Climate change in Baden-Württemberg 1. Baden-Württemberg climate scenarios 2021-2050 It is undisputed that the anthropogenically caused greenhouse effect will, among other things, raise the mean global temperature by 1.4 to 5.8 C within the next 100 years. This is a central statement in the 3 rd report of the Intergovernmental Panel on Climate Change (IPCC) issued in January 2001. This climate change will be accompanied by considerable effects on the water balance. In general, a temperature rise leads to intensification of the water cycle, which can be reflected in increased evaporation and higher precipitation. This report summarises the currently available findings on the effects of climate change for the State of Baden-Württemberg. These findings have been acquired within the scope of the KLIWA project ( Klimaveränderung und Konsequenzen für die Wasserwirtschaft Climate change and consequences for water resource management ). KLIWA is a cooperation project of the States of Bavaria and Baden-Württemberg with the Deutsche Wetterdienst. The aim of KLIWA is to make statements for the next few decades, i.e. for the time horizon 2021-2050, on the possible effects of climate change on the water balance. These statements are then to be used as the basis for determining the consequences for water resource management, i.e. to provide suggestions for water management action. Suitable regional climate scenarios were prepared for the assessment using a realistic emissions scenario of the IPCC. These scenarios were used in Baden-Württemberg as the input variables for the available water balance models (WBM), in order to be able to make statements on the effects of climate change on the water balance. The climate scenarios were prepared for the area of the States of Baden-Württemberg and Bavaria. A basic study by ETH Zurich prepared on behalf of KLIWA drew the conclusion that as yet there is no optimum method for the preparation of regional climate scenarios from the global climate models. The KLIWA climate scenarios workshop held on 14.5.2001 in Berlin also showed that at present no assured methods are available for determining regional climate scenarios. Therefore, the KLIWA partners decided to apply three different methods, in order to obtain a bandwidth of possible development. The following methods were chosen: the regional dynamic REMO climate model of the Max-Planck-Institut für Meteorologie (MPI), Hamburg a special statistical method (taking into account weather situations) of the firm Meteo-Research, Berlin the statistical method of the Potsdam-Institut für Klimafolgenforschung (PIK), Potsdam. In order to obtain comparable results, the orders placed with the climate modellers to a large extent specified identical boundary conditions: measured data 1951-2000, global climate model ECHAM 4, verification period 1971-2000, IPCC emissions scenario B2, scenario period 2021-2050.
2 2. Results The following statements concentrate on the area of the State of Baden-Württemberg and are based on the results of all three methods. Due to broad climatological homogeneity, the extensive diagrams show the whole KLIWA area of Baden-Württemberg and Bavaria. The methods applied to determine the regional climate scenarios are being further developed. The results therefore still contain uncertainties; however the trends of the changes to the most important hydro-meteorological variables determined such as temperature and precipitation are in the same direction in all three methods. Air temperature Air temperature in Baden-Württemberg will continue to rise considerably in future. In summer months the mean daytime temperature will be approx 15 C (Figure 1), in winter it will be approx 4.5 C (Figure 3). The rises in the hydrological winter of approx 2 C (Figure 4) are greater than those in the hydrological summer of approx 1.4 C (Figure 2). The hydrological winter lasts from November until April, the hydrological summer from May to October. The temperature rise can also be identified in the individual months, and not only in the mean, but also in the maximum and minimum daytime temperatures. It is highest in the months December to February. The differences between the current state (1961-2000) and the future state (2021-2050) are exemplarily shown for Freudenstadt climate station in Figures 5 to 7. The expected temperature rise in winter is of special significance, as temperature has a major effect on the intermediate storage of precipitation as snow and therefore can be decisive for future expected flow conditions. Summer days and hot days The number of summer days (days with Tmax > 25 C) in Baden-Württemberg will rise considerably. This can be seen in Figure 8; where the mean values at the State s individual climate stations are shown. The number of hot days (days with Tmax > 30 C) almost doubles in places (Figure 9). Frost and icy days As a result of climate warming, the number of frost days (days with Tmin < 0 C) (Figure 10) and the number of sub-zero days (days with Tmax < 0 C) will reduce considerably, the latter by more than half in most cases (Figure 11). Late frost in spring Late frosts in spring, depending on when they occur, can cause major damage to agriculture. Due to the expected warming, on average the last frost in spring will occur earlier than at present, so that the risk of frost damage for agriculture could reduce. Figure 12 shows the current general situation (in days from start of the year); the future shift (in days) is indicated at the gauging station locations.
