<p>Heat extremes are becoming more intense in a warming world. However, as global warming outcomes depend significantly on the interpretation and implementation of existing emission reduction pledges, estimating their future risks can be difficult. One of the most vulnerable groups to future heat extremes are those living in cities, whose future heat risk is exacerbated by a myriad of factors. Here, we estimate the exposure of future urban populations to heat extremes which are unprecedented in the current climate across four police-relevant warming levels: 1.9^∘C, 2.6^∘C, 2.8^∘C, and 3.1^∘C as outlined in the UN emissions gap report. Using established statistical methods used in extreme event attribution studies, alongside high resolution urban population projections we estimate that an additional 3.0–3.1 billion urban residents will be exposed to currently unprecedented heat extremes by the end of the twenty-first century if existing mitigation policies persist with no further action, relative to the most ambitious implementation scenario. Our results highlight the benefit of successfully implement the most ambitious mitigation commitments from the 2021 Glasgow Climate Pact.</p>