CHARACTERISTICS AND FORMS OF THE URBAN CLIMATE
2.11.1 Climate Change in Germany

Since 1901, the average temperature in Germany has risen by almost 0.9 K. The years from 2000 to 2009 were the warmest decade recorded by meteorologists for at least 130 years. Temperatures have especially increased in the Southwest of Germany, with an average annual increase of 1.2 K in Saarland for example. In contrast to this, the registered increase in Mecklenburg-Western Pomerania was only 0.4 K.

What has also increased is the amount of rain. Meteorologists have reported an increase of precipitation of about 9 % since the beginning of the 20th century. The last 15 years have been particularly wet – with only four exceptions.

While there is nowadays more rain in spring (until June), the months of July and August become drier and drier. Higher amounts of precipitation have also been recorded in winter months, but as the variation from one year to another is very big, a clear statistical statement is not possible yet.

What will the future look like? Global climate models are too broad for precise local predictions. The horizontal grid is at least 120 kilometres wide, sometimes even more than 200 kilometres. This is why regionalisation methods are used. Some use measured information from climate stations for forecasts; others refine global data by means of physico-numerical methods and apply it to a smaller grid with minimum intervals of currently ten kilometres. Four regional climate models (REMO, CLM, WETTREG and STAR) are used for Germany.

Predictions about the future always include uncertainties. In the context of global and regional climate models, this means that several hardly predictable factors could either compensate each other or add up.

The amount of greenhouse gases reaching the atmosphere depends on many factors and cannot be predicted exactly. These factors include the development of the population, of the economy and of energy prices, the type of land use and the extent to which technologies for reducing greenhouse gas emissions will be realised. Another uncertainty is the actual dimension of the harmful effect of various greenhouse gases.

In order to deal with the uncertainties but remain able to act, the IPCC has defined emission scenarios. These describe possible developments of our society and environment and form the basic conditions for climate models. But it is absolutely clear that models can be no more than approximations of reality, preferably as close as possible. Our "real" climate is far more complex and cannot be entirely represented with all factors and correlations. The farther the model is meant to look into the future and the smaller the regional grid is, the more uncertain the prognosis becomes. The accuracy of a prediction can be increased, however, by overlapping several models. After all, four regional models with three emission scenarios each have been used in Germany , further calculations will follow.

For further information on climate change implications for Baden-Württemberg, please see the Ministry for the Environment, Nature Conservation and Traffic (2012).

Figure 2/27 and Figure 2/28 show the possible development of the number of days with heat stress in this century at the example of Greater Stuttgart as well as implications for the number of affected people.