Compared to previous decades, the intensity and frequency of heatwaves in Germany has increased. In recent years, dry periods during the growing season also appear to be occurring more frequently and lasting longer [1]. This development is in line with global trends caused by anthropogenic climate change. In its Sixth Assessment Report, the Intergovernmental Panel on Climate Change (IPCC) also reports an increase in heatwaves in most land regions of the world and a trend towards an increase in agricultural and ecological droughts in Western and Central Europe [2].

Trends

In the first phase of ClimXtreme (2020-2023), the subproject DYNPROHEAT ("Role of multi-scale dynamical processes in shaping recent and future extreme heat waves over Germany") analysed heatwaves in Europe between 1979 and 2019 using normalized heat indices in order to better compare heatwaves [3].

Subproject PERSEVERE ("Persistent Summer Extremes over Europe due to wave-Resonance Events") notes a significantly stronger trend in the increase in heatwaves for Central Europe compared to other regions at the same northern latitude [4]. They identify the formation of a double jet as a decisive factor in this intensification.

In the second phase of ClimXtreme, DecHeat2 (A8, "Attribution of the Decadal Variability of European Extreme Heat and Drought") predicts that extreme heat and drought conditions, such as those expected in Europe at the end of the 21st century, may already occur in the near future. In the 2030s, the probability of individual years of this intensity is already 10%. The Atlantic Multidecadal Variability has a major influence here [5].

Mechanisms and factors affecting heat extremes in the context of climate change

In the aftermath of an extreme weather event, we often ask what role climate change played. The answer can depend heavily on how the event is defined. 

• Weather conditions

For example, many recent studies of heatwaves or heavy rainfall events have considered not only temperature or rainfall accumulation, but also the weather conditions that enabled the observed extreme values.

In the first part of the subproject “Coord” (B1.1, "Advanced Detection/Attribution Studies"), the researchers adopted a systematic approach to conditional studies and compared them to unconditional ones. A key finding was that the outcome of the analysis could be dramatically altered if the weather conditions moderated the climate change signal or contained such a signal themselves. Such cases must be considered carefully before synthesising a single statement from several different conditional or unconditional studies. In the second part of the project, they proposed attributing complete heatwave time series to anthropogenic forcing via a non-stationary Markov process, thus avoiding the need to define individual extreme events. When applied to European heatwaves, this approach provides conclusive evidence of human influence across all European regions, driven primarily by the general shift in temperature distribution due to global warming.

• Atmospheric circulation patterns

In an effort to join probabilistic and physical perspectives to attribute extreme events, The subproject DesAttHeat (A5, "Towards an improved description and attribution of the most extreme Central European Heat Waves") combined storyline simulations and weather “analogues” of historical events to understand how global warming changes extreme events. By fixing the large-scale weather pattern, it showed that similar atmospheric circulation patterns can still occur, but in a warmer climate they lead to more frequent and more intense heatwaves. This suggests that beyond changes in atmospheric circulation, rising background temperatures and other small-scale mechanisms play a key role in amplifying future extremes.

According to the subproject DynSyTEx (B3.7, "Dynamical Systems perspective on weather regimes and heatwaves in a changing climate"), heatwaves in July and August are associated with more persistent atmospheric circulation patterns (blocking high-pressure systems) and differ from heatwaves in early and late summer. As global warming progresses, heatwaves occurring from July through to early autumn in particular will become more frequent. In addition, the period during which heatwaves are associated with particularly persistent blocking weather patterns is extending into early autumn.

• Extensions of the subptropical pressure belt

While heatwaves over Europe are often linked to atmospheric blocking, they can also be caused by poleward extensions of the subtropical high pressure belt, so-called subtropical ridges. DesAttHeat (A5) showed that these two dynamical „flavours“ of heatwaves exhibit different heat-generating mechanisms. Moreover, ongoing analysis of model projections for a warmer future climate suggest that subtropical ridges may become more frequent in summer and play a more important role in inducing heatwaves over Cental Europe, stressing the need to extend our research about dynamical drivers of European heatwaves beyond atmospheric blocking.

• Regional aerosol trends

Regional aerosol trends can have a clear influence on heat extremes. PATTETA (B1.3, "Process-based Attribution of Extreme Temperatures to Anthropogenic drivers") showed that not explicitly including aerosols in the statistical attribution of heat extremes may lead to biases which can be reduced by adding the regional aerosol optical depth as additional covariate [6]. Furthermore, the project showed that regional anthropogenic aerosol reductions amplify the probability of record-breaking heat extremes [7].

Attribution

The project AXE_G_2 (B1.2, "Application and extension of the pre-operational attribution system for weather and climate extremes") performed a rapid probabilistic attribution study for a heat and a drought event, investigating the precipitation deficit above Northern Germany [8]. No significant trend caused by anthropogenic climate change was found. In opposite, the heat study analyzing the hot days in July 2025 showed a large anthropogenic influence to this event with an increase in temperature by 1,6 °C (0,2 to 2,9) °C in the climate models and 4,2 °C (2,9 to 5,9 °C) seen in observations [9].

Impacts

DecHeat2 (A8, "Attribution of the Decadal Variability of European Extreme Heat and Drought") show that unprecedented fire-weather extremes in Southwestern Europe are associated with stronger spring soil drought and positive surrounding Sea Surface Temperature (SST) anomalies than more moderate extremes. Deep soil moisture emerges as the dominant precursor, with the strongest signals found for the most persistent events (Dietz et al., in prep.).

CROP4Europe (C03, "Impacts of compound weather extremes on crops in Germany") developed a fast AI-based crop-yield modelling tool that can produce yield development for grain maize and spring barley across the wider European domain using only daily min and max temperature and precipitation data, making accurate large-scale seasonal and climate-risk assessments extremely faster than the ECroPS crop growth model which was surrogated. The tool reproduces key crop-growth patterns well enough to support operational screening of yield anomalies, including the identification of regions with potential yield losses linked to heat and moisture stress. CROP4Europe also connects generated crop-yields and yield impacts to broader climate drivers intermediated by extreme events indices related to heat and moisture, helping stakeholders understand not only where yield risks occur, but also which climate conditions may be driving them, supporting seasonal reporting, risk monitoring, and future impact assessments.

Conclusion

The ClimXtreme research project has shown that heatwaves and droughts in Germany and Europe are becoming more frequent as a result of anthropogenic climate change, and that their physical drivers are changing. In addition to the general shift in temperature distribution, large-scale circulation patterns, persistent block patterns, subtropical high-pressure ridges and regional influences such as changes in aerosol concentrations contribute to the development and intensification of heatwaves. At the same time, it is becoming clear that extreme events may not only occur more frequently, but also earlier and under conditions that were previously expected to prevail only in the late 21st century. The studies presented also show that the attribution of individual events depends heavily on the definition chosen and the weather conditions taken into account, which is why the analyses must be interpreted with care. 

For Central Europe, the overall picture points to a clear indication of human influence on heatwaves. The implications extend far beyond meteorology: soil moisture, sea surface temperatures and conditions in the spring influence the intensity of extreme events, whilst agriculture and crop yields are increasingly at risk. 

Faster, data-driven tools such as CROP4Europe are opening up new possibilities for operational risk analysis and adaptation strategies. Overall, the findings underscore the need to treat heatwaves and droughts as key challenges for the coming decades.

If you have specific questions, please contact the research groups using the contact information provided, and if you have general questions, please contact info@climxtreme.de.