Hydrological impacts from transient high-impact low-likelihood climate scenarios for Great Britain

Information about the potential for progression of climate change outside the range of conventional climate projections could be crucial for appropriate adaptation planning, particularly for water-related hazards like floods and droughts. The development and application of plausible high-impact low-likelihood (HILL) climate scenarios aims to address this. Here, a recently developed set of transient HILL climate scenarios for the UK is used with a grid-based hydrological model to assess the potential impacts on river flows across Great Britain and how these differ from those under conventional climate projections. Changes in seasonal mean flows and 10-year return period high and low flows are investigated. The results vary, both between the five transient HILL scenarios applied, and for different flow indices and regions of the country. Some of the flow impacts from conventional climate projections are at least partially ameliorated under some HILL scenarios, but made worse under others. For example, high flow increases in the north are ameliorated under ‘Stronger Arctic Amplification’ but worsen under ‘Collapse of Sub-Polar Gyre’. The most concerning HILL scenario is possibly ‘Collapse of AMOC’, which could cause substantial changes in seasonal mean and high flows to the north of Britain, related to increased snow. Studies such as this should add impetus to mitigation efforts, to reduce the chance of occurrence of HILL scenarios, as well as aiding adaptation planning, particularly for critical infrastructure.