Over the past two decades, more than one-fifth of the world’s oceans, an area exceeding 75 million square kilometers, have undergone a phenomenon known as ocean darkening, according to groundbreaking new research.
Ocean darkening occurs when changes in water clarity reduce the depth of photic zones, the sunlit layers of the ocean that support 90% of marine life. These zones are crucial for ecological interactions driven by sunlight and moonlight, and any disruption could have far-reaching consequences.
The study, published in Global Change Biology, analyzed satellite data and numerical models to track annual shifts in the depth of the photic zone worldwide. Between 2003 and 2022, researchers found that 21% of the ocean had experienced significant darkening.
Some regions suffered particularly dramatic losses in light penetration:
- Over 9% of the ocean, equivalent to the size of Africa, saw photic zones shrink by more than 50 meters.
- In 2.6% of the ocean, light penetration dropped by over 100 meters.
At the same time, the researchers observed that around 10% of the ocean had become lighter, illustrating complex shifts in marine ecosystems.
The team, led by scientists from the University of Plymouth and Plymouth Marine Laboratory, has been studying artificial light at night (ALAN) for over a decade. However, they say ocean darkening is likely due to different factors.
Near coastlines, darkening appears to be linked to agricultural runoff, increased rainfall, and sediment loading. In open ocean regions, shifts in algal bloom patterns and rising sea surface temperatures have reduced light penetration, affecting the ecosystem.
The changes are especially pronounced in climate-sensitive regions, such as the Arctic, Antarctic, and the Gulf Stream. Coastal areas, including enclosed seas such as the Baltic, have also experienced widespread darkening due to excess nutrients fueling the growth of plankton.
The study found varying trends in UK waters:
- The darkening was observed in parts of the North Sea, Celtic Sea, and the eastern coasts of England and Scotland.
- In contrast, much of the English Channel and waters near Scotland’s northern isles became lighter.
Although the full ecological impact remains uncertain, researchers warn that ocean darkening could disrupt marine food chains, affect biodiversity, and reduce the ocean’s ability to support key ecosystem services.
As climate change accelerates, understanding these shifts will be crucial for protecting ocean health.
Journal Reference
- Davies, T. W., & Smyth, T. (2025). Darkening of the Global Ocean. Global Change Biology, 31(5), e70227. DOI: 10.1111/gcb.70227