Brits' environmental footprints are less than 3% of the global total—and declining. Three percent is the upper bound of how much the UK can reduce emissions itself. Reducing 90% of UK emissions by 2050 will cost roughly $5 trillion. But this barely makes a dent in the current global emissions trajectory.

Because of diminishing returns, it is very hard to imagine reducing UK emissions to zero. This would mean replacing every last lightbulb with LEDs powered by zero carbon energy and having everyone fly in electric planes. It is much easier to conceive of an UK scientists or engineers improving technology, say, carbon capture technology so that the diffuse benefits reduce global emissions by >3%.