Likelihoods of European hottest temperatures in net-zero stabilised climates

While projections of European heat extremes have been widely explored, only recent efforts address heat extremes specifically in net-zero emissions futures and with a global rather than regional focus. In addition, existing studies extend to net-zero futures spanning a few decades, but new Earth system model simulations point to substantial net-zero emissions changes over multi-centennial timescales. Therefore, we address the knowledge gap on characterising European heat extremes in long-term net-zero stabilised climates. We quantify and attribute yearly hottest temperatures (TXx) in European regions using extended Earth system model simulations with ACCESS-ESM-1.5. Analysing these 1000-year net-zero emissions simulations branched over the coming decades at different times of a transient scenario, we address the long-term effects of delayed mitigation on European heat extremes. After favourably evaluating our model for European hottest days against the ERA5 reanalysis using rank frequency analysis, we compare present-day hottest days to their long-term net-zero future likelihood. Across all European regions, any delay in achieving net-zero emissions shifts the distribution of yearly hottest days towards higher temperatures, and these extreme temperatures remain elevated for centuries. Most European regions show two- to five-fold frequency increases for heat events as strong as currently observed records, while the Mediterranean region could experience more than 30-fold increases for current records. When comparing extreme heat distributions at global mean temperature warming levels from transient periods to levels in early and late stabilised periods, we find warm shifts (about one degree) in transient climates, while colder distributions result from earlier mitigation at higher (3°C) global warming levels. We provide the first assessment of European hottest temperatures in net-zero stabilised climates, paving the way for further investigations of other extreme event types or regions in net-zero long-term timescales.