This can lead to a major problem for humans because we rely on a stable, predictable planet for just about everything.

Every civilization is based on predictability.

Even if you're hunter-gatherers, the animals will move at this time.

So we're all making predictions.

a massive assumption about stability of the Earth's system, and that stability is not given.

It wasn't always like that.

We live within this two-degree variation, and it's that stability of the two degrees that has enabled humans to multiply, to develop, to do sedentary agriculture, and to be where we are today.

But even if there is uncertainty about the exact timing and thresholds, it is absolutely clear that human activity is a major cause of the system-level changes we're seeing today.