Heat in Chemnitz
Heat: a key challenge
Increasing heat stress is one of the key challenges facing the city of Chemnitz. Heatwaves are among the most dangerous natural hazards worldwide. However, they are rarely given sufficient consideration due to their lack of spectacular and sudden nature compared to other hazards such as tropical cyclones or flash floods. The great heatwave of 2003 showed that heat protection needs to be improved both across Europe and at local level. At that time, it was estimated that up to 70,000 people lost their lives.
Cities heat up faster
Climate change hits us even harder in cities than in the countryside. During heatwaves, cities become so-called heat islands, meaning that it gets even hotter there than in the surrounding area. In densely built-up city centres, such as Chemnitz, buildings and streets absorb the sun's rays, store the energy and gradually release the heat back into the surrounding air. Concrete and metal surfaces store heat particularly well and for a long time. Glass surfaces can reflect sunlight like a magnifying glass, focussing it on one point and causing the temperature to rise.
In addition, warm air is trapped if it cannot escape due to particularly dense and high buildings or if there are no large open spaces such as forests, parks and bodies of water. If the removal of warm air and the inflow of fresh, cold air along so-called "fresh air corridors" is also impaired, no cooling can take place at night. The higher the proportion of sealed soil, the less soil is available for the absorption and evaporation of rainwater. Instead, the water flows unhindered into the sewage system and is no longer available for evaporation and thus for cooling the immediate surroundings. In addition to the construction method, the colours of surfaces can also influence the temperature: Dark-coloured facades heat up faster than light-coloured ones. Air conditioning systems or car exhaust fumes also cause cities to sweat due to their waste heat.
Further information
A comparison of the average annual temperatures in Chemnitz over the thirty-year periods 1961 to 1990 and 1991 to 2020 shows that the temperatures in the period 1991 to 2020 were always higher than the annual averages in the reference period 1961 to 1990, with the exception of six years. According to the projections of the Saxon State Office for the Environment, Agriculture and Geology (LfULG), there will be a further increase in average annual temperatures in the future. By 2100, the projected temperature change is plus 4.7 °C (compared to the reference period), accompanied by a sharp increase in hot days and summer heat. Permafrost will become less and less likely and cold spells will decrease.
The deviation in the average annual mean temperature clearly shows that it has always been warmer in Chemnitz in the last 30 years than in the climate reference period, with the exception of 1996 and 2010. In the period 1961 to 1990, the average temperature in Chemnitz was 8.1 °C. In the second period from 1991 to 2020, the average temperature was already 9.2 °C.
The diagram shows the development of heat days in Chemnitz. On the one hand, the real measurement data from 1970 to 2000, supplemented by the projected development of heat days according to the two different IPCC scenarios RCP 2.6 (light red vertical bars) and RCP 8.5 (dark red vertical bars). The projected heat days are each shown as the mean value for a ten-year period, for example from 2000 to 2010, from 2010 to 2020 and so on. While the RCP 2.6 scenario projects an average of 7.5 heat days by 2100, the RCP 8.5 scenario projects 32.5 heat days per year.
The light grey vertical bars show the lowest or highest projected number of heat days in the relevant ten-year period. For example, in the RCP2.6 scenario, a minimum of two and a maximum of 21 heat days can be expected in the period from 2030 to 2040. The horizontal light red and dark red bars show the mean values for the relevant 30-year period.
Development of precipitation in Chemnitz
Observation in millimetres | 846 | 202 | 279 | 185 | 180 |
Deviation in per cent | |||||
1991 to 2020 | +1 | -9 | +2 | +4 | +8 |
2021 until 2050 | +3 | +8 | -6 | +3 | +9 |
2071 to 2100 | 0 | +11 | -19 | +1 | +17 |
1982 (year with lowest rainfall) | -31 | -20 | -24 | -64 | -22 |
2010 (rainiest year) | +38 | 0 | +64 | +58 | +22 |
In its annual climate profiles, the LfULG publishes on the REKIS platform how temperature and precipitation could develop specifically in Central Germany for the periods 2021 to 2050 and 2071 to 2100 if greenhouse gas emissions continue unabated. A look at the development of precipitation levels shows that an increase in annual precipitation is to be expected in the medium and long term. There are changes within the seasons: For the period 2071 to 2100, precipitation is projected to decrease by up to 18 per cent in summer, while precipitation increases massively in winter and spring, rising by an average of 17 per cent in winter and 11 per cent in spring. The consequences of the seasonal fluctuations are longer dry periods interrupted by individual (heavy) rainfall events, increased erosion of dry soils and increased sediment input into the sewer network.
