“That plunged us into a snowball Earth that nearly killed off all of life. The great oxidation event 2.7bn years ago saw primitive bacteria start producing oxygen which led to the destruction of the potent greenhouse gas methane in the atmosphere. “But there are examples where the Earth system did just the opposite, where it spun out of control, and did so because of life itself,” Mann says. Prof Michael Mann points to ‘stabilising mechanisms’ in the Earth’s climate – but also examples of when it ‘spun out of control’.