David Wallace-Wells:

There's sort of three big things.

The first is speed. I think we had long thought that climate change was happening very slowly, that it was unfolding, at fastest, at about a decade timescale, more usually like centuries, and we didn't have to worry about it in our own lives, maybe even our children's lives, but it was something to worry about for our grandchildren.

More than half of all the emissions that we have put into the atmosphere in the entire history of humanity, we have done in the last 30 years. And that means that we're doing this damage in real time and in the space of a generation. So the speed is really overwhelming.

And if we lived off the coasts, we often thought that it was a matter of sea level rise, and we'd be safe. We could live inland, that would be OK. In fact, it's an all-encompassing threat that's not compartmentalizable to the coast. It's much bigger than sea level rise.

It impacts the economy, which could be 30 percent lower than it would be without climate change by the end of the century, and it impacts public health, conflict. We could have twice as much war because of climate change at the end of the century. That's the second thing, the scope.

And then the severity. Most scientists talked about two degrees as the sort of threshold of catastrophe.