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The Atlantic meridional overturning circulation (AMOC) is an important part of our climate system. The AMOC is predicted to weaken under climate change; however, theories suggest that it may have a tipping point beyond which recovery is difficult, hence showing quasi-irreversibility (hysteresis). Although hysteresis has been seen in simple models, it has been difficult to demonstrate in comprehensive global climate models. Here, we outline a set of experiments designed to explore AMOC hysteresis and sensitivity to additional freshwater input as part of the North Atlantic Hosing Model Intercomparison Project (NAHosMIP). These experiments include adding additional freshwater (hosing) for a fixed length of time to examine the rate and mechanisms of AMOC weakening and whether the AMOC subsequently recovers once hosing stops.Initial results are shown from eight climate models participating in the Sixth Coupled Model Intercomparison Project (CMIP6). The AMOC weakens in all models as a result of the freshening, but once the freshening ceases, the AMOC recovers in half of the models, and in the other half it stays in a weakened state. The difference in model behaviour cannot be explained by the ocean model resolution or type nor by details of subgrid-scale parameterisations. Likewise, it cannot be explained by previously proposed properties of the mean climate state such as the strength of the salinity advection feedback. Instead, the AMOC recovery is determined by the climate state reached when hosing stops, with those experiments where the AMOC is weakest not experiencing a recovery.

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.

Climate changes under net zero emissions will take many centuries to play out, particularly in the Southern Hemisphere and in the ocean and cryosphere. New millennial-length Earth System Model simulations are required to better understand committed changes and their dependence on delays in reaching net zero emissions, especially with respect to local and regional extremes.

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 are expected to be detectable is less well understood. Here, we use a set of 1000‐year‐long net zero carbon dioxide emissions simulations branching from different points in the 21st century to examine detectability of large‐scale, regional and local climate changes as time passes under net zero emissions. We find that even after net zero, there are continued detectable changes to climate for centuries. While local changes and changes in extremes are more challenging to detect, Southern Hemisphere warming and Northern Hemisphere cooling become detectable at many locations within a few centuries under net zero emissions. We also study how detectable delays in achieving emissions cessation are across climate indices. We find that for global mean surface temperature and other large‐scale indices, such as Antarctic and Arctic sea ice extent, the effects of an additional 5 years of high greenhouse gas emissions are detectable. Such 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 improve understanding of post‐net zero climate changes. Plain Language Summary The rapid pace of climate change is observed in many aspects of the Earth system including local warming and rainfall changes, increases in some extremes, and decreasing ice in polar regions. These observable climate change effects have been part of the motivation for the Paris Agreement and the push to achieve net zero emissions. There is a growing understanding that we should expect some aspects of the climate to continue changing under net zero and that there are benefits to getting to net zero sooner, but it has been unclear to date whether these changes will be obvious or masked by noise in the climate. Here we use simulations to examine how apparent climate changes may be under net zero and the effects of delays in achieving net zero. We find that over time, detectable changes in the climate system still occur under net zero. Many people live in places where we identify detectable local climate changes under net zero emissions. Delays in getting to net zero have identifiable effects across many aspects of the climate system. Achieving net zero should not be expected to halt all climate changes, but it is a necessary step in reducing climate change impacts. Key Points We examine detectability of global, regional and local climate change measures using millennial‐scale net zero CO2 emissions simulations Detectable changes under net zero are found in temperature and precipitation means and extremes, Atlantic Meridional Overturning Circulation recovery, and sea ice extent Delays to emissions cessation have widespread consequences for many centuries