Human contribution to the record-breaking June 2019 heat wave in EU

Last June, a heat wave affected several European countries like France, Austria, Germany, The Czech Republic, Italy, and Spain causing the postpone of the national school examen in France, different wildfires excelling that took place in Spain destroying thousands of hectares, train delays due to the heat damage to the train tracks... 

It always occurred after a cold period. This change is caused by the specific dynamical conditions with a mid-tropospheric cut-off low system that formed off the coast of the Iberian Peninsula. The system created a transport of heat from low-level Saharan air and induced extreme temperatures in mountainous areas.

The people in the past do not get used to the extreme heat and its collateral effects were minor, but nowadays it affects more in terms of mobility, mortality, and economic damages.

Analysis

The focus of the study is to analyze a country temperature average in western Europe, as well as a single station, and finally in last year’s analysis of heat in northern Europe. The city considered was Toulouse due to the reach record temperature, and it was close to the region with peak temperatures.  The different analysis were:  

Figure 1 : The hottest 2-day mean temperature in the Meteo France

It shows the previous hot event around 1950, 1976, 2005, 2017, and this year. The green line shows a combination of the increasing trend due to greenhouse gases and a cooling phase around 1980, mainly due to air pollution.

Figure 2:  The June-August land temperature averaged over 35-75 oN, 15 o W- 20o E with a 15-yr running mean. The greenline denotes and additional 10-yr running mean

The western European summer temperature smoothed with a 15 yr running mean a similar balance of greenhouse heating and aerosol cooling.

 Figure 3: Return period plot with observations and fit drawn for the climates of 1901 and 2019 

It shows an estimation of the total warming since a time when anthropogenic influences were small.  

Figure 4: Position parameter (thick line) and 6 and 40 yr return time (thin lines) as a function of the smoothed western European land Temperature.

It shows that taking the greenhouse gas warming and aerosol cooling into account, extreme heat in June has a probability of about 3% each year in the current climate with a 95% uncertainty margin of 0.5% to 7%. The increase in temperatures is estimated to be four oC using the scaling of the measured temperature in Toulouse with that of the European temperature average.

This implies a much higher warming trend in France in June compared to that of the average European land summer temperature, which has warmed by about two degrees. The difference factor is the soil moisture drying.

In summary, the analysis of heat waves in different regions shows a similar trend in average summer temperatures, which is attributed to anthropogenic warming. To end up, the increase in heat waves is due to human impact.

Models

In order to evaluate the role of climate change, it was done a comparison of different climate models using two methodologies.

1.  It treats climate model simulations from the transient model runs in the same ways as the observation by fitting an extreme value distribution shifted with smoother European summer temperature from the same model .
2.   It uses simulation of the present day and compares then with a simulation of a counterfactual climate as the present could have been without anthropogenic climate change or simulation of the late 19 the century.  

Hazard synthesis  

It is found that the probability has increased by a factor five. It is observed a trend in temperature of the heat during with a similar frequency is around 4o C whereas the climate models show a much lower trend 

It is confident about the positive trend and the factor that the probability has increased by at least a factor five. There are significant uncertainties in the trends and systematic differences between the representation of extreme heat waves in the climate models and the observations positive trend and the factor that the probability has increased by at least a factor five. There are a large uncertainties in the trends and systematic differences between the representation of extreme heat waves in the climate models and in the observations.

Figure 5: Left: Increase in probability for the highest 3-day mean temperature averaged over France. Blue: observations, red: climate models. Right: same for Toulouse. Note that the climate models have systematic biases in their representation of heat waves.

Figure 6: The increase in temperature is shown in the figure above, left for the average over France, right for Toulouse. There is no overlap between the trend in the observations around 4 oC, and the modeled trends, around 2 oC .

Vulnerability and exposure

The consequences of the heat wave are always reported in the media excepting the mortality that is calculated on a statistical analysis of reported deaths over a long period. In general, the heat deaths affect more to the elderly, chronically ill and very young people.

This abstract has been elaborated by the AEInnova's environmental department in collaboration with external sources.