There is growing interest in how the climate would change under net zero carbon dioxide emissions pathways as many nations aim to reach net zero in coming decades. In today's rapidly warming world, many changes in the climate are detectable, even in the presence of internal variability, but whether climate changes under net zero would be detectable is less well understood. Here, we use a bespoke set of 1000-year-long net zero carbon dioxide emissions simulations branching from different points in the 21st century to examine the detectability of large-scale and local climate changes as time passes under net zero emissions. We find that many changes under net zero become detectable within centuries. While local changes and changes in extremes are more challenging to detect, warming in the Southern Hemisphere and cooling in the Northern Hemisphere becomes detectable at many locations within a few centuries under net zero emissions. We also study detectability of differences in climate indices due to delays in achieving emissions cessation. We find that for global mean surface temperature and other large-scale indices, such as Antarctic and Arctic sea ice extent, the effects of even a five-year delay in emissions cessation are detectable. Short delays in emissions cessation result in significantly different local temperatures for most of the planet, and most of the global population. The long simulations used here help with identifying local climate change signals. Multi-model frameworks will be useful to examine confidence in these changes and ultimately improve understanding of post-net zero climate changes.

Projections of European heat extremes have been widely explored in the context of continued global warming. However, analyses of recent Earth system model simulations point to substantial climatic changes over multi-centennial timescales in net-zero emissions futures. Focusing on Europe, we address the gap in characterising heat extremes in long-term net-zero stabilised climates. We quantify the long-term effects of delayed mitigation on annual maximum daily maximum temperatures (TXx) in European regions using 1000-year-long stabilised simulations with ACCESS-ESM-1.5, reaching net-zero CO₂ emissions at different times over the coming decades. We evaluate ACCESS-ESM-1.5 against the ERA5 reanalysis for European maximum temperatures using rank frequency analysis and compare present-day maximum temperatures to their long-term future likelihood. Across all European regions, any delay in achieving net-zero emissions shifts the distribution to higher annual maximum temperatures, remaining elevated at the same levels for centuries. European regions show two- to five-fold frequency increases for heat events as strong as current records, while the Mediterranean region could experience 30-fold increases if emissions cessation is delayed until 2060. When comparing extreme heat distributions at global warming levels, we find substantial differences between transient and net-zero emissions quasi-stable climate states, with larger differences at higher warming levels. We provide the first comprehensive assessment of European extreme hot temperatures in net-zero stabilised climates, paving the way for further investigations of other extreme event types or regions in net-zero futures.