17.10.2012 | Potsdam: During periods of transition to cooler climatic conditions there was a distinct increase in the number of extreme floods. Such events were observed particularly during the Great Migration and the Early Middle Ages (AD 450-750) and the transition to the Little Ice Age (AD 1140-1520). In contrast, there was less flooding during the medieval warm phase (AD 1000-1140) and the coldest period of the Little Ice Age (AD 1600-1700).
This is the result is published by geoscientists of the GFZ German Research Centre for Geosciences in the current journal GEOLOGY. “We have used deposits from Lake Mondsee in Austria to create the first accurate calendar of floods and debris flow over the past 1600 years in the Northeast Alps”, explains Tina Swierczynski, a scientist at the GFZ. “Because of their annually laminated layers, the finely stratified lake sediments provide accurate information about flood events in times before instrumental records.” Using microscopic analysis and modern geochemical element scanning, it was possible to identify traces of floods in the lake sediments to seasonal accuracy and to determine the frequencies of the floods.
The reason for more frequent floods during the cooling phase is probably a stronger inflow of moist Atlantic and Mediterranean air masses in the northern Alpine region due to a change in atmospheric circulation. The formation of a stable high-pressure cell over Siberia during colder periods, however, leads to a blockage of northbound low pressure cells and therefore less flooding in the Austrian Alpine foothills.
In terms of the comprehension of a relationship between climate changes and the occurrence of floods, there was a surprising result: “This study demonstrates that the frequency of floods in certain regions changes over long periods and no simple relation to climate change exists”, declares the head of the study, Professor Achim Brauer (GFZ).
As present-day observations show that floods occur with regional variations, more long flood time series of lakes in other regions are currently being developed at the GFZ (Helmholtz Association) to create a more accurate spatial picture of the occurrence of floods. Floods pose as a threat for human settlement and therefore require an accurate risk assessment. Above all, the question of the natural frequencies of such events is difficult to answer because the existing time series usually lie no more than 100 years back.
The research was funded by the Potsdam Research Cluster for Georisk Analysis, Environmental Change and Sustainability PROGRESS which in turn is funded by the BMBF.
Tina Swierczynski et al. (2012), “A 1600 yr seasonally resolved record of decadal-scale flood variability from the Austrian Pre-Alps”, Geology, November 2012, v. 40, p. 1047-1050, doi:10.1130/G33493.1