Flood data from 500 years: Rivers and climate change in Europe

Date:
July 22, 2020
Source:
Vienna University of Technology
Summary:
Studying historical documents from 5 centuries, scientists were
able to compare flood events from the past with recent flood events
in Europe.

This combination of historical and hydrological research provides
evidence for the strong influence of climate change on rivers and
floodings. Floods tend to be larger, the timing has shifted and
the relationship between flood occurrence and air temperatures
has reversed.



FULL STORY ========================================================================== Overflowing rivers can cause enormous problems: Worldwide, the annual
damage caused by river floods is estimated at over 100 billion dollars --
and it continues to rise. To date it has been unclear whether Europe is currently in a flood-rich period from a long-term perspective.


========================================================================== Austrian flood expert Prof. Gu"nter Blo"schl from TU Wien (Vienna)
has led a large international study involving a total of 34 research
groups that provides clear evidence that the past three decades were
among the most flood-rich periods in Europe during the past 500 years,
and that this period differs from others in terms of its extent, air temperatures and flood seasonality. Compared to the past, floods tend
to be larger in many places, the timing has shifted and the relationship between flood occurrence and air temperatures has reversed. In the past,
floods tended to occur more frequently in cold phases, while today,
global warming is one of the main drivers of their increase. The results
of the study have now been published in "Nature" magazine.

Historical data from half a millennium "From our previous research,
we already knew how climate change has affected Europe's floods in the
past 50 years," says Alberto Viglione from the Politecnico di Torino,
one of the key authors of the publication. "For forecasts of the next
decades, however, it is also important to understand whether this is
a completely new situation or whether this is just a repetition of
something that has already occurred. So far, the available data had not
been sufficient to ascertain whether this is the case or not. We have
examined this question in great detail and can now say with confidence:
Yes, flooding characteristics in recent decades are unlike those of the previous centuries." For the study, tens of thousands of historical
documents containing contemporary flood reports from the period 1500 to
2016 were analysed. The TU Wien team has worked with historians from
all over Europe. "The particular challenge of this study consisted
in making the very different texts of the different centuries and
different cultural regions comparable," explains Andrea Kiss from the
Vienna University of Technology, researcher and historian herself, and
one of the key authors of the publication. "We managed to achieve this comparability by putting all the texts in their respective historical
contexts with painstaking attention to detail." Formerly cold, now warm:
River floods now function differently The data analysis identified nine flood-rich periods and associated regions.

Among the most notable periods were 1560-1580 (western and central
Europe), 1760-1800 (most of Europe), 1840-1870 (western and southern
Europe) and 1990- 2016 (western and central Europe). Comparisons with air temperature reconstructions showed that these historical flood periods
were substantially cooler than intervening phases.



========================================================================== "This finding seems to contradict the observation that, in some areas
such as in the northwest of Europe, the recent warmer climate is aligned
with larger floods," says Gu"nter Blo"schl. "Our study shows for the
first time that the underlying mechanisms have changed: While, in the
past, floods have occurred more frequently under colder conditions, the opposite is the case now. The hydrological conditions of the present
are very different from those in the past." The timing of the floods
within the year has also changed. Previously, 41% of Central European
floods occurred in the summer, compared to 55% today. These shifts are
related to changes in precipitation, evaporation and snowmelt and are
an important indicator for distinguishing the role of climate change
from that of other controls such as deforestation and river management.

These findings have been made possible by a new data base compiled by the
study authors that includes the exact dating of almost all flood events reported by written sources. So far, one had to often rely on other,
less precise sources of information, such as lake sediments. It is the
first study worldwide to evaluate historical flood periods for an entire continent in such detail.

Better data -- better forecasts Because of the shift in the flood
generating mechanisms, Gu"nter Blo"schl advocates the use of tools for flood-risk assessment that capture the physical processes involved,
and management strategies that can incorporate the recent changes in
risk. "Regardless of the necessary efforts of climate change mitigation,
we will see the effects of these changes in the coming decades,"
says Blo"schl. "Flood management must adapt to these new realities."
Video: https://www.youtube.com/watch?time_continue=1&v=fQMqw- P51qs&feature=emb_logo

========================================================================== Story Source: Materials provided by Vienna_University_of_Technology. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Gu"nter Blo"schl, Andrea Kiss, Alberto Viglione, Mariano Barriendos,
Oliver Bo"hm, Rudolf Bra'zdil, Denis Coeur, Gaston Demare'e,
Maria Carmen Llasat, Neil Macdonald, Dag Retso", Lars Roald,
Petra Schmocker-Fackel, Ine^s Amorim, Monika Běli'nova',
Gerardo Benito, Chiara Bertolin, Dario Camuffo, Daniel Cornel,
Radosław Doktor, Li'bor Elleder, Silvia Enzi, Joa~o Carlos
Garcia, Ru"diger Glaser, Julia Hall, Klaus Haslinger, Michael
Hofsta"tter, Ju"rgen Komma, Danuta Limano'wka, David Lun, Andrei
Panin, Juraj Parajka, Hrvoje Petrić, Fernando S.

Rodrigo, Christian Rohr, Johannes Scho"nbein, Lothar Schulte, Lui's
Pedro Silva, Willem H. J. Toonen, Peter Valent, Ju"rgen Waser,
Oliver Wetter.

Current European flood-rich period exceptional compared
with past 500 years. Nature, 2020; 583 (7817): 560 DOI:
10.1038/s41586-020-2478-3 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200722112655.htm

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