Earth's Oceans Were Green a Billion Years Ago. They Could Soon Be Green Again.

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19-02-2025

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A new study from scientists at Nagoya University in Japan analyzes how the Earth likely sported a green hue in its early days.
Due to evolutionary pressures and the light spectrum available to photosynthesizing cyanobacteria on early Earth, the world's oceans were likely much greener than they are today.
Although the Earth's been decidedly blue for 600 million years, rising populations of phytoplankton caused by rising temperatures are once again causing the world's oceans to turn green.
On September 5, 1977, while venturing out of the Solar System, NASA's Voyager 1 spacecraft swung its gaze one last time to its home planet and snapped a photo. Now known as 'The Pale Blue Dot'—a famous moniker coined by the scientist Carl Sagan—the image shows the Earth suspended in a sunbeam, wholly insignificant against the black void of never ending space.
However, for a majority of its history, the Earth wouldn't have looked blue at all. In fact, for billions of years, any visiting extraterrestrial would've more likely suggested the name 'Pale Green Dot.' From 3 billion years ago to roughly 600 million years ago—right at the dawn of complex life on the planet—the Earth's oceans would've been significantly more green than they are today. Scientists from Nagoya University in Japan investigated why ancient Earth would've donned such a greenish hue, and found that cyanobacteria were the driving factor. The results of the study were published in the journal Nature Ecology & Evolution.
'[The 'Pale Blue Dot'] description is a consequence of the Rayleigh scattering of sunlight in the atmosphere, in conjunction with the reflection and scattering across the expanse of the ocean,' the authors wrote. 'Nevertheless, one might inquire: does only a blue hue of a planet serve as an indicator of its potential to nurture life?'
A collection of various factors—particularly the make-up of the world's oceans during its early days—determined the planet's overall hue. In Earth's first few billion years, the planet's oceans were filled with iron hydroxide, which is an inorganic compound that absorbs blue light. Meanwhile, water present in these ancient oceans would've absorbed red light, creating a 'green light window,' Taro Matsuro, the study's lead author, told New Scientist.
Cyanobacteria, much like plants, use chlorophyll to photosynthesize sunlight, which results in organism absorbing red and blue light and reflecting green. However, these organisms also contain pigments called 'phycobilins' that absorb red and green light. Matsuro and his team wanted to understand why, and what it tells us about the time in which these cyanobacteria evolved.
The scientists created models to determine what spectrum of light would have been available to ancient photosynthesizing life, and found that the spectrum matched the light absorbed by phycobilin pigments. When replicating the conditions of Archaean Earth, cyanobacteria with these phycobilin pigments grew more rapidly, suggesting that evolution would have favored their inclusion.
'If we assume an atmosphere similar to today's, the green hue reflected by the ocean would have mixed with the blue from Rayleigh scattering, likely creating a more bluish-green color rather than the blue we see today,' Matsuo told New Scientist. He also says that the ocean was likely larger than it is today, so its effect on the planet's hue would've been more profound.
However, just like old fashion trends can suddenly become the 'new hot thing,' the Earth's oceans might one day return to the green side of the color spectrum—though, likely through entirely different means. A 2019 study conducted by MIT suggested that by the century's end, half the world's oceans would turn green due to rising phytoplankton populations as the world warms. In 2023, a follow-up study confirmed that 56 percent of the world's oceans had already greened in just the past 20 years.
The only constant is change—a bit of wisdom that relates to the world's oceans, as well.
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