NEW YORK – 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. The study, published in Global Change Biology, found that 21% of the world’s oceans have darkened between 2003 and 2022.
Immediate Impact
Using satellite data and mathematical modeling, researchers determined that about 75 million square kilometers of the ocean—an area equivalent to the combined landmass of Europe, Africa, China, and North America—experienced reduced light penetration.
Key Details Emerge
This change affects the photic zone, the upper layer of water where there’s enough sunlight to support photosynthesis and where 90% of marine species live. According to The Guardian, while sunlight can penetrate up to about a kilometer, the top 200 meters receive sufficient light. In newly darkened areas, this crucial band of life-supporting water is now shallower, meaning less space for organisms like phytoplankton to photosynthesize. They form the base of marine food webs and generate nearly half of Earth’s oxygen.
9% of the world’s oceans have seen the photic zone become 50 meters shallower, and in 2.6% of the ocean, it is now 100 meters shallower.
Expert Analysis
Study authors Thomas Davies and Thomas Smyth suggest ocean darkening could have consequences ranging from impacts on the air we breathe to increased competition for food among marine animals, who may need to move closer to the surface for light and sustenance. Davies notes, “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.”
By the Numbers
- 21% of oceans darkened from 2003 to 2022
- 75 million sq km affected
- 9% saw photic zone shrink by 50 meters
- 2.6% saw photic zone shrink by 100 meters
Background Context
While coastal darkening is often linked to runoff and sediment, as well as 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. The BBC highlights other potential factors, including algal bloom dynamics, changes in sea surface temperatures, and artificial light.
Regional Implications
Interestingly, about 10% of the ocean, including a zone off Ireland’s west coast, has become lighter during the same period. However, scientists emphasize that the overall trend remains concerning.
What Comes Next
The timing of these findings is particularly significant as global climate discussions continue to address the multifaceted impacts of climate change. Future research will likely focus on pinpointing the specific causes of open-ocean darkening and developing strategies to mitigate its effects on marine ecosystems.
The announcement comes as the world grapples with the broader implications of climate change on biodiversity and the health of our oceans. As scientists continue to investigate, the urgency to address these changes grows ever more pressing.