Economists tend to agree that an important way to reduce carbon emissions is to tax them, providing incentives for everyone to cut down. But the changes those taxes will bring about will be disruptive in the short run. That means any discussion of imposing a carbon tax in a country soon runs into either free-rider or fairness problems.

The US and other industrialised countries worry that developing countries will freeride and keep increasing emissions, even as everyone else works hard to bring them down. Developing countries, for their part, believe there is profound inequity in asking Uganda, which had per capita emissions of 0.1 tonnes of carbon dioxide in 2017, to bear the same burden of adjustment as the US, which emitted 16 tonnes per person, or Saudi Arabia, which emitted 19 tonnes.

Yet, the least costly way of reducing global emissions would be to give everyone similar incentives: Uganda should not build dirty coal plants as it grows, and Europe should close down the plants it already has. How do we balance these concerns while saving the world?