In den Wintern 2013/14 und 2014/15 ereigneten sich in Großbritannien schwere Überschwemmungen nach Starkregenfällen. Insbesondere 2013/14 wurden die Einwohner regelrecht überrascht, hatte der britische Wetterdienst (Met Office) doch ursprünglich einen sehr trockenen Winter prognostiziert. Einige Medien und Forscher fackelten nicht lange und hatten den Schuldigen schnell ausgemacht: Den menschengemachten Klimawandel (siehe z.B. Der Standard, Stuttgarter Zeitung, Süddeutsche Zeitung). Nun wissen wir aber, dass gewissenhafte Forscher seine Zeit braucht. Daher konnte man umso gespannter sein, was eine seriöse Auswertung der historischen Überschwemmungsschäden in Großbritannien ergeben würde.
Am 20. März 2017 war es schließlich so weit. Neil Macdonald und Heather Sangster veröffentlichten im Fachblatt Hydrology and Earth System Science eine Analyse der großen Überschwemmungen in Großbritannien während der vergangenen 250 Jahre. Dabei entdeckten sie, dass es auch in der Vergangenheit immer wieder hochwasserreiche Phasen gegeben hat, z.T. sogar intensiver als in den letzten Jahren. Insofern bewegt sich das Überschwemmungsgeschehen der letzten Jahre auf der britischen Insel noch voll und ganz im Bereich der natürlichen Schwankungsbreite. Auf der Suche nach möglichen Antrieben für die beobachtete Variabilität der Hochwässer wurden die Forscher im Atlantik fündig. Ozeanzyklen wie die Atlantische Multidekadenoszillation (AMO) und Nordatlantische Oszillation (NAO) scheinen hier eine wichtige Rolle zu spielen. Anbei der Abstract der wichtigen neuen Studie:
High-magnitude flooding across Britain since AD 1750
The last decade has witnessed severe flooding across much of the globe, but have these floods really been exceptional? Globally, relatively few instrumental river flow series extend beyond 50 years, with short records presenting significant challenges in determining flood risk from high-magnitude floods. A perceived increase in extreme floods in recent years has decreased public confidence in conventional flood risk estimates; the results affect society (insurance costs), individuals (personal vulnerability) and companies (e.g. water resource managers). Here, we show how historical records from Britain have improved understanding of high-magnitude floods, by examining past spatial and temporal variability. The findings identify that whilst recent floods are notable, several comparable periods of increased flooding are identifiable historically, with periods of greater frequency (flood-rich periods). Statistically significant relationships between the British flood index, the Atlantic Meridional Oscillation and the North Atlantic Oscillation Index are identified. The use of historical records identifies that the largest floods often transcend single catchments affecting regions and that the current flood-rich period is not unprecedented.
Eine Studie von Foulds et al. 2013 im Fachblatt Hydrological Processes untersuchte die Überschwemmungsgeschichte in Südwest England und fand besonders häufige Hochwässer Ende des 19. bis Anfang des 20. Jahrhunderts. Die Autoren beschrieben einen Zusammenhang mit der NAO:
The chronology and the hydrometeorology of catastrophic floods on Dartmoor, South West England
Extreme floods are the most widespread and often the most fatal type of natural hazard experienced in Europe, particularly in upland and mountainous areas. These ‘flash flood’ type events are particularly dangerous because extreme rainfall totals in a short space of time can lead to very high flow velocities and little or no time for flood warning. Given the danger posed by extreme floods, there are concerns that catastrophic hydrometeorological events could become more frequent in a warming world. However, analysis of longer term flood frequency is often limited by the use of short instrumental flow records (last 30–40 years) that do not adequately cover alternating flood-rich and flood-poor periods over the last 2 to 3 centuries. In contrast, this research extends the upland flood series of South West England (Dartmoor) back to ca AD 1800 using lichenometry. Results show that the period 1820 to mid-1940s was characterized by widespread flooding, with particularly large and frequent events in the mid-to-late 19th and early 20th centuries. Since ca 1850 to 1900, there has been a general decline in flood magnitude that was particularly marked after the 1930s/mid-1940s. Local meteorological records show that: (1) historical flood-rich periods on Dartmoor were associated with high annual, seasonal and daily rainfall totals in the last quarter of the 19th century and between 1910 and 1946, related to sub-decadal variability of the North Atlantic Oscillation and receipt of cyclonic and southerly weather types over the southwest peninsula; and (2) the incidence of heavy daily rainfall declined notably after 1946, similar to sedimentary archives of flooding. The peak period of flooding on Dartmoor predates the beginning of gauged flow records, which has practical implications for understanding and managing flood risk on rivers that drain Dartmoor.
Auch Paul Homewood dokumentierte anhand von vielen Statistiken ein regenreiches frühes 20. Jahrhundert in Großbritannien (hier und hier). Nach dem ersten überschwemmungsreichen Winter 2013/14 fasste eine Gruppe um Chris Huntingford die Modellierungsherausforderung in einem Artikel in Nature Climate Change im August 2014 zusammen. Man müsse zunächst alle möglichen Treiber verstehen, bevor man große Modellierungsanstrengungen unternähme. Unter anderem müsse man die Ozeanzyklen, solare Aktivitätsschwankungen, Veränderungen im arktischen Meereis, Landnutzung und die Bevölkerungsdynamik unter die Lupe nehmen:
Potential influences on the United Kingdom’s floods of winter 2013/14
During the winter of 2013/14, much of the UK experienced repeated intense rainfall events and flooding. This had a considerable impact on property and transport infrastructure. A key question is whether the burning of fossil fuels is changing the frequency of extremes, and if so to what extent. We assess the scale of the winter flooding before reviewing a broad range of Earth system drivers affecting UK rainfall. Some drivers can be potentially disregarded for these specific storms whereas others are likely to have increased their risk of occurrence. We discuss the requirements of hydrological models to transform rainfall into river flows and flooding. To determine any general changing flood risk, we argue that accurate modelling needs to capture evolving understanding of UK rainfall interactions with a broad set of factors. This includes changes to multiscale atmospheric, oceanic, solar and sea-ice features, and land-use and demographics. Ensembles of such model simulations may be needed to build probability distributions of extremes for both pre-industrial and contemporary concentration levels of atmospheric greenhouse gases.
