Good day My name is Peter Hoffmann and I have been working at PIK in the Hydroclimatic Risks working group since 2012. My focus is on regional climate change and changes in meteorological phenomena. In my presentation, I would like to introduce you to the phenomenon of heatwaves. Here you see the general weather situation that led to record temperatures in Western Europe at the end of June. Extremely hot air masses flowed toward us from the south. The weather was correspondingly hot and dry in many parts of the country. It clearly shows that local weather phenomena usually have a large-scale context.
The effects were noticeable locally. The figure here shows temperature variability over the last two years relative to a climatic context of the period 1961–1990. The resulting temperature anomalies can be presented relative to the mean seasonal cycle. The dominance of positive anomalies is clearly visible. On average, this results in an anomaly of nearly 2°C. The highest deviation of nearly 10 degrees coincides with the prominent heatwave at the end of June 2026.
To compare temperature conditions, value distributions following a Gaussian distribution shape are used. In a warmer climate, the distribution shifts toward higher values. Looking at the right tail of the distribution, this means a higher probability of extreme values. And that is not all. The climatic shift of the distribution also changes weather variability. Indirect effects play an important role here, driven significantly by dynamic factors. Weather variability increases because the moderating weather influence from the North Atlantic is blocked more frequently than before.
Using the Potsdam climate measurement series since 1893 as an example, the trend in annual maximum temperatures can be clearly illustrated. While values before 1960 were still well below 38°C, we have seen an accumulation of values above that threshold in recent decades. It was only a matter of time before 40 degrees was exceeded. In 2026, that moment arrived. The old record was broken by a significant margin. Since annual maximum values are usually triggered by similar weather patterns, it clearly shows how the intensity of heat extremes has changed locally.
There was sufficient time to prepare for such a weather situation. Almost a week in advance, the development of the heat phase was already foreseeable. Immediately beforehand, predictions became even more precise. It could have been even more intense.
Looking at long-term relationships among climatological variables makes it clear that with every degree of mean temperature rise, extreme days also increase. Learning from the past means that in Potsdam's case, 5 hot days are added for every 1 degree rise in annual mean temperature. Considering only recent decades, it could be even more. Added to this is the growing dispersion of values.
Similar relationships are evident when examining indicators that also account for heat perception. This includes the Humidex, which incorporates humidity conditions. The muggier it feels, the harder it is for the body to regulate its heat balance. One thing becomes clear: regardless of the indicator, heat stress increases with rising mean temperatures.
<small> ### **Statistical & Absolute Metrics** * absolutely measured peak temperature values (41.8°C – June 27, 2026 – Saxony-Anhalt) * defined thresholds (Germany average approx. 22 hot days) * location-specific rarity values ($T_{max}$ and $T_{min}$ were in the 99th percentile range at the end of June) * temporal deviation from normal conditions (daily mean anomaly was +10 K) ### **Temporal & Spatial Dynamics** * duration and residence time ($T_{min}$=29.4°C in Saxony) * geographical extent * natural variability and trends * statistical return period (extreme values > 41°C in Germany have return periods >> 100 years) ### **Impact & Climate Context** * actual effect of heat on the human body * amplification degree due to anthropogenic factors </small>
Heatwaves are among the large-scale phenomena. Unlike heavy rain, they are usually supra-regional and can last for days to weeks. Development becomes visible several days ahead in forecasts, enabling timely awareness for the population and key actors.
Characteristic temperature patterns are tied to specific large-scale weather setups, reinforced by air mass transport. In an Omega block pattern, hot air masses flow far northward from the Mediterranean on the rear side during summer. The higher the average temperatures, the more intense heatwaves can become.
Heatwaves triggered by high-pressure patterns are part of natural weather variability. In mid-latitudes, the westerly influence from the North Atlantic dominates. It prevents stable high-pressure systems from building up into dangerous heatwaves in summer. Cold air masses remain in the north, and warm air masses stay in the south.
Climate change creates warming contrasts that indirectly affect weather variability. The jet stream is known to be sensitive to this. When the contrast between pole and equator diminishes, the dominant westerly influence slows down. As a result, weather patterns persist longer, especially in summer. This allows heatwaves to gain severity through the direct temperature effect.
Already today, we observe more frequent blocking of the North Atlantic influence. This triggers increased air mass transport across latitudes and climate zones. Consequently, cold air masses break out southward and warm air masses push far northward. This favors weather extremes.
What foreseeable consequences does this have for us? Let us compare the distribution of hot days in the past versus today: 1. The mean value has doubled. 2. Today's distribution has shifted, widened significantly, and opened at the top. 3. This cannot be explained by the temperature effect alone. 4. Summer 2026 marks the current peak of the distribution.
The heatwave at the end of June ranked 5th compared to historical heatwaves in Europe in terms of duration, intensity, and area. All heatwaves after 2001 are marked in orange. Before that, only 3 heatwaves out of 25 met the criterion. Anomalies reached over 10 degrees above normal conditions.
It is the interplay of various effects that shapes the magnitude of heatwaves. Climatological: higher mean values as a baseline Meteorological: critical weather setups that drive the inflow from the south