We’ve always called Earth the blue planet, but that color might not be permanent. Previous research claims our oceans were once green, and if environmental conditions shift dramatically, a new study says they could eventually turn purple.
The study, published in Nature Ecology & Evolution, explores how ocean color has changed over billions of years, driven by the chemistry of the water and the light filtering through it. Between 3.8 and 1.8 billion years ago, during the Archean eon, the seas weren’t blue at all. Back then, high levels of dissolved iron from volcanic vents and eroded rock turned the oceans green.
These iron-rich waters filtered out blue and red light, leaving green as the dominant color. Cyanobacteria, which were among the earliest life forms, adapted to this environment with a pigment called phycoerythrobilin, which allowed them to absorb green light more efficiently than chlorophyll alone.
But scientists are especially intrigued by what might happen next. The research team from Nagoya University modeled how modern ocean color could shift again, depending on future environmental changes. Under scenarios involving intense volcanic activity and rising sulfur levels, Earth’s oceans could take on an entirely different hue: purple.
That’s because sulfur compounds and other particles in low-oxygen waters can reflect light differently, shifting the visible spectrum. In sulfur-rich conditions, purple wavelengths may dominate the underwater environment. Combined with warming climates and decaying organic material washing into the seas, these shifts could be enough to tint vast stretches of the ocean.
Purple oceans wouldn’t just be a visual transformation. They’d represent significant changes in Earth’s atmosphere, water chemistry, and possibly the types of microorganisms that dominate marine ecosystems—all side effects of our losing battle with climate change. Just as ancient cyanobacteria adapted to green seas, future life could evolve to thrive in a purple-tinged world.
Importantly, the study suggests these color shifts aren’t just relics of the distant past or hypothetical sci-fi futures. They are tied to real geological and atmospheric forces that can and do reshape the planet. Watching how the oceans respond to environmental pressures might give us deeper insight into what’s happening beneath the surface, both literally and ecologically.
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