A heat period like last week occurs on average once every two years in todays climate and is therefore not very exceptional. It was different in the past: in De Bilt, the chance of such an event has increased fivefold since the beginning of the last century, and the corresponding temperatures have risen by about 3 degrees. This is evident from an attribution study by the KNMI. 

With climate attribution, we investigate the role climate change has played in the likelihood and intensity of an extreme weather event. In this study, we use the measurements in De Bilt and climate models that realistically simulate extreme weather. 

Such high temperatures once every two years 

From August 12 to 15, a yellow alert for heat was in effect. During this period, the average daily temperature was 22.7 °C. The highest temperature was recorded on August 14, when the mercury in De Bilt reached 31.4 °C. 

In todays climate, a period of four days with such high average daily temperatures occurs approximately once every two years. This number of years is a statistical value and should not be taken literally; after all, this summer we have already had previously experienced a heat period with approximately the same recurrence time. A recurrence time of once every two years corresponds to a 50 percent chance of it occurring in a given year. 

Five times more often 

A period with this heat is now 3 °C warmer than a comparable situation in a pre-industrial climate

In the measurements, we see that the chance of heat like last week has increased. The climate models confirm this, although they slightly underestimate this trend compared to the measurements. If we combine the estimates from the climate models and measurements, we see that an event like this (a four-day average temperature of 22.7°C) now occurs five times more often than in 1901. A period with this heat is now 3 degrees warmer than a comparable situation in a pre-industrial climate. 

The expectation is that this warming will continue. The maximum temperature on the hottest summer day in De Bilt averaged 33°C between 1991 and 2020. According to the KNMI23 climate scenarios, the hottest summer days will warm up to a temperature between 33.8°C and 40°C by 2100, depending on the climate scenario we follow (figure 1). The number of tropical days with a maximum temperature of 30°C or more is also increasing. Between 1991 and 2020, there were an average of 5 tropical days per year in De Bilt. By 2100, this will increase to between 9 and 39 days (figure 2). 

Double heat wave 

Not only is the number of heat waves increasing, but the temperature is also rising, and they are lasting longer on average

A yellow alert for heat is not the same as a heat wave. We speak of a heat wave when there are five consecutive days in De Bilt with a maximum temperature of 25.0 °C or higher, of which at least three have a maximum temperature of 30.0 °C or higher. Since the KNMI has been measuring the weather in De Bilt since 1901, we can see how often such temperatures occurred in the past (figure 3). Each dot represents an occurrence of a heat wave. Not only do we see the number of heat waves increasing, but the maximum temperature during the heat wave is also rising, and they are lasting longer on average (the color of the dots indicates the duration of the heat wave). 

This year, there have already been two heat waves. This is quite rare and has only occurred in the past in 1941, 2006, 2018, and 2019. Thus, this summer is the fifth summer since the beginning of measurements in which a double heat wave has been recorded.