10 Mar 2022
10 Mar 2022
Status: this preprint is currently under review for the journal HESS.

Projected changes in droughts and extreme droughts in Great Britain strongly influenced by the choice of drought index

Nele Reyniers1, Timothy J. Osborn1,2, Nans Addor3, and Geoff Darch4 Nele Reyniers et al.
  • 1Climatic Research Unit, School of Environmental Sciences, University of East Anglia, Norwich, United Kingdom
  • 2Water Security Research Centre, University of East Anglia, Norwich, United Kingdom
  • 3Geography, College of Life and Environmental Sciences, University of Exeter, Exeter, United Kingdom
  • 4Anglian Water Ltd., Huntingdon, United Kingdom

Abstract. Droughts cause enormous ecological, economical and societal damage, and are already undergoing changes due to anthropogenic climate change. Understanding, anticipating and communicating these changes is essential to a wide range of stakeholders. In this study, the projected impacts of climate change on future atmospheric droughts in Great Britain were assessed for two warming levels (2 °C and 4 °C above pre-industrial levels) using the UKCP18 regional climate projections. As projected changes can be very sensitive to the choice of drought index, two indices were compared: the Standardized Precipitation Index (SPI), and the Standardized Precipitation Evapotranspiration Index (SPEI, which unlike the SPI, accounts for increasing potential evapotranspiration). The SPI and SPEI were used to quantify drought frequency, extent and duration of all droughts and of only extreme droughts. To provide context, aridity and seasonal precipitation and potential evapotranspiration changes were also assessed, as well as seasonal contributions to dryness at a yearly time scale. The UKCP18 regional simulations project (strongly) increasing drought frequency and extent due to climate change based on the SP(E)I almost everywhere in Great Britain. Importantly, the relative increase in frequency and extent is much more pronounced for extreme droughts than for more moderate droughts. Increasing longer-term dry conditions can be attributed mostly to more frequent dry and extremely dry summers, for which normal to wet winters are decreasingly able to compensate (even where winters are projected to become wetter). In general, using the SPEI results in far greater increases in drought frequency and extent than using the SPI. These differences are so substantive that at +2 °C the SPEI6-based projected changes reach a similar magnitude to the SPI6-based changes at +4 °C. Finally, projected changes in the distribution of drought durations depend on the drought index, region and warming level. These results illustrate that the choice of atmospheric drought index can have a decisive influence on changes in projected drought characteristics, and therefore users of these indices should be aware of the importance of potential evapotranspiration in their intended context when choosing a drought index. The stark differences between SPI- and SPEI-based projections highlight the need to understand the interplay between increasing atmospheric evaporative demand and moisture availability under a changing climate.

Nele Reyniers et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on hess-2022-94', Anonymous Referee #1, 07 Jun 2022
  • RC2: 'Comment on hess-2022-94', Marie-Claire ten Veldhuis, 18 Sep 2022

Nele Reyniers et al.

Data sets

Projected changes in droughts and extreme droughts in Great Britain are strongly influenced by the choice of drought index: UKCP18-based SPI and SPEI data Reyniers, Nele; Osborn, Timothy J.; Addor, Nans; Darch, Geoff

Nele Reyniers et al.


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Short summary
How will climate change impact droughts? This question is important, but also very complicated, in part because droughts are notoriously less straightforward to define than other extreme events such as floods or heatwaves. We show that two popular measures of drought give very different outlooks for how often, how widely, when and for how long droughts may occur in a warmed future. Nevertheless, both agree on increasing droughts in Great Britain, highlighting the need for rapid mitigation.