Andrea J. Dittus

When anthropogenic CO ₂ emissions cease, global temperature does not stabilize immediately. The resulting global temperature change, the zero emissions commitment, is central to carbon budget assessments and understanding reversibility of global warming, yet its dependence on prior warming levels remains unclear. Here we show, using zero-emission simulations with the UK Earth System Model (UKESM1.2) across global warming levels of 1.5–5 K, that the zero emissions commitment stays near zero up to 2 K but becomes positive at higher warming levels, reaching 0.4–0.6 K after ~ 150 years. This dependence indicates that stabilizing or reversing warming becomes progressively more difficult as peak temperature rises. We identify the mechanism behind this behaviour: stronger warming from higher radiative disequilibrium and weakening carbon sinks, driven by saturated land carbon uptake and the reduced radiative impact of CO ₂ drawdown at higher background CO ₂ concentrations. Higher peak warming therefore commits the planet to additional warming, limiting temperature-overshoot reversibility.

This paper introduces the CANARI HadGEM3 Large Ensemble (CANARI LE), a 40-member Single-Model Initial-condition Large Ensemble (SMILE) designed to advance the study of climate variability and change. It is based on HadGEM3-GC3.1, the physical climate model used by the UK Met Office and partners for submissions to CMIP6 (Climate Model Intercomparison Project Phase 6). The CANARI LE simulations span 1950–2100 and are driven by CMIP6 historical and SSP3-7.0 forcings. Compared to existing Large Ensembles, the CANARI LE offers several distinctive features: (i) its resolution (1/4° in the ocean, 60 km in the midlatitude atmosphere), (ii) sub-daily output at the surface and on atmospheric pressure levels, and (iii) the provision of boundary conditions for regional modelling. In this study, the design of the CANARI LE is documented and key aspects of historical model performance are evaluated, while applications of the SSP3-7.0 simulations are left for forthcoming studies. Model evaluations are carried out for global mean surface temperature, North Atlantic jets and storm tracks, Northern Hemisphere stratospheric circulation, Atlantic meridional overturning circulation and subpolar gyre, Arctic sea ice, and El Niño-Southern Oscillation teleconnections. In addition, an example is provided deriving analogues of the North Atlantic 2023 summer marine heatwave showing that the CANARI LE captures unprecedented events of this magnitude and spatial scale. The CANARI LE is a community resource that lends itself for the study of physical coupled climate processes and weather systems in a changing climate. The ability to drive regional models will potentially allow for risk assessments of unprecedented detail in climate impacts and adaptation studies.

This paper introduces the CANARI HadGEM3 Large Ensemble (CANARI LE), a 40-member Single-Model Initial-condition Large Ensemble (SMILE) designed to advance the study of climate variability and change. It is based on HadGEM3-GC3.1, the physical climate model used by the UK Met Office and partners for submissions to CMIP6 (Climate Model Intercomparison Project Phase 6). The CANARI LE simulations span 1950–2100 and are driven by CMIP6 historical and SSP3-7.0 forcings. Compared to existing Large Ensembles, the CANARI LE offers several distinctive features: (i) its resolution (1/4° in the ocean, 60 km in the midlatitude atmosphere), (ii) sub-daily output at the surface and on atmospheric pressure levels, and (iii) the provision of boundary conditions for regional modelling. In this study, the design of the CANARI LE is documented and key aspects of historical model performance are evaluated, while applications of the SSP3-7.0 simulations are left for forthcoming studies. Model evaluations are carried out for global mean surface temperature, North Atlantic jets and storm tracks, Northern Hemisphere stratospheric circulation, Atlantic meridional overturning circulation and subpolar gyre, Arctic sea ice, and El Niño-Southern Oscillation teleconnections. In addition, an example is provided deriving analogues of the North Atlantic 2023 summer marine heatwave showing that the CANARI LE captures unprecedented events of this magnitude and spatial scale. The CANARI LE is a community resource that lends itself for the study of physical coupled climate processes and weather systems in a changing climate. The ability to drive regional models will potentially allow for risk assessments of unprecedented detail in climate impacts and adaptation studies.

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.

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.

