European weather and climate extremes in a net-zero world

The Paris Agreement, adopted by 196 parties at COP21 in 2015, sets out the goal to limit global warming to no more than 2 degrees celsius and pursue efforts to limit warming to 1.5 degrees celsius. Many countries across the world now have declared net-zero targets, including the UK. Understanding the climate of a world where the net-zero emission targets have been met, and global temperatures are stabilising, is an important and developing area of research.

Evidence is emerging that regional temperature and precipitation trajectories can be substantially different in a stabilised climate, compared to the rapidly warming world we currently experience. For example, recent work suggests that the projected summer rainfall decline in the Mediterranean in a warming climate could be halted if emissions of greenhouse gases were reduced to net-zero (Dittus et al. 2024).

This PhD project will focus on understanding how weather and climate extremes across Europe and the UK are projected to change in a world with net-zero emissions of greenhouse gases. Existing research suggests that differences in sea-surface temperature patterns and associated atmospheric circulation changes are important factors in explaining the differences between transient climate change projections and stabilised warming scenarios.

Building on existing climate model simulations, this project will run an ensemble of medium resolution, atmosphere-only climate model simulations with two different sets of sea-surface temperature patterns: one representing a rapidly warming world, and another representing the sea-surface temperature patterns post-net-zero emissions, both at the same level of globally averaged temperatures (e.g., 2 or 3 degrees celsius of warming). Using this approach, atmospheric circulation changes associated with differences in the sea-surface temperature patterns between both sets of ensembles will be identified.

The second part of the project will focus on understanding changes in weather extremes across the European domain, initially broadly including multiple types of extreme events across the ensemble, then focussing on one type of extreme weather event specifically such as large-scale extreme precipitation events. By definition, extreme weather events are rare, and extremely large ensembles of simulations are needed to sample the range of physically plausible events (to the extent that models are able to simulate them).

To address this issue and explore a range of physically plausible events in a warming world beyond the observed range, a small number of case studies will be selected to be 'boosted': a specific event will be re-run many times, re-running the same event by adding a very small perturbation to the atmospheric initial conditions 15-20 days before the event occurs (e.g. Gessner et al. 2021, Fischer et al. 2023).

This approach will identify how extreme such an event could get if the weather trajectory had been slightly different. Such an approach has not yet been used in the context of a 'net-zero' stabilised climate.

Weather extremes in a warming world are often highly damaging, endangering lives, livelihoods and infrastructure. Understanding how the characteristics of such extremes will be different in a world where global temperatures have stabilised is important for preparing for future weather and climate extremes.