About two and one-half billion years ago…
Once the oceans’ supply of iron was exhausted, oxygen began to seep from the sea into the air. With very little competition for resources, cyanobacteria continued to proliferate and pollute. The free oxygen they produced reacted with the air, gradually breaking down the methane which kept the Earth’s atmosphere warm and accommodating. It took at least a hundred thousand years– a short duration in geological terms– but the Earth was eventually stripped of her methane, and with it her ability to store the heat from the sun. Temperatures fell well below freezing worldwide, and a thick layer of ice began to encase the oxygen-saturated planet.

The survivors of the oxygen catastrophe eventually adapted to consume the abundant oxygen and produce carbon dioxide. This greenhouse gas very gradually made its way into the atmosphere, increasing in concentration and nudging temperatures back into the hospitable range over millions of years. Had temperatures been slightly colder during the first major ice age– if Earth had been in a slightly more distant orbit– the planet would have remained an icy wasteland because the carbon dioxide would have frozen solid before it could promote the greenhouse effect.