How climate change affects rain and floods

An Austrian research team has now shown for the first time that short-term precipitation and flood events on the scale of a few hours are particularly affected by the temperature increase caused by climate change. For events on a longer time scale, the relationship is more complicated. This finding was made using detailed data collected in Austria over more than a century -- but it can also be applied to other regions of the world. It also allows conclusions to be drawn about which regions will experience changes in the probability of flooding, and how they will be affected. The results have now been published in the journal Nature.

Excellent data

Climate change is affecting water cycles all over the world. A look at Austria is particularly revealing: "We are in the very fortunate situation of having excellent data available," says Prof. Günter Blöschl of TU Wien (Vienna), who led the research project. Since 1900, precipitation in Austria has been recorded by two separate institutions: by the meteorological service, now Geosphere Austria, and by the Austrian Hydrography, which is managed by the Ministry of Agriculture. The Austrian data are therefore particularly reliable and can be used to make forecasts for other countries.

These data sets have now been analysed in a cooperation of TU Wien, the Federal Ministry of Agriculture, Forestry, Environment and Water Management (BML), GeoSphere Austria and the University of Graz.

Significantly more frequent precipitation events on short timescales

The analysis showed that short-term precipitation events lasting only a few hours have increased significantly in the last 30-40 years -- by about 15%. "This had already been predicted by climate models, albeit with uncertainties. We have now been able to confirm it," says Günter Blöschl.

The increase was equally strong on both sides of the Alps -- this is an important result, as these are two different regions in climatological terms. "This clearly shows that large-scale weather systems are not the decisive factor for these short-term precipitation events, because they would be different near the Mediterranean than north of the main Alpine ridge," says Blöschl. "Instead, the temperature increase caused by climate change leads to more intense precipitation locally. This is partly because warmer air can hold more moisture, but also because there is more energy in the system, and stronger warming at ground level leads to stronger upward movement of air masses. Then they also cool down faster, which leads to more rain."

A more complicated picture on a longer time scale

This is a universal phenomenon: other regions are affected by this mechanism in much the same way as Austria. However, if longer-term rainfall events lasting several days are analyzed, a different picture emerges. In that case, global weather phenomena such as El Niño, a climate process influenced by ocean temperatures, play a much more decisive role. Rainfall events on this timescale are therefore not changing everywhere in the same way. In the Mediterranean, in some regions of Italy, Spain and Greece, long periods of precipitation may even become rarer as a result of climate change.

This difference between short and medium-term rainfall events also means that different regions are affected quite differently by floods. "Smaller rivers with smaller catchment areas are strongly influenced by short-term intense rainfall. The risk of short-term flooding is therefore much higher in areas close to such rivers," explains Günter Blöschl. The situation is different for larger rivers like the Danube. They are less influenced by local precipitation on an hourly scale; here, weather events on a daily scale play a more important role, causing floods to increase or change little depending on the hydro-climatic situation.

The data collected in Austria thus clearly show that different types of rainfall and flood events are definitely influenced by climate change -- but not always in the same way. If you want to assess the risk of flooding correctly, you have to distinguish between different time scales.