Chemnitz heat action plan
The current status of the specialist section of the heat action plan includes background information on the impact of heat on human health, analyses current climatic developments for Chemnitz and shows which districts of the city of Chemnitz are particularly heat-stressed. It also analyses which of the heat-stressed urban areas are also vulnerable to heat, due to additional risk factors such as a high population density or a high number of elderly people living in the district.
The heat section is a prerequisite for starting the second work package in summer 2024: Targeted measures are to be developed in a broad coordination process to ensure that the population of the city of Chemnitz and the city's infrastructure are well prepared for future periods of heat.
The measures in the heat action plan are based on the recommendations for the creation of heat action plans to protect human health from the Federal Ministry for the Environment, Nature Conservation, Nuclear Safety and Consumer Protection (BMUV). The following fields of action are proposed:
- Coordination and interdisciplinary systems
- Utilisation of a heat warning system
- Cross-target group information and communication
- Reduction of indoor heat
- Preparation of health and social systems with special consideration of risk groups
- Urban planning and construction
- Monitoring and evaluation
Heat prevention projects (selection)
For the future-proof adaptation of blue-green infrastructures in terms of sustainable heavy rain and heat prevention, the city administration founded the "Water-sensitive urban development" working group on the basis of the city council resolution BA-080/2020, initially within the administration, which deals with topics of decentralised rainwater management, heavy rain prevention and the water-sensitive design of planning processes.
The following locations for new drinking fountains are currently under construction and are expected to be completed at the beginning of 2025: Konkordiapark and in Küchwaldpark (entrance). A drinking fountain on the Brühl is currently being planned and should also be realised in 2025. The City of Chemnitz's Parks Department is in charge of the project. All drinking fountain locations are to be published in the Cool City Map. This is currently in preparation.
A special map is currently being developed for the themed city map, in which drinking fountains, refill stations, parks, water features and water areas will be marked - for immediate cooling off on the go in the city.
>>Furtherinformation
A small puzzle booklet is currently being created for children of kindergarten age on the topic of "Heat in Chemnitz". The booklet is intended to help raise awareness of the topic of heat in a playful way and teach important behaviour in hot weather.
Glossary
Summer days are days on which the temperature rises above 25 °C or higher. We speak of heat days (hot days) when the maximum daily temperature exceeds 30 °C. Every hot day is therefore also a summer day. A tropical night is when the air temperature is always at least 20 °C in the time window from 18:00 in the evening to 6:00 in the morning of the following day (definition according to DWD).
The RCP 2.6 scenario corresponds to the so-called "climate protection scenario". The scenario corresponds to a goal of the Paris Agreement. By drastically reducing emissions, global warming of more than 2 °C in 2100 is not exceeded. The RCP 8.5 scenario is the "worst-case" scenario. Here, the temperature increase by 2100 is around 4.8 °C compared to the pre-industrial state. RCP stands for "representative concentration path".
There is no fixed and standardised definition of a heatwave. The term "heatwave" refers to a prolonged period of at least two to three days of unusually hot and dry or hot and humid weather, usually with recognisable effects on people and natural systems. During this period, there is insufficient cooling at night. Since there is no absolute universal value, such as a specific temperature that defines extreme heat, heat events (or heat waves) are relative to a location's climate. This means that the same meteorological conditions may constitute a heatwave in one place but not in another.
The perceived temperature is used to describe the heat sensation of an average adult outdoors as objectively as possible. In addition to the air temperature, this is also influenced by humidity, wind speed and solar radiation. Clothing and activity also play a role. Overall, the individual thermal comfort range also varies according to geographical location, season and acclimatisation (the body's physiological ability to adapt to the environment).
It indicates the potential water supply and is calculated from the precipitation minus the potential evaporation. The higher the temperatures and number of hours of sunshine, the higher the degree of evaporation at the respective location; the "thirst" of the atmosphere is then very high.
The reference period 1961 to 1990 recommended by the World Meteorological Organisation (WMO) is used to assess long-term climate development, as this period is only partially affected by the accelerated warming currently being observed. To record the climate and its changes, it is customary to calculate mean values over a period of 30 years in order to exclude the influence of natural variability from the statistical analysis of the climate. In general, climate reference periods make it possible to relate the current weather conditions to both the current climate status of a region and the long-term development of the climate in the region.