Free oxygen broke down atmospheric methane, one of the most powerful of greenhouse gases, while photosynthesizing cyanobacteria consumed huge amounts of the other crucial greenhouse gas, carbon dioxide. As oxygen levels rose and levels of greenhouse gases fell, early in the Proterozoic eon, Earth froze in the first of several snowball-Earth episodes. Glaciers spread from the poles to the equator, turning the Earth white, and a white Earth reflected more sunlight, cooling it even further in a terrifying positive-feedback loop. Eventually, most of Earth’s oceans and continents were covered by
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