The Oceans Are Greening Toward the Poles: What This Means for Life on Earth
- sulevaivelina
- Jun 25
- 2 min read
A new study published in Science by Zhao et al. (2025) reveals a striking global trend: the oceans are becoming greener at higher latitudes, while tropical and subtropical waters are turning bluer. This change is driven by shifting distributions of phytoplankton—microscopic algae that form the base of the marine food web.
What Did the Researchers Find?
Using satellite data on chlorophyll—the pigment that phytoplankton use for photosynthesis and that gives ocean water its green tint—scientists observed a 20-year trend:
Increased chlorophyll concentrations (i.e., more phytoplankton) in polar regions, particularly in the Northern Hemisphere.
Decreased chlorophyll concentrations in tropical and subtropical zones, resulting in a "bluing" of those waters, signaling lower biological productivity.
What’s Causing This?
The primary driver appears to be rising sea surface temperatures, which:
Extend the growing season in polar waters by reducing sea ice and increasing light availability.
Increase ocean stratification in the tropics, limiting the upward movement of nutrient-rich deep water.
In the Southern Hemisphere, the greening is less intense—likely due to iron limitations that hinder phytoplankton growth despite warming.
What Are the Implications?
This isn’t just a color change. Phytoplankton are essential to marine ecosystems. According to the authors:
Marine life in the tropics may decline due to decreasing productivity.
Fisheries may shift poleward, following the biomass, which could spark geopolitical and economic tensions, particularly for low-income fishing communities.
Even small changes in phytoplankton can trigger large ecosystem shifts through trophic amplification—where disruptions at the base of the food web impact fish, birds, and marine mammals at the top.
Source:Raphael M. Kudela, “Poleward greening of the global oceans,” Science, Vol. 388, Issue 6671, pp. 1337–1338 (2025). Poleward greening of the global oceans | Science
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