High-Impact Low-Likelihood Climate Scenarios for Risk Assessment in the UK

There is an increasing interest amongst policymakers in understanding the implications of high-impact low-likelihood (HILL) risks for climate mitigation, adaptation and resilience. Whilst extreme sea level rise scenarios have been used and there is awareness of some HILL risks, in practice there are currently few scenarios which can be applied in risk assessments. Here we present two sets of HILL climate scenarios for the UK, complementing existing UK climate projections. Both are based around storylines describing physically-plausible changes, were developed using observations, models and theory, and describe HILL drivers of change as inputs to impact models or stress tests. The storylines provide a narrative framework for understanding risk, and indicative quantifications provide the basis for quantitative risk assessments. One set describes six storylines for transient climate change to 2100 and beyond, reflecting plausible forcings and system responses outside the range conventionally assumed. These describe enhanced global warming, rapid reductions in aerosol emissions, volcanic eruptions, enhanced Arctic Amplification, changes to ocean circulation, and accelerated sea level rise. The other set describes extreme monthly and seasonal anomalies, representing hot, cold, wet, dry and windy extreme years. This set includes storylines describing persistently anomalous weather.