More than one-fifth of Earth’s oceans have grown darker over the past two decades, a change largely unnoticed but deeply alarming. New research published in Global Change Biology shows light penetration in the ocean has fallen significantly from 2003 to 2022.
The depth of the photic zone has decreased in large parts of the global ocean. This zone is the sunlit layer essential for marine life. In total, 21% of the global ocean has darkened. This change limits the space for plankton, fish, and other species that rely on sunlight.
The study reveals that over 9% of ocean regions saw photic zones shrink by more than 50 meters in just twenty years. In some areas, the loss was even greater, with over 2.6% of oceans losing more than 100 meters of light-filled depth. This drop in light affects creatures whose daily cycles are shaped by sunlight and moonlight, impacting feeding and breeding behavior.
UNDERSTANDING OCEAN DARKENING: WHAT’S DRIVING THE SHIFT?
Researchers from the University of Plymouth and Plymouth Marine Laboratory combined satellite data with advanced light modeling. They analyzed NASA’s Ocean Colour Web and developed algorithms to measure light depth and track changes across ocean zones.
They also used solar and lunar irradiance models to evaluate how daylight and moonlight patterns have shifted in marine areas. Their findings show that day-time light loss is far more pronounced than at night. However, both play vital roles in marine ecology.
In coastal zones, increased sediment and nutrient runoff from agriculture and heavy rains are reducing water clarity and sunlight reach. Runoff triggers plankton blooms and pollution that cloud waters, disrupting the balance of marine food chains and ecosystem services.
In the open ocean, warming waters and changing plankton populations are key culprits, linked closely to the broader climate crisis.
DARKENING OCEANS DISRUPT THE BALANCE OF LIFE BENEATH THE WAVES
The photic zone is where sunlight powers photosynthesis, producing oxygen and fueling life for up to 90% of marine species. Shrinking this zone compresses habitats, forcing species into tighter spaces and escalating competition for food and breeding grounds.
Dr. Thomas Davies, the study’s lead author, said ocean darkening could disrupt ecological relationships and endanger ocean productivity. “We’re losing light in places that matter most,” Davies said. “This isn’t abstract—it’s a threat to food, oxygen, and biodiversity.”
The phenomenon poses risks to fish stocks, plankton populations, and climate regulation functions that rely on sunlight-driven processes. Marine species will likely shift closer to the surface to access light. This shift raises risks of overfishing and exposure to surface-level threats.
ARCTIC AND ANTARCTIC SHOW MOST DRASTIC CHANGES
The study points to the Gulf Stream, Arctic, and Antarctic as hotspots. Photic zones have dramatically declined in these areas due to warming.
These regions are already experiencing severe climate change, now compounded by loss of light crucial to polar marine ecosystems. In enclosed seas like the Baltic, heavy rainfall and runoff load waters with sediment and nutrients, accelerating light-blocking algal growth.
These trends suggest that darkening is not uniform but heavily influenced by local geography, weather, and human land use. Professor Tim Smyth, co-author of the study, warned that shrinking light zones could trigger major shifts in entire marine ecosystems.
“If animals must move closer to the surface for light, we could see food webs collapse or migrate,” Smyth said.
The findings mark a new frontier in understanding ocean health. They serve as a reminder that climate change is altering more than temperatures and ice.
SATELLITE EYES REVEAL A HIDDEN CRISIS IN THE DEEP
Using satellite sensors, researchers tracked each 9km square of ocean and mapped changes in light penetration over two decades.
This high-resolution approach revealed both widespread darkening and smaller areas where oceans have actually grown lighter. Roughly 10% of oceans became brighter, largely in regions where water clarity improved or nutrient flows stabilized.
But these gains were far outweighed by the extent and severity of darkening across key regions tied to biodiversity and food security. The data supports growing evidence that Earth’s oceans are dynamic, sensitive systems deeply affected by both local and global forces.
WHAT’S NEXT? A CALL FOR URGENT MONITORING AND RESPONSE
Scientists emphasize that ocean darkening must be monitored alongside temperature, acidification, and sea level rise.
It’s a less visible, but no less urgent, indicator of planetary stress—impacting life from microscopic algae to global fisheries.
Without adequate light, the ocean’s ability to store carbon, produce oxygen, and feed billions is at serious risk. Governments and global bodies must factor ocean darkening into marine conservation strategies and climate action plans.
Investing in clearer data and ocean monitoring systems can help detect and respond to these under-the-radar ecosystem shifts. As Earth warms and weather patterns intensify, understanding and addressing light loss could help protect the ocean’s vital role. In short, the sea is losing its light—slowly, quietly, but significantly. And that loss may change the future of life on Earth.
