Future changes in heatwave severity, duration and frequency due to climate change for the most populous cities

A novel approach to quantify the present and future heatwave hazard is presented which can discern characteristics beyond what can be achieved from current approaches, such as distributions of severity, duration and frequency, including very low probability events that may not have been seen. A statistical model is built that represents the seasonal cycle, climate change, magnitude and temporal behaviour of all temperatures above a moderately high time varying threshold at a site from which very large samples of temperature time series can be drawn. From these, user defined heatwaves can be extracted and precise empirical statistics calculated, allowing application to a wide range of problems such as heatwave impact on the pollination of food crops. Fitting the heatwave model to climate model simulations allows the changing severity, duration and frequency of heatwaves from the past to the future to be quantified. This approach is validated by reproducing the heatwave climatology of a pre-industrial 3 500 year GCM control run from a 110 year future emission run of the same GCM. Using this methodology, future heatwave changes for the most populous cities from 20 countries are derived from a 44 member ensemble of 28 GCMs from the CMIP5 archive forced with the RCP 8.5 emission scenario. Compared to 2006, absolute temperatures of 4-10 day long heatwaves are projected to be between 3.4 to 6.6 degrees C hotter in 2099, though the magnitude of the increases are independent of heatwave rarity and duration. For 13 of the cities no significant future changes in the distribution of durations are found relative to a contemporaneous threshold. Six cities show future heatwaves will have a tendency to be longer and one to be shorter. Half of the cities show increases in severity, the time integrated temperature anomalies above the contemporaneous threshold. The largest changes are found for Paris with 100 year return level severity changes equivalent to mean heatwave temperatures increasing by 3.4 degrees C or by 1.7 degrees C for 5 and 10 day heatwaves respectively, and the rate of 10 day heatwaves increasing by 130%.