The use and interpretation of 'extreme' and 'worst case' climate scenarios in the UK

Arnell, N. W. ORCID: https://orcid.org/0000-0003-2691-4436, (2024) The use and interpretation of 'extreme' and 'worst case' climate scenarios in the UK. Report. University of Reading (Prepared for the UK Climate Resilience Programme. Project CR20-4.)

Abstract/Summary

This report reviews the current use and interpretation of ‘extreme’ and ‘worst-case’ climate scenarios in the UK, based on guidelines for risk and adaptation assessment, actual assessments that have been undertaken, Adaptation Reports produced as a requirement of the Climate Change Act, and approaches used in emergency planning. The key findings are: • There are no overall guidelines on the use and interpretation of climate scenarios in the UK, although there is an increasing trend towards concentrating on scenarios characterising 2oC and 4oC worlds. • Guidance and best practice documents exist for most sectors – with the major exception of agriculture – but the details and requirements vary. This means that different organisations within a sector have sometimes interpreted scenarios differently. • The RCP8.5 emissions scenario and corresponding UKCP18 projections have been interpreted differently between organisations, ranging from ‘business as usual’ through to ‘worst case’. RCP8.5 is often interpreted – mistakenly – as representing a 4oC world. • Similarly, ‘worst case’ has been interpreted differently: sometimes it is taken to be RCP8.5, and sometimes a more extreme change. Terminology varies considerably between different sets of guidelines and between organisations. • Several sets of sector guidelines refer to the use of ‘credible maximum scenarios’, without specifically defining what these are. • Organisations have used climate information differently, over different time scales, in their assessments and adaptation plans: some use scenarios to describe broad directions of change, whilst others require more quantitative projections for both strategy and design. The three key implications are: • It is necessary to develop clarity on the interpretation of RCP8.5, and on its implementation in UKCP18 with the ‘hot’ HadGEM models. • It is necessary to develop procedures to construct pathways consistent with 2 and 4oC worlds, partly to reflect increasing policy interest and partly to provide a reference for worst-case and extreme scenarios. • It is necessary to develop some agreed consistent high-level nomenclature to describe climate scenarios for planning adaptation and resilience – for example distinguishing between ‘low’ and ‘high’ scenarios to represent uncertainty over emissions (and representing model uncertainty through ranges or extremes), and ‘worst case’ scenarios representing deep uncertainty in the response of the climate system to increasing emissions.

Item Type	Report (Report)
URI	https://centaur.reading.ac.uk/id/eprint/127917
Official URL	https://www.metoffice.gov.uk/research/approach/col...
Divisions	Science > School of Mathematical, Physical and Computational Sciences > Department of Meteorology
Publisher	University of Reading
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