The Dutch heat wave of July 2019 in a warmer world: How much hotter could it get?

Summertime heat waves are extreme meteorological events with a high impact. Three defining aspects are their intensity, duration and spatial extent. All three will change for the worse in a warming world. We illustrate how these changes could play out for the heat wave that produced the hottest day to date in the Netherlands (40.7^◦C, Gilze-Rijen 25 July 2019, a record-shattering event of more than 2 degrees). This is done using a chain of hydrostatic and non-hydrostatic regional climate models of increasing horizontal resolution (12km-2.5km-500m-150m) in combination with the Pseudo Global Warming (PGW) approach. Various scenarios are explored using an ensemble approach to examine robustness. Results indicate that if the 2019 July heat wave were to occur in a +2K warmer world: (i) temperatures would likely reach 45^◦C in many places; (ii) the cumulative heat-wave intensity sum would double; (iii) the time to “cool off” in between heat waves would reduce to a level where the total number of days spent in heat waves roughly equals the number of cool days (Tx<25^◦C); (iv) the area where the 40^◦C threshold is passed will increase strongly; (v) the heat wave will last longer as a simple consequence of the higher temperatures (i.e., an earlier start and a later end). Further persistence increases occur if large-scale circulation changes are supportive; (vi) the temperature response is between 1.5-2 degree per degree global warming, with higher values occurring in scenarios with a stronger future drying. (vii) Finally, during heat waves cities become ‘islands of heat’ where the daily maximum temperatures and the night-time minima are 1-5^◦C higher than in nearby more rural areas. A first impression of these differences is obtained from experimental simulations with the convection permitting model HCLIM43 in ultra-high ‘resolution-of-the-future’ mode (500-150m).