Emergence and de-emergence of climate change at regional scale in coupled climate models

The impacts of climate change won’t be felt everywhere equally. Considering just temperature, even if the globe warmed up uniformly, a single-degree temperature rise will be more significant for regions with less natural variability than those with more. Hence, it is important to quantify climate change relative to background conditions. The work in this thesis does just that, using the framework of emergence (change relative to background variability) to assess the significance of climate change at the regional level. My analysis is primarily forward-looking, comparing results from models running simulations of the future climate. I investigate the exposure that different regions of the world will have to unusual temperatures in the latest generation of coupled climate models across a range of scenarios. While the negative impacts of continued warming are of obvious concern, less attention has been paid to what might happen to the climate if we pass the point of net-zero carbon dioxide emissions and temperatures start coming back down. What this looks like at a regional scale has implications for setting climate policy goals and adapting to future changes. Population-based emergence of climate change in the Coupled Model Intercomparison Project Phase 6 (CMIP6) model ensemble was assessed and compared to the previous generation of models. My results show climate change emerging soonest and most strongly at low latitudes and for lower-income nations, regardless of the emissions pathway taken. In terms of signal-to-noise (S/N) ratios of average annual temperatures, this ensemble exhibits earlier and stronger emergence under the Shared Socio-economic Pathway (SSPs) than the previous generation did under corresponding Representative Concentration Pathway (RCPs). Spatial patterns of emergence also change between generations of models, and I find that these global and regional changes are caused by a combination of higher Equilibrium Climate Sensitivity (ECS) in the CMIP6 ensemble, as well as changes to emissions pathways, component-wise effective radiative forcing (ERF), and region-scale climate responses between model generations. Extending the framework of climate change emergence, I investigate the phenomenon of de-emergence, where temperatures rise beyond a given threshold and subsequently drop back below that threshold as forcing reverses. Large-scale carbon dioxide removal (CDR), also referred to as negative emissions, has been proposed to lower global-average temperatures and mitigate damage to the Earth system, though the regional effects of this have been under-studied to date. I analyse the results of eight Earth System Model (ESM) that participated in the Carbon Dioxide Model Intercomparison Project (CDRMIP). I find that decades after CO2 concentrations return to pre-industrial levels following a gradual quadrupling, the only regions that most models agree return to pre-industrial temperatures are an area across northwestern Eurasia and a small area off the coast of West Antarctica. Around half of the models project de-emergence for areas of North America and East Asia. I investigate potential causes for these patterns and find that the land areas that cool more also exhibit enhanced latent heat flux, net carbon uptake, and precipitation, implying enhanced evapotranspiration. The region off West Antarctica appears to experience a cooling influence from changes to sea ice and meltwater. Both regions experience changes to cloud radiative effects that cause net cooling. While the above experiment is highly idealised, overshooting a global temperature target before returning back to it using negative emissions is increasingly being discussed as a real-world possibility in light of ongoing emissions inconsistent with achieving the goals of the Paris Agreement. While global temperature is widely expected to be reversible under such conditions, regional climate responses are much less well understood. I analyse results from two CMIP6 overshoot scenarios run by an ensemble of Earth system models to assess changes in temperature and precipitation across the globe. I find that overshooting a temperature target by a larger amount leads to a warmer Southern Hemisphere, a cooler Northern Hemisphere, and larger decreases of precipitation in North Africa and increases in East Asia, compared to a smaller temperature overshoot. Some differences, notably increases in extreme temperatures, persist for centuries after the overshoot. The work in this thesis contributes to our understanding of the climate by exploring climate change emergence and de-emergence under multiple experiments, using a range of climate models. My results confirm the previously identified inequity of climate change-related impacts in the decades between now and the 2050 target for net-zero emissions held by many countries. The regional disparities in de-emergence rates also raise questions about the equity and fairness of overshoot scenarios as a reversal of global temperatures will not be felt evenly across the globe. I bring novel analytical frameworks for assessing the significance of climatic changes, particularly in the under-explored scenario of a post-net zero world.