huge numbers of the planet’s marine species and the ecosystem services provided by the ocean as a whole.
The study was conducted by researchers from the University of Plymouth and Plymouth Marine Laboratory, who have spent more than a decade examining the impact of artificial light at night (ALAN) on the world’s coasts and oceans.
They say that is not directly connected to ocean darkening, however, with the changes likely being as a result of a combination of nutrient, organic material, and sediment loading near the coasts, caused by factors such as agricultural runoff and increased rainfall.
In the open ocean, they believe it will be down to factors such as changes in algal bloom dynamics and shifts in sea surface temperatures, which have reduced light penetration into surface waters.
A Wake-Up Call for Ocean Health
Dr. Thomas Davies, Associate Professor of Marine Conservation at the University of Plymouth, said: “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.
“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. 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. Taking all of that into account, our findings represent genuine cause for concern.”
Marine Competition and Ecosystem Disruption
Professor Tim Smyth, Head of Science for Marine Biogeochemistry and Observations at the Plymouth Marine Laboratory, added: “The ocean is far more dynamic than it is often given credit for. For example, we know the light levels within the water column vary massively over any 24-hour period, and animals whose behavior is directly influenced by light are far more sensitive to its processes and changes.
“If the photic zone is reducing by around 50 m 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. That could bring about fundamental changes in the entire marine ecosystem.”
NASA Tools Track Ocean Light Levels
To assess changes in the photic zone, the researchers used data from NASA’s Ocean Colour Web, which breaks the global ocean down into a series of 9 km pixels.
This satellite-derived data enabled them to observe changes on the ocean surface for each of these pixels, while an algorithm developed to measure light in sea water was used to define the depth of the photic zone in each location.
They also used solar and lunar irradiance models to examine particular changes that might impact marine species during daylight and moonlight conditions, demonstrating that changes in photic zone depth at night were small compared to daytime, but remained ecologically important.
Global Hotspots of Change
The most prominent changes in photic zone depth in the open ocean were observed at the top of the Gulf Stream, and around both the Arctic and Antarctic, areas of the planet experiencing the most pronounced shifts as a result of climate change.
Darkening is also widespread in coastal regions and enclosed seas, such as the Baltic Sea, where rainfall on land brings sediment and nutrients into the sea, stimulating plankton growth and reducing light availability.
Mixed Trends Around the UK
Around the UK, however, the picture was mixed.
The study indicated that areas of the North Sea and Celtic Sea, the eastern coasts of England and Scotland, the coastlines of Wales, and the northern elements of the Irish Sea have all become darker over the past two decades.
However, much of the English Channel and regions stretching from the north of Scotland to the Orkney and Shetland Islands have become lighter.
Reference: “Darkening of the Global Ocean” by Thomas W. Davies and Tim Smyth, 27 May 2025, Global Change Biology.
DOI: 10.1111/gcb.70227
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6 Comments
So. ..what are we supposed to do about this “crisis” in the photic zone? Maybe it’s being helpful for life sensitive to the Sun’s damaging UV radiation…reef corals? Maybe it’s been taking place irregularly for eons. Articles like this seem to be more common with “climate change” being in the headlines.
Water is a strong absorber of UV EMR. Sometimes beached cetaceans get sun burned, but I have never read of air breathers coming up for air getting sun burned as long as they have even a thin film of water on their skins — even in the tropics where the sun’s rays are stronger.
“Taking all of that into account, our findings represent genuine cause for concern.”
Why is the change “startling?” It reflects the old conundrum about whether the glass is half-full or half-empty. It seems that those who make their living through research grants, invariably find the glass half-empty.
From another article on this yesterday, in The Guardian, “It [darkening] is often seen along coastlines where upwellings of cold, nutrient rich water rise to the surface, …” It should be obvious that the reason for the darkening is that the nutrient-rich water is fostering blooms of the photosynthetic organisms at the bottom of the food chain. That is, the biological productivity is enhanced, not threatened. One of the most biologically productive areas of the Earth’s oceans is the Antarctic Ocean, where filter-feeders like to fill up on krill. I doubt that “global heating” is a significant player when the organisms are cold tolerant. It may well be that warming is responsible for the increased productivity because chemical and biological activity usually increases with increasing temperatures. However, if the researchers don’t design their experiments to look for it, they probably won’t find it.
“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.”
He is missing the obvious. The changes in ocean color and clarity, which parallel each other the last 20-years, reflects the fact that something else, perhaps warming and/or fertilizer runoff, is providing an environment more conducive to biological productivity. It is evidence against his dour speculation about the future. There are justifiable concerns about Red Tides, but they don’t mention them.
Once again, the public is provided with cherry-picked data and biased interpretations. It seems that the current generation of ‘technicians’ have not been exposed to Chamberlain’s “Method of Multiple Working Hypotheses.” Let’s hope so, because the alternative explanation of willfully ignored some hypotheses implies fraudulent behavior.
What a useless headline.
Tell me, what percentage of the ocean got “lighter”? Perhaps around 21% ?
This is unsophisticated research based on 25-year old technology; MODIS was launched in 1999. It is based on spot readings with low-angles of incidence (nadir). Water has very non-linear specular reflectance, with it being at a minimum at solar noon (angle of incidence=0 deg). Specular reflectance increases with the angle of incidence, reaching a maximum of 100% at glancing (90 deg) angles. Thus, the reflectance increases with latitude and the hours before and after noon, with the reflectance spectrum duplicating the surface spectrum at the maximum reflectance. This is important because the light that penetrates the surface of the water, and is available to the photosynthetic organisms, is (1-R), where “1” is the normalized impinging intensity per unit area and R varies between 0 and 1. R is best calculated with Fresnel’s equation. The authors are apparently using a NASA product based on diffuse reflectance only, which is largely the suspended organic and inorganic particles.
I saw nothing in the published paper to suggest that the authors were assessing the actual light flux versus depth, instead reporting a percentage change based on Beer’s Law, using a cutoff of 1% of the original subsurface intensity, when absolute intensity is critical to the organisms. What should be of concern is the range of absolute light intensity that is usable by the organisms (assuming that they all have different optimal ranges) and what the total flux is, integrated over the hours of exposure.
They do, however, receive an ‘A-‘ grade for their honesty: “The implications of ocean darkening for marine ecology and the ecosystem services provided by the surface oceans are currently unknown but likely to be severe.” Albeit, they don’t justify their assessment of “LIKELY to be severe.”