Global Politics Explainer

CWA # 1762
Global Politics Explainer
Ocean Darkening: 
What is the phenomenon? What are its effects? And who are more vulnerable?

Lekshmi MK
21 July 2025

Photo Source:

According to a recent study published by the University of Plymouth on 27 May, its study on the oceans revealed that one-fifth of the total ocean area has become darker. The study found that 21 per cent of the surface waters of the ocean faced reductions in light penetration since 2003. The findings were released few weeks before the United Nations Ocean Conference III held in Nice (June 2025). 

The reduced light penetration raises concerns about the health of marine ecosystems. Yet the issue failed to gain significant attention in diplomatic discussions. Scientists and environmental groups have called for photic zone degradation to be included in frameworks such as Sustainable Development Goal 14 (SDG 14) and the United Nations Framework Convention on Climate Change (UNFCCC)’s adaptation and mitigation pathways. The same concern has also been raised by the fishing communities in South Asia, West Africa, and Southeast Asia. They reported noticeable changes in fish behaviour and water clarity.  

What is ocean darkening? 
According to the University of Plymouth’s 2025 study, since 2003, 21 per cent of surface waters globally experienced significant declines in light penetration, marking ocean darkening as a widespread and growing issue. 
Ocean darkening refers to the reduction in sunlight penetration into the upper layers of the ocean. This phenomenon occurs due to increased sediment, pollutants, dissolved organic matter, and algal blooms. Due to this phenomenon, ocean surface absorbs or scatter sunlight. As a result, less light reaches deeper waters, affecting photosynthesis and marine life. 
Ocean darkening is measured using satellite remote sensing technologies such as NASA’s MODIS (Moderate Resolution Imaging Spectroradiometer). This instrument can detect changes in ocean colour and light reflectance from ocean surfaces. 

What are the effects of ocean darkening?
First, the ocean light and ecosystem impact. Ocean darkening reduces sunlight penetration in the upper layer of the ocean. This effect weakens the photic zone that supports phytoplankton, the foundation of marine food chains. Ultimately, ocean darkening destabilises marine ecosystems. Second, livelihood and coastal concerns. In regions like South Asia, West Africa, and Southeast Asia, small-scale fisher communities reported shifting fish behaviour and declining water clarity. These changes disrupt fishing patterns, reduce catch volumes, and reduce income and food security. Particularly, women-led fisheries and artisanal fishers are vulnerable. They are lacking the tools to adapt.
Third, global climate and governance relevance. Ocean darkening is still absent from climate governance frameworks like the UNFCCC or SDG 14. Its neglect in global diplomacy and science-policy agendas makes it an urgent environmental challenge.

Who are vulnerable to ocean darkening, and why?
Ocean darkening disproportionately affects countries that rely heavily on marine ecosystems. Large populations of India, Indonesia, the Philippines, and Senegal depend on small-scale fisheries. These countries face direct impacts as ocean darkening disrupts fish behavior. These changes threaten coastal communities with local food security and income sources. They also survive with limited adaptation capacity. Small Island Developing States (SIDS) like Fiji or the Maldives are also highly vulnerable. Ocean darkening contributes to other stressors such as warming seas, acidification, and sea-level rise. 

Most of the above countries also lack advanced tools to track ocean optics, making early detection difficult. Without data, darkening remains invisible in environmental assessments and national adaptation plans. 

As a result, countries least responsible for global pollution often bear the consequences. This impact intensifies climate injustice and widening the gap between global ecological responsibility and local vulnerability. Most developing countries are not equipped to monitor underwater light changes, and is not well funded. Without keen advocacy by scientists, civil society, and exposed coastal communities, darkening oceans will continue to be a blind spot for governments.

Why is ocean darkening missing from climate negotiations and funding priorities?
Ocean darkening remains largely absent from international climate negotiations and funding schemes. It has less visibility while other issues like coral bleaching, plastic pollution, or sea-level rise gain more attention. Most climate management frameworks, such as the UNFCCC and SDG 14, refer to ocean health in general terms. But they do not address changes in the ocean’s photic zone. Light penetration is not routinely monitored in climate models or Nationally Determined Contributions (NDCs) like carbon emissions or temperature rise.

References
 Dr Thomas Davies and Professor Tim Smyth, "Ocean darkening, understanding the scale and impact of darker oceans," University of Plymouth, 27 May 2025
 Karen McVeigh, "Is the ocean ‘having a moment’? This was the UN summit where the world woke up to the decline of the seas," The Guardian, 14 June 2025
 Alan Williams, "21% of the Ocean Is Losing Sunlight – What That Means for Marine Life," Scitechdaily, 28 May 2025

About the author
Lekshmi M K is a research intern at the National Institute of Advanced Studies, Bengaluru. Her research interests include ocean governance, geopolitics of the Arctic and Antarctic regions, the impact of climate change on maritime security, and environmental diplomacy.
 

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