In den Schlussfolgerungen erläutern die Autoren, dass es bei den Überschwemmungen in Großbritannien derzeit keinen Trend gäbe. Zudem könne nicht ausgeschlossen werden, dass verminderte Schneemengen in einer wärmeren Welt insgesamt zu einer Verringerung des Überflutungsrisikos führen wird:
Although there is some observational evidence of intensification of UK rainfall, there is little suggestion of emerging trends in precipitation-driven (that is, non-tidal) high-magnitude UK floods; less UK ice and snowmelt in a warming world may suppress flood risk.
Wie in vielen Bereichen des Lebens meldeten sich ein paar Monate später sogleich Bedenkenträger. Man habe doch sowieso keine Chance, die britischen Überschwemmungen modellmäßig in den Griff zu bekommen, gaben Geert Jan van Oldenborgh und Kollegen im Mai 2015 in Nature Climate Change zu Protokoll, denn keiner der von Huntingford und Kollegen angeführten Parameter hätte für die bisherige Entwicklung irgendeinen statistischen Wert gezeigt:
Drivers of the 2013/14 winter floods in the UK
“To conclude, due to the lack of any strong associations with precipitation in southern England, we find it difficult to believe that any of the proposed drivers could have been responsible for the extreme event in winter 2013/14.“
Ein starkes Stück, denn historische Studien hatten doch eigentlich einen Zusammenhang mit AMO und NAO festgestellt (siehe oben). Huntigford et al. 2015 antworteten geduldig. Man müsse hier etwas großmaßtäblicher im Atlantik vorgehen und sich nicht nur auf eine kleine Region wie Südengland beschränken:
The disagreement is essentially one of spatial scale: their arguments are based on a particularly localized measure of rainfall for southern England, rather than the Atlantic atmospheric circulation or more UK-wide precipitation and river flows that we discussed.
Recht haben sie!
Überhaupt gab es in der britischen Winterflut-Diskussion etliche geistige Kurzschlüsse. Auf dem Höhepunkt der ersten winterlichen Fluten trat der damalige Premierminister David Cameron vor die Mikrofone und erklärte kurzerhand den Klimawandel zum Schuldigen der Misere. Auch die Chefin des Met Office, Julia Slingo, vertrat diese Ansicht. Ihre Mitarbeiter waren darüber ’not amused‘ und kritisierten sie für diesen Schnellschuss heftig. Hatte die Met Office Chefin durch zuviel Verwaltungsarbeit den wissenshaftlichen Boden unter den Füßen verloren? Andrew Montford berichtete am 18. Februar 2014 im Spectator über den Vorfall:
Why the Met Office has hung its chief scientist out to dry
Last week the Met Office and the Centre for Ecology and Hydrology issued an admirable joint report on the floods and their possible connection to climate change, concluding that it is not possible to make such a link. ‘As yet’, it said, ‘there is no definitive answer on the possible contribution of climate change to the recent storminess, rainfall amounts and the consequent flooding’. In many ways this was not much of a surprise, since only the wild activist fringe among the climate science community have tended to try to make the link in the past.
Taking such a level-headed view, the Met Office report represented a valuable opportunity to bring some calm to an increasingly frenzied debate over the flooding. However, unfortunately for everyone, the good work was all undone by the Met Office’s own chief scientist, Professor Dame Julia Slingo. Newly ennobled in the New Year’s honours list, Slingo seems to have found the temptation to put a global warming spin on everything that crosses her desk too much, and she blurted out to journalists the extraordinary claim that ‘all the evidence suggests there is a link to climate change’.
Weiterlesen im Spectator
Es ist für die Politik natürlich sehr bequem, dem Klimawandel die Schuld für die Fluten in die Schuhe zu schieben. In Wirklichkeit hatte man jahrelang bei der Hochwasser-Sicherung geschludert, wichtige Investionen unterlassen. Die FAZ fand am 28. Dezember 2015 klare Worte:
Hochwasser in England: In den Wogen der Kritik
Angesichts der heftigen Überschwemmungen in Nordengland muss sich David Cameron bohrende Fragen nach dem Hochwasserschutz gefallen lassen. Der Premier stapft mit Gummistiefeln durch die Fluten – und schiebt alle Schuld auf den Klimawandel.Noch nie dagewesen“ und „warum“ sind die zwei Begriffe, die in der jüngsten britischen Hochwasserkatastrophe refrainartig wiederholt werden. Warum führe die Regierung beispielsweise zwölf Milliarden Pfund an Entwicklungsländer ab, wo doch die eigenen Steuerzahler der Hilfe bedürften? Warum wurde eine wesentliche Flutbarriere in York nicht in Betrieb gesetzt? Und warum hat die Regierung den Rat der Experten nicht beachtet, die Maßnahmen zur Anpassung an den Klimawandel empfohlen hätten? Lauter Fragen, auