Large differences in climate outcomes are projected by the end of this century depending on whether greenhouse gas emissions continue to increase or are reduced sufficiently to limit total warming to below 2 degrees C (ref. 1). However, it is generally thought that benefits of mitigation are hidden by internal climate variability until later in the century2. Here we show that if the likelihood of extremely hot seasons is considered, the benefits of mitigation emerge more quickly than previously thought. It takes less than 20 years of emissions reductions in many regions for the likelihood of extreme seasonal warmth to reduce by more than half following initiation of mitigation. Additionally we show that the latest possible date at which the probability of extreme seasonal temperatures will be halved through emissions reductions consistent with the 2 degrees C target is in the 2040s. Exposure to climate risk is therefore reduced markedly and rapidly with substantial reductions of greenhouse gas emissions, demonstrating that the early mitigation needed to limit eventual warming below potentially dangerous levels benefits societies in the nearer term not just in the longer-term future.