Climate change attribution of the compound 2013/14 winter storms flooding in Somerset, UK

The sequence of compounding winter storms of 2013/14 in the United Kingdom (UK) caused a range of significant impacts across the country, totalling an economic cost of approximately £1.3 billion (Environment Agency, 2016). Heavy rain, totalling 545mm over the season, caused widespread flooding, and coastal impacts were exacerbated by high spring tides and strong winds. The Somerset Levels particularly felt the impact of the flooding, accounting for 30% of the total UK area of flooded agricultural land. This event is a case study for the COMPASS project, where the main goal is to produce a flexible and harmonised methodological framework for such compound extremes with a focus on impact attribution.

Using the flood model SFINCS (Super-Fast Inundation of Coasts), developed by Deltares, we model the total flood extent for a small region of the Somerset levels for the season. We drive this model with both factual and counterfactual (“natural”, with anthropogenic warming removed) simulations of the winter precipitation, using the HadGEM3-A large-ensemble attribution runs (Ciavarella et al., 2018). We find the likelihood of the magnitude of the observed flood extent to be 1.21 times more likely due to climate change, based on return periods. We also find that a flood event under “natural” forcing but with the same return period as the factual event would be slightly less severe in its extent, 113.40km² compared to 114.02km².

Although these results are not statistically significant, this agrees with generally inconclusive results from other studies on the 2013/14 UK winter storms, such as that of Schaller et al. (2016) who found a small, non-significant increase due to climate change in the number of properties impacted by flooding in the Thames river catchment. Potential modelling improvements to refine results for Somerset include increasing resolution and adding flood defences to better represent the coastal inundation. Investigation of attribution of the flood extent to post-industrial sea level rise also opens another avenue for exploring the compound nature of the event.

The Horizon-Europe COMPASS project (Compound events attribution to climate change: towards an operational service) is exploring climate and impact attribution of different complex extreme events, and scoping an operational attribution service. It aims to develop transferable attribution methods for operational attribution of compound extremes to support climate change evidence and policy.