3 Precipitation Despite the spatial diffuseness, differences of regional precipitation can be identified and therefore enable comments to be made. Figures 13 and 14 show that the rainfalls in Baden-Württemberg will hardly change in summer (mainly slight falls in the order of magnitude of less than 10%). Precipitation in winter however will increase considerably. Depending on the region, the sharp increase is varied and is up to 35% (Figures 15 and 16). The number of days with high precipitation (greater than 25 mm) will also increase in winter. The example of Freudenstadt climate station shows that in the months of December to February, on average the number of days with N > 25 mm approximately doubles (Figure 20). Dry periods For agriculture it is significant that in future the number of dry periods (at least 11 consecutive days with precipitation less than 1 mm) per year will fall. Equally, the number of dry days (precipitation less than 1 mm) is lower (Figure 17). This behaviour is also indicated in the monthly values, as can be seen in the example of Freudenstadt climate station (Figure 19). The mean duration of the dry periods will also not increase in the future (Figure 18). This trend towards a reduction has already been determined for recent decades in the investigation into the long-term behaviour of precipitation. Weather conditions In winter the frequency and duration of the westerly weather conditions significant for flood formation, especially the so-called westerly cyclonal (WC)", will increase. In summer great changes are not to be expected. Runoff The results of the regional climate scenarios were employed as as input quantities for the water balance models, in order to assess the impact of climate change on runoffs and flows with the aid of statistical calculations (extreme value statistics). The evaluations of the Landesanstalt für Umwelt, Messungen und Naturschutz Baden-Württemberg (LUBW State Institute for Environment, Measurements und Nature Conservation Baden-Württemberg) show that the flood runoffs will increase at virtually all the gauging stations of the River Neckar basin. Therefore, from today s point of view it is necessary to take into account the effects of climate change when designing new water management facilities in the form of a climate factor. This occurs by increasing the design value. Within the catchment area of the River Neckar, which has been investigated to date as a pilot area, the increase in the hundred year high water flow HQ 100 is around 15%. HQ 100 is the high water flow or flood that on average, from a statistical point of view, occurs every 100 years. In future, it will therefore be necessary to multiply the HQ 100 value for the River Neckar basin with the climate factor of 1.15.
4 3. Conclusion Overall, in a critical assessment of the results for the target year 2050 it can be stated: Warming continues. The air temperature will continue to rise, especially in winter. Precipitation will increase in the winter months. The water cycle, especially the runoff into the surface waters, is affected by these changes. An increase is also to be expected in the duration and frequency of westerly weather conditions in the winter. In Baden-Württemberg the LUBW performs calculations for the water balance models in the remaining river basins too (Donau, Tauber, Lake Constance tributaries, high and upper Rhine tributaries). The aim is to use pragmatic approaches to specify climate factors for these areas too. The uncertainties in the results of the model chain: Global climate models - regional climate models water balance models and the subsequent extreme value statistics are still large; nevertheless, the results of the simulation calculations in the River Neckar catchment area allow us to expect an increase in the mean high water as well as the extreme runoffs. A worsening of floods in the Neckar basin due to climate change therefore appears probable for the target year 2050. Further information on this topic is available on the internet under www.kliwa.de.
Figure 1: Future mean air temperature [ C] in summer (2021-2050) Figure 2: Change in future air temperature [ C] in summer compared to today
Figure 3: Future mean air temperature [ C] in winter (2021-2050) Figure 4: Change in future air temperature [ C] in winter compared to today
Figure 5: Monthly mean daytime temperature in C, Freudenstadt climate station Figure 6: Monthly maximum daytime temperature in C, Freudenstadt climate station Figure 7: Monthly minimum daytime temperature in C, Freudenstadt climate station
Figure 8: Number of summer days (Tmax > 25 C) per year to date and in the future Figure 9: Number of hot days (T max > 30 C) per year to date and in the future
Figure 10: Number of frost days (T min < 0 C) per year to date and in the future Figure 11: Number of icy days (T max < O C) per year to date and in the future
Figure 12: Current mean point in time of the last late frost in spring and its future shift in time at the gauging stations
Figure 13: Future mean total precipitaion [mm] in summer (2021-2050) Figure 14: Change in future total precipitation [%] in summer compared to today
Figure 15: Future mean total precipitation [mm] in winter (2021-2050) Figure 16: Change in future total precipitation [%] in winter compared to today
Figure 17: Number of dry days (precipitation < 1 mm) per year in current and future state Figure 18: Mean duration of dry periods in current and future state
Figure 19: Monthly mean number of days with precipitation < l mm, Freudenstadt climate station Figure 20: Monthly mean number of days with precipitation >25mm, Freudenstadt climate station