What does a commitment to effective action look like in practice? Well, we see the solution in three parts. First, we must test every policy. We must know how many tonnes of greenhouse gas as a policy we take from the atmosphere, taking into account downstream effects to decide whether to go on with the policy or not. Every policy should be tested and each policy should be tested again after it is launched—and not just once but regularly, because reducing emissions is not just where to cut emissions but how much. For every policy the first tonne of cuts is usually the cheapest. Further cuts in emissions become more expensive. Each policy has a point at which the next tonne is no longer competitive with other sources. Our ambitious targets mean we need to know when each policy has reached those critical points. 