Rivers inundating flood plains cause huge damage: Worldwide, the annual flood damage is estimated at over $ 100 billion - and it continues to rise. The extent to which climate change affects the severity of floods has not been clear so far. Globally, consistent trends did not seem to exist.

The Austrian flood specialist Prof. Günter Blöschl from the Vienna University of Technology has headed a large international study in which a total of 35 European research groups, including IG PAS, were involved. The study found that changes in the flood magnitudes observed in recent decades can be clearly attributed to climate change. However, climate change does not have the same effect everywhere: flood events are becoming increasingly severe in north-western Europe, while the size of floods has decreased in southern Europe and Eastern Europe, although they may also increase in small areas. The results have now been published in the prestigious journal "Nature".

Climate change as a decisive factor

"From our previous research, we already knew that climate change is shifting the timing of floods, i.e. when they occur in the year," says Günter Blöschl. "But the key question is: Does climate change also control the magnitude of flood events? Previously, the data were not sufficient to ascertain whether this is the case at large scales (across Europe). We have studied the matter in detail and can now say with confidence: Yes, the impact of climate change is clearly visible here."

The study evaluated data from 3738 flood measurement stations across Europe, from the period from 1960 to 2010. "It has long been suspected that climate change has an impact on the magnitude of flood waters because a warmer atmosphere can store more water" explains Günter Blöschl. "However, this is not the only effect, flood changes are more complicated."

The evaluation of the data revealed different trends in different regions of Europe: in central and north-western Europe, between Iceland and Austria, floods are increasing because precipitation is increasing and soils are becoming wetter. In southern Europe, on the other hand, flood levels tend to decrease - as climate change results in declining precipitation and the higher temperatures cause increased evaporation of water in the soil. However, for small rivers floods may be becoming larger due to more frequent thunderstorms and land management changes (e.g. deforestation). In the more continental climate of Eastern Europe, flood levels are also decreasing, which is due to shallower snow packs in winter associated with the higher temperatures. "There are consistent patterns of flood change across Europe and these are in line with predicted climate change impacts," says Blöschl. "This indicates that we are already in the midst of climate change."

Large changes

The magnitude of the flood changes is remarkable: they range from a decrease in expected flood levels of 23% per decade to an increase of 11% per decade (relative to the long-term averages). If these trends continue into the future, major effects on flood risk can be expected in many regions of Europe.

The identified flood changes are consistent with the results of the climate model projections. "This suggests that climate change is not a matter of the distant future, but is already happening" - emphasizes Marzena Osuch.
We invite you to read the article at: https://www.nature.com/articles/s41586-019-1495-6