We messed up our figures on how much carbon dioxide is too much

What would we have to do to limit global warming to 2°C, or better still to 1.5°C?

For the past decade or so, climate scientists have answered this vital question in terms of the total amount of carbon dioxide we can pump into the air. If we emit more than a certain total, temperatures will rise by more than 2°C, greatly increasing extreme weather and further accelerating the melting of ice sheets.

But the whole idea of…