How ambitious is the world? The Paris climate conference last December astounded many by pledging not just to keep warming “well below two degrees Celsius,” but also to “pursue efforts” to limit warming to 1.5C. That raised a hugely important question: What’s the difference between a two-degree world and a 1.5-degree world?

Given we are already at one degree above pre-industrial levels, halting at 1.5C would look to be at least twice as hard as the two-degree option.

So would it be worth it? And is it even remotely achievable?