Do dry periods in winter exacerbate forest fire conditions in summer


Dr. Bruno Rudolf

To person

Born in 1948, graduate meteorologist, has been working for the German Weather Service since 1975. Current focus of work: hydrometeorology, water cycle, regional and global precipitation climatology, preventive flood protection and low water, meteorological advice for water management.

Increasing drought endangers the quality of the soil - in southern regions, but also in Germany. The causes are varied, they range from a lack of precipitation to increasing water consumption due to increasing population numbers.

Low water on the Middle Rhine 2011 (& copy picture-alliance / dpa)

The hot summers of recent years have highlighted the possible consequences of a drought in Europe. In the summer of 2003, heat and drought cost the lives of more than 30,000 people across Europe. In Germany, too, fields and forests withered, streams dried up, the water of the Rhine was so low that shipping was not possible for weeks. The overall economic damage of this hot summer exceeded that of the Elbe floods in 2002. The consequences of a drought are varied and often immense: damage to vegetation, crop losses, forest fires, low water levels, increased concentrations of pollutants in bodies of water, scarcity of drinking water, diseases, famine.

Problems can appear after just a few weeks of drought

Droughts are not a local phenomenon; they affect larger regions, and their extent and duration vary greatly. They are categorized differently according to the different effects in the natural and economic areas: meteorological due to the precipitation deficit, hydrological due to the water levels of rivers and groundwater, agricultural due to the effects on vegetation. Agriculture usually reacts to drought more quickly than large bodies of water. Each country has its own standard. The UK is already experiencing three rain-free weeks, while in some other countries much longer periods of drought are considered normal.

Which factors play a role in the development of droughts?

Water is the basis of all life. The total mass of water on earth of around 1.4 billion cubic kilometers has not changed for millions of years, but the geographical distribution and the division into ice, water vapor and liquid water has. In the 20th century the total was distributed as follows: oceans 97 percent (salt water), ice sheets 2.3 percent, land area with groundwater, lakes and rivers 0.62 percent, atmosphere 0.001 percent. Although the precipitation is negligible compared to the total water supply on earth, it alone provides the fresh water required for life on the continents. It fills lakes, rivers, glaciers and groundwater, which means that water is available for nature and people even in arid regions and times.

The development of a drought is a complex process, one prerequisite is a prolonged lack of precipitation. This is caused by a weather situation in our region in which a very wide high pressure area lies over Central Europe and blocks the damp west wind flow. As in the summer of 2003, such a situation can be stable for months. Precipitation and clouds do not occur, and high levels of solar radiation and temperature accelerate the evaporation of the soil moisture that was still present at the beginning. The soil dries out faster and deeper, so that soon the cooling effect of evaporation is missing, which leads to a further intensification of drought and heat.

Global cycles have local effects

In general, fluctuations in the circulation are the cause of dry or wet periods. The best-known phenomenon is "El Nino": A change in the high and low pressure centers over the tropical Pacific leads to a shift in the large-scale wind systems and ocean currents. The most significant effects can be seen in tropical South America at Christmas time (El Nino = Christ Child) with relatively high water temperatures in the Pacific (resulting in fish death) and extreme rainfall in the western Andes. These fluctuations in circulation have a worldwide connection. While Peru and Ecuador are inundated by rain floods, other areas, particularly Australia and Indonesia, are suffering from exceptional drought. El Nino occurs roughly every four to seven years; a particularly strong event was last observed in 1997/1998.

The air becomes warmer, the precipitation more changeable

Multi-year mean monthly precipitation amount for June, July, August (northern hemisphere summer; unit: millimeters per month). Source: German Weather Service
Since the beginning of the 20th century, the average global annual mean temperature has risen by a good 0.7 degrees. The warming did not occur evenly, but rather with an increasing tendency over the past 30 years. In Germany, the average temperature rose by almost 1 degree Celsius. The ten absolutely warmest years have occurred since 1990. For the future, the results presented by the Intergovernmental Panel on Climate Change (IPCC) in 2007 suggest further global warming. Depending on the development of greenhouse gas emissions, the air temperature will rise by a further 0.5 to 2.5 degrees Celsius by 2050, and by around 2 to 5 degrees Celsius by 2100 compared to today.

While the climate models provide a clear change signal with a high degree of sharpness for the temperature, the change in precipitation is very small on average, but the range of the model results is much greater. This suggests an increase in both dry and wet weather conditions. According to this, we have to reckon with a higher variability of the precipitation, i.e. with more frequent heavy precipitation as well as with more frequent or longer dry phases.