Vast stretches of the world’s oceans have grown noticeably darker over the past two decades, a shift scientists warn could upend marine ecosystems and threaten global ocean health. A recent study published in Global Change Biology found that 21% of the world’s oceans have darkened between 2003 and 2022. Using satellite data and mathematical modeling, researchers determined that about 75 million square kilometers of the ocean—an area comparable to the combined size of Europe, Africa, China, and North America—experienced reduced light penetration.
This alarming trend affects the photic zone, the upper layer of water where sunlight is sufficient to support photosynthesis, and where 90% of marine species live. According to the study, while sunlight can penetrate up to about a kilometer, it is the top 200 meters that receive enough light to sustain life. In the newly darkened areas, which include sections of the open ocean as well as coastal regions, this crucial band of life-supporting water has become shallower, providing less space for organisms like phytoplankton to photosynthesize. These organisms form the base of marine food webs and generate nearly half of Earth’s oxygen.
The Impact on Marine Life
The study found that in 9% of the world’s oceans, the photic zone has become 50 meters shallower, and in 2.6% of the ocean, it is now 100 meters shallower. This reduction in the photic zone could have far-reaching consequences for marine life. As the zone becomes shallower, marine animals may face increased competition for food and may need to move closer to the surface to access light and nutrients. This shift could disrupt existing food chains and alter the distribution of marine species.
“Ocean darkening occurs when changes in the optical properties of the oceans reduce the depth to which sufficient light penetrates to facilitate biological processes guided by sunlight and moonlight.”
While coastal darkening is often linked to runoff and sediment and nutrient-rich upwellings of cold water from the deep, the causes of open-ocean darkening are less established. Researchers suspect that changing ocean currents and climate-driven warming play a part. Other potential factors include algal bloom dynamics, changes in sea surface temperatures, and artificial light.
Scientific Perspectives and Concerns
Study authors Thomas Davies and Thomas Smyth emphasize the potential consequences of ocean darkening, ranging from impacts on the air we breathe to increased competition for food among marine animals. Despite the concerning trend, the study also notes that about 10% of the ocean, including a zone off Ireland’s west coast, has actually become lighter over the same period. However, scientists caution that the overall trend remains worrying.
Experts suggest that the darkening of the ocean could have implications beyond marine ecosystems. The reduction in photosynthesis could lead to decreased oxygen production, affecting both marine and terrestrial life. Additionally, changes in the distribution of marine species could impact fisheries and the economies that rely on them.
Looking Forward: Addressing the Challenge
The findings of this study highlight the urgent need for further research into the causes and consequences of ocean darkening. Understanding the underlying mechanisms driving this phenomenon is crucial for developing strategies to mitigate its impact. Scientists advocate for increased monitoring of ocean conditions and the implementation of measures to reduce pollution and climate change, which are likely contributors to the darkening trend.
As the world grapples with the challenges posed by climate change, the darkening of the oceans serves as a stark reminder of the complex and interconnected nature of Earth’s ecosystems. Addressing this issue will require a coordinated global effort to protect the health of our oceans and the myriad forms of life they support.