By Sabine Undorf

Heatwaves in Scotland, such as the one in late June 2018 are often thought of positively by the public, but can have a wide range of negative impacts – especially if we are unprepared. Our on-going project brings together climate and social science researchers to investigate the impacts of recent weather events in the Greater Glasgow region. With the new UK Climate Projections UKCP18 released in November 2018, we wanted to know what they could tell us about future heatwave frequency in Scotland.

What is a heatwave?

To assess heatwave frequency, we analysed the regional projections from both UKCP18 and the previous projections UKCP09. We define a heatwave in southern Scotland as a sequence of five days or more during which minimum temperatures do not fall below 15°C and maximum temperatures exceeds 28°C averaged across the region. We then count the number of heatwave day per year, i.e. the number of days that are part of a heatwave. This definition is based on previous work with Scottish infrastructure stakeholders and the thresholds in North East England used in the Heatwave plan for England 2018.

How will heatwave frequency change in future?

The results showed that in both datasets the number of heatwave days are projected to increase in future (Figure 1). The red and blue lines show the central estimate of the UKCP09 and UKCP18 projections, respectively, while the respective shading represents both modelling uncertainty and natural climate variability; uncertainty in future emissions of greenhouse gases and air pollutants is not included in this figure.

The new UKCP18 projections show a wider range of plausible future heatwave-day frequencies, with now much larger frequency increases also considered possible. Levels of heatwave day frequency projected for the end of the century in UKCP09 occur several decades earlier for UKCP18. More than ten heatwave days per year seem plausible as soon as the 2050s given the new projections.

Figure 1: Future number of heatwave days per year as projected by the (blue) UKCP09 and (red) UKCP18 datasets for Scotland. The respective shadings show the full plausible range of the number of heatwave days in any one decade, including internal variability. The respective lines show the best estimate for the number of heatwave days per decade with decadal averaging. Heatwave days are days which are part of a heatwave, defined as a sequence of five days or more during which minimum temperatures do not fall below 15°C and maximum temperatures exceeds 28°C averaged across the region.

How will the frequency of sustained periods of warm nights change?

The Heatwave plan for England identifies that for health impacts, sustained periods with hot nights may be even more important than hot days. We therefore repeated our analysis but instead of days that are part of a heatwave, we count days that are part of a period during which minimum temperatures stay higher than 15°C but maximum temperatures are not necessarily high.

We found that these event days are projected to become very common in future (Figure 2). While the maximum projected frequency was in the order of twenty per year in the UKCP09 dataset, this number is double in the UKCP18 dataset and is plausible already in the 2050s.

Figure 2: As Fig 1, but for days during sustained periods of warm nights rather than days during a heatwave.

Taking action to reduce negative impacts

We find these projected changes in the frequency of hot and warm events not only for the distant, but also for the near, future. A range of impacts was reported during the heatwave last summer, including wildfires, buckling railway tracks, and overheating hospitals. All sectors of society, including, but not limited to, healthcare, infrastructure, and the environment need to consider adaptation measures to reduce the negative impacts of these potential changes.

Many Scottish businesses, local authorities and organisations are already working actively on this, for example the Climate Ready Clyde initiative in the Glasgow City Region, but there is no formal heatwave plan for Scotland. Our findings for the projected increase of both heatwave days and warm nights underline the need to consider developing one.

Sabine Undorf works as climate scientist at the University of Edinburgh together with Kathleen Allen, Joseph Hagg, Marc Metzger, and Simon Tett.