Hot extremes following net-zero CO₂ emissions in UKESM: physical drivers and role of vegetation

Abstract. Reaching net-zero CO₂ emissions is essential to halt continuing global warming and attempt to stabilise global temperatures. However, large uncertainties remain on the sign and the magnitude of the long-term responses of the climate system following anthropogenic emissions cessation. This study contributes to improving our understanding of the climate system post CO₂ emissions cessation, by exploring the global and regional temperature evolution in UKESM1.2 following the TIPMIP protocol. Zero CO₂ emission simulations, starting from global warming levels of +1.5 °C to +5 °C above pre-industrial are analysed to understand the impact of historical cumulative emissions and associated global warming level on post zero-emissions trends. We find that the global average surface air temperature (GSAT) keeps increasing in all zero CO₂ emission UKESM1.2 projections. The increase is more pronounced at higher warming levels, approaching 0.25 °C per century in the +3.0 °C to +5.0 °C scenarios. Most of the warming occurs in the Southern Hemisphere, particularly in the Southern Ocean, while the Northern Hemisphere land experiences a slight cooling trend. These regional cooling trends are more marked for the annual temperature maxima, with several regions across 45–65° N experiencing cooling of >1 °C per century. We find the strongest regional cooling trend following emissions cessation in the higher warming scenarios. Here, we investigate the drivers behind the cooling trend in northeastern North America, where the cooling magnitude exceeds 1.5 °C per century. We find that the cooling trend is almost completely explained by thermodynamic drivers. We reconcile this finding with the UKESM1.2 dynamic vegetation changes, as the evergreen vegetation cover increases across all regions experiencing substantial cooling in the hot extremes. This finding highlights the significant regional contribution that vegetation changes can have for the attenuation of annual temperature maxima, supporting the case for their careful consideration in future mitigation and adaptation strategies. However, these results also show the limitations of highly idealised scenario protocols like TIPMIP, which set crop and pasture distributions, as well as other anthropogenic forcings, to pre-industrial values, allowing vegetation to expand freely. This highlights the importance of developing new zero emissions protocols considering other forcing agents beyond CO₂.