High-impact low-likelihood climate scenarios for the UK: background report

Arnell, N. W. ORCID: https://orcid.org/0000-0003-2691-4436, Hawkins, E. ORCID: https://orcid.org/0000-0001-9477-3677, Shepherd, T. G. ORCID: https://orcid.org/0000-0002-6631-9968, Haigh, I. D., Harvey, B. ORCID: https://orcid.org/0000-0002-6510-8181, Wilcox, L. ORCID: https://orcid.org/0000-0001-5691-1493, Shaffrey, L. ORCID: https://orcid.org/0000-0003-2696-752X and Turner, A. G. ORCID: https://orcid.org/0000-0002-0642-6876, (2025) High-impact low-likelihood climate scenarios for the UK: background report. Report. Department of Meteorology, University of Reading (Produced for the UK Climate Resilience Programme. Project CR20-4)

Abstract/Summary

This report outlines the evidence used to define a series of high-impact low-likelihood climate scenarios for the UK. The scenarios consist of a narrative storyline and an accompanying illustrative quantitative characterisation. The scenarios themselves are described in more detail in the accompanying scenarios report. The storyline approach provides a framework for the presentation and development of scenarios. The storylines and scenarios are based on theoretical analysis, interpretation of climate model output and observed historical experience. The scenarios are designed to complement the UKCP18 climate projections. The quantitative characterisation of each scenario can be updated as new evidence becomes available. The scenarios describe potential high-impact low-likelihood drivers of impact, rather than high-impact low-likelihood outcomes. One set of scenarios describe transient changes in climate and sea level to 2100, outside the range of changes projected under ‘conventional’ climate scenarios either because the climate forcings or the climate system response are different. The scenarios are based on six storylines describing (i) accelerated climate change (a world with an increase in global average temperature considerably greater than 4oC above pre-industrial levels), (ii) a substantial reduction in planet-cooling aerosols, (iii) a series of major volcanic eruptions, (iv) enhanced amplification of changes in the Arctic leading to substantial circulation changes, (v) abrupt ocean circulation change, and (vi) enhanced sea level rise due to rapid melting of the Greenland and Antarctic Ice Sheets. Each narrative scenario storyline is accompanied by an indicative quantitative characterisation. The physical mechanisms behind each storyline are presented in this report, along with an assessment of the confidence of their implications for UK weather, climate and sea level. The transient scenarios provide a high-level picture of how future UK climate could be different to that implied by conventional UKCP18 climate projections. The second set of scenarios describe plausible extreme monthly and seasonal anomalies, specifically extreme temperature, rainfall and windspeed. Each scenario is associated with a ‘backstory’ describing the driving climatic conditions. The scenarios also describe compound extreme months and seasons and sequences of extremes. Each scenario is expressed in quantitative terms as an anomaly from a long-term average and can be implemented at a range of spatial scales. The scenarios are developed using a combination of theoretical understanding of the drivers of extreme months and seasons in the UK, model simulations and empirical analysis based on observed climate data and a wide range of indicators of climatic variability. This set of scenarios allows an assessment of risks from plausible extreme events. The assumptions underpinning the scenarios are examined and justified in the report.

Item Type	Report (Report)
URI	https://centaur.reading.ac.uk/id/eprint/127920
Divisions	Science > School of Mathematical, Physical and Computational Sciences > NCAS Science > School of Mathematical, Physical and Computational Sciences > Department of Meteorology
Publisher	Department of Meteorology, University of Reading
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