High-impact low-likelihood climate scenarios for the UK: scenario 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. B., 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: scenario report. Report. University of Reading (Produced for the UK Climate Resilience Programme. Project CR20-4)

Abstract/Summary

This report presents two sets of high-impact low-likelihood (HILL) climate scenarios for the UK. The scenarios are designed to supplement and contextualise the UKCP18 climate projections, and to meet requirements of users for plausible high-end or worst-case climate scenarios. One of the sets of scenarios – transient scenarios - describe the evolution of change in climate to 2100. This set consists of six plausible and distinctive narrative storylines describing qualitatively different potential drivers of high-impact, low-likelihood changes in UK climate and sea level. The storylines are designed to describe changes outside the extreme range conventionally assumed, taken here to be either a world reaching an increase of 4oC above pre-industrial levels by 2100 or one where changes in climate are outside the range as represented by the majority of current climate models. Such changes could occur because the forcings of climate change are outside the conventional range, or because the large-scale climate system response is outside the range projected by most current climate models. The scenarios are similar in principle to conventional climate change scenarios. Each is presented as a single quantification. Two (enhanced global warming and enhanced sea level rise) are defined as standalone scenarios, and the other four are presented as perturbations to be applied to UKCP18 climate projections from any strand with any emissions scenario. The transient scenarios provide a high-level picture of how future UK climate could be different to that implied by conventional UKCP18 climate projections. They can be applied individually or in combination. The second set consists of a number of extreme anomaly scenarios representing plausible extreme monthly and seasonal temperature, rainfall and windspeed anomalies, both individually and in combination. These scenarios are designed to be applied to a long-term climate mean to represent an individual month or season, or sequence of months. They can be applied to conventional climate projections or the high-impact low-likelihood transient scenarios developed here to define plausible extreme months or seasons. Each is associated with a ‘backstory’ describing plausible circulation patterns (such as jet stream position) that generate the anomalous extremes. This set of scenarios allows an assessment of risks from plausible extreme months and seasons. They are not intended to replace sector-specific worst case event scenarios. Both the transient and extreme anomaly scenarios are developed through a combination of theory, historical experience, expert judgment and the interpretation of climate model output, including UKCP18 products. The scenarios are not assigned quantitative likelihoods, but their plausibility is characterised in terms of understanding of (i) the drivers of the scenario (forcing or large-scale response) and (ii) the consequences of the scenario for the UK if it were to occur. The narrative storylines and backstories provide a framework for further elaboration, for example to add specific detail relevant to a user or to develop narrative descriptions of potential consequences. The quantitative characterisations of the scenarios presented here can be used in quantitative risk assessments but are not to be interpreted too literally. They can be updated or supplemented within the framework presented here as new information becomes available. This report describes the transient and extreme anomaly scenarios in Parts A and B respectively. A separate technical report outlines the use and interpretation of high-impact low-likelihood climate scenarios in the UK, describes the framework developed for constructing the scenarios, and summarises the evidence used to construct them. The transient and extreme anomaly scenarios describe plausible high-impact low-likelihood changes in average temperature, rainfall and windspeed (and sea level for the transient scenarios). Plausible changes in other climate variables (such as humidity) can be inferred if necessary from the narrative storylines.

Item Type	Report (Report)
URI	https://centaur.reading.ac.uk/id/eprint/127919
Divisions	Science > School of Mathematical, Physical and Computational Sciences > NCAS Science > School of Mathematical, Physical and Computational Sciences > Department of Meteorology
Publisher	University of Reading
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