This paper introduces the CANARI HadGEM3 Large Ensemble (CANARI LE), a 40-member Single-Model Initial-condition Large Ensemble (SMILE) designed to advance the study of climate variability and change. It is based on HadGEM3-GC3.1, the physical climate model used by the UK Met Office and partners for submissions to CMIP6 (Climate Model Intercomparison Project Phase 6). The CANARI LE simulations span 1950–2100 and are driven by CMIP6 historical and SSP3-7.0 forcings. Compared to existing Large Ensembles, the CANARI LE offers several distinctive features: (i) its resolution (1/4° in the ocean, 60 km in the midlatitude atmosphere), (ii) sub-daily output at the surface and on atmospheric pressure levels, and (iii) the provision of boundary conditions for regional modelling. In this study, the design of the CANARI LE is documented and key aspects of historical model performance are evaluated, while applications of the SSP3-7.0 simulations are left for forthcoming studies. Model evaluations are carried out for global mean surface temperature, North Atlantic jets and storm tracks, Northern Hemisphere stratospheric circulation, Atlantic meridional overturning circulation and subpolar gyre, Arctic sea ice, and El Niño-Southern Oscillation teleconnections. In addition, an example is provided deriving analogues of the North Atlantic 2023 summer marine heatwave showing that the CANARI LE captures unprecedented events of this magnitude and spatial scale. The CANARI LE is a community resource that lends itself for the study of physical coupled climate processes and weather systems in a changing climate. The ability to drive regional models will potentially allow for risk assessments of unprecedented detail in climate impacts and adaptation studies.

We describe a new Earth system model (ESM) experiment protocol, as part of the international Tipping Points Modelling Intercomparison Project (TIPMIP) project. We propose this as a protocol for the Coupled Model Intercomparison Project 7 (CMIP7). The protocol requires ESMs to run in CO ₂ -emission mode, with atmospheric CO ₂ a predicted variable. Forcing for the protocol consists solely of a constant emission of CO ₂ , based on each model's transient climate response to cumulative emissions of carbon dioxide (TCRE) value, to give a common global mean surface warming rate of 2 °C per century. This positive emission (ramp-up) experiment is started from the pre-industrial state of a given model. When the ramp-up run first exceeds a specified level of global warming (2 and 4 °C) relative to the model's pre-industrial global mean surface air temperature (GMSAT), CO ₂ emissions are set to zero and the positive emission run is branched into a zero-emission run. The zero-emission runs continue for 300 years. 50 years into each zero-emission run, CO ₂ emissions are set to the negative of the positive emission rate and the model run until GMSAT cools below the original pre-industrial value. Additionally, when the negative emission run started from the global warming level (GWL)=4 °C first drops below GWL=2 °C, a zero-emission run is branched off this, completing the set of experiments. Using this protocol, we are able to control the rate of global warming, and potentially also the rate of cooling, across participating models. TIPMIP experiments will support a range of analyses, including; an assessment of abrupt/rapid Earth system change under net zero CO ₂ emissions at a range of global warming levels, the long-term Earth system response to net zero CO ₂ emissions at these warming levels, the response to net negative CO ₂ emissions and the efficacy of negative emissions to drive cooling, and the reversibility of Earth system change under a pathway of positive (warming), zero, and negative (cooling) CO ₂ emissions.

The tropical Pacific is warming more in the west than the east. This observed strengthening of the tropical Pacific east‐to‐west Sea Surface Temperature (SST) gradient is poorly reproduced in climate models—a prominent model bias with far reaching global impacts. We explore the tropical Pacific SST gradient response to anthropogenic aerosols in large‐ensembles of CMIP6 simulations between 1950 and 2014. We find that anthropogenic aerosols are cooling the tropical Pacific—the cooling is more pronounced in the east Pacific—while greenhouse gases have the opposite response. Tropical Pacific gradient strengthening is not due to the anthropogenic aerosol effective radiative forcing magnitude or top of atmosphere energy imbalance. This suggests that pathways connecting the regional radiative response to the tropical Pacific are important. In the future, all models predict a weakening gradient, irrespective of historical trends. Anthropogenic aerosol forcing plays an important role in driving changes in tropical Pacific SSTs.

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.