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We’re not the first living beings to drastically alter Earth’s climate. The earliest photosynthetic microorganisms belched out enough methane to warm the planet by 15°C. This bout of global warming may have saved Earth from freezing over, and created a comfortable climate for early organisms.
When Earth formed 4.5 billion years ago, the sun was 25 per cent dimmer than it is today. This suggests the early planet should have been a big snowball, but geological evidence indicates it was just as warm as now, if not warmer.
One explanation for this “faint young sun paradox” is that greenhouse gases like carbon dioxide warmed Earth by trapping the sun’s heat. But carbon dioxide levels probably weren’t high enough to fully account for the balmy climate.
Now, Chris Reinhard and Kazumi Ozaki at the Georgia Institute of Technology in Atlanta and