Detectability of Post-Net Zero Climate Changes and the Effects of Delay in Emissions Cessation

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.