Arnell et al. (2025) High‐Impact Low‐Likelihood Climate Scenarios for Risk Assessment in the UK

Identification

• Journal: Earth s Future
• Year: 2025
• Date: 2025-12-01
• Authors: Nigel W. Arnell, Ed Hawkins, Theodore G. Shepherd, Ivan D. Haigh, Ben Harvey, Laura J. Wilcox, Len Shaffrey, Andrew G. Turner
• DOI: 10.1029/2025ef006946

Research Groups

• Department of Meteorology, University of Reading, Reading, UK
• National Centre for Atmospheric Science, University of Reading, Reading, UK
• School of Ocean and Earth Science, National Oceanography Centre, University of Southampton, Southampton, UK

Short Summary

This paper develops two sets of High-Impact Low-Likelihood (HILL) climate scenarios for the UK, comprising transient changes to 2100 and extreme monthly/seasonal anomalies, to complement existing climate projections and facilitate practical risk assessment for adaptation and resilience planning. These scenarios provide physically plausible storylines and indicative quantifications for "worst-case" climate outcomes beyond conventionally assumed ranges.

Objective

• To develop and present two sets of High-Impact Low-Likelihood (HILL) climate scenarios for the UK, complementing existing UK climate projections, to address the lack of practical scenarios for risk assessment in adaptation and resilience planning.

Study Configuration

• Spatial Scale: United Kingdom (UK), with adjustments for regional (13,000–25,000 km²) and local (5 km × 5 km) scales for extreme anomaly scenarios.
• Temporal Scale:
  • Transient scenarios: To 2100 and beyond.
  • Extreme anomaly scenarios: Monthly and seasonal.
  • Specific scenario durations: Volcanic eruption (up to 5 years), ocean circulation change (maximum effect persisting after 2050), sea level rise (by 2100).

Methodology and Data

• Models used:
  • UKCP18 climate projections (global, regional, local strands, probabilistic projections).
  • UKCP18 HadGEM PPE-15 RCP8.5 ensemble.
  • Model simulations for aerosol effects (Luo et al., 2020).
  • Model simulations for volcanic impacts (Bethke et al., 2017).
  • Hosing experiments for AMOC collapse (Jackson et al., 2015; Mecking et al., 2016).
  • CMIP6 model simulations for sub-Polar Gyre collapse (Swingedouw et al., 2021).
• Data sources:
  • HadUK-Grid observed climate data set (Hollis et al., 2019; Met Office et al., 2018).
  • NASA IPCC AR6 sea level projection tool (Garner et al., 2021; Kopp et al., 2023).
  • Empirical relationships between drivers and extreme anomalies.
  • Process understanding and climate theory.

Main Results

• Two sets of High-Impact Low-Likelihood (HILL) climate scenarios were developed for the UK, each consisting of narrative storylines and indicative quantifications.
• Transient Scenarios (to 2100 and beyond):
  • HILL-1 (Enhanced global warming): Global temperature increases well above 4 °C by 2100.
  • HILL-2 (Rapidly reduced aerosol forcing): Additional warming across the UK by up to 0.75 °C, peaking in 2040.
  • HILL-3 (Volcanic eruption): Cooling across the UK by up to 2.5 °C and rainfall reduction by up to 20% for up to 5 years following a VEI-7 eruption.
  • HILL-4 (Stronger Arctic Amplification): Reduction of winter temperature by 1.5 °C and winter rainfall by 20% points by 2100 due to circulation changes.
  • HILL-5 (Ocean circulation change):
    • HILL-5a (AMOC collapse): Maximum temperature reduction of 5-6 °C by 2050, summer rainfall reduction of 25-35%, and increased rate of sea level rise.
    • HILL-5b (SPG collapse): Maximum temperature reduction of 2.5 °C.
  • HILL-6 (Enhanced sea level rise): Increase in sea level around the UK coast of 2.0-2.2 m by 2100 relative to 1981–2000.
• Extreme Anomaly Scenarios (monthly and seasonal):
  • Five basic scenarios: Hot (4-6 °C above mean), cold (4-7 °C below mean), wet (2.5-3 times mean rainfall), dry (10% of mean rainfall), and windy (60-80% higher windspeeds).
  • Four persistent anomaly storylines: Persistently cyclonic (wet and windy), persistently anticyclonic (dry and calm), cyclonic in winter/anticyclonic in summer (mild winter, hot summer), and anticyclonic in winter/cyclonic in summer (cold winter, cool summer).
• Extreme temperature anomalies are approximately 1 °C more extreme at the local (5 km × 5 km) scale than at the regional scale, and extreme wet rainfall anomalies are approximately 50% larger.

Contributions

• Provides the first comprehensive and practical set of High-Impact Low-Likelihood (HILL) climate scenarios for the UK, extending beyond previously available extreme sea level rise scenarios.
• Offers a template for the development of similar "worst-case" climate scenarios in other regions.
• Complements existing UK climate projections (UKCP18) by addressing risks outside conventionally estimated ranges.
• Delivers both narrative storylines for qualitative risk assessment and indicative quantifications for quantitative risk assessment and stress testing.
• Addresses a critical policy need for robust scenarios to inform adaptation and resilience planning against extreme climate risks.

Funding

• UK Climate Resilience Programme (Project CR20-4, funded by the UK Met Office)
• Co-Centre award number 22/CC/11103 (funded by Research Ireland, Northern Ireland's Department of Agriculture, Environment and Rural Affairs (DAERA), and UK Research and Innovation (UKRI))

Citation

@article{Arnell2025HighImpact,
  author = {Arnell, Nigel W. and Hawkins, Ed and Shepherd, Theodore G. and Haigh, Ivan D. and Harvey, Ben and Wilcox, Laura J. and Shaffrey, Len and Turner, Andrew G.},
  title = {High‐Impact Low‐Likelihood Climate Scenarios for Risk Assessment in the UK},
  journal = {Earth s Future},
  year = {2025},
  doi = {10.1029/2025ef006946},
  url = {https://doi.org/10.1029/2025ef006946}
}