"For those of you who are interested in statistics, this is a five-sigma event. So it's five standard deviations beyond the mean. Which means that if nothing had changed, we'd expect to see a winter like this about once every 7.5 million years.

[...]

She fears a further change in the balance could trigger a tipping point from where it's difficult to reverse the trajectory. "We might end up in a new state," she said. "That would be quite concerning to the sustainability of human conditions on Earth, I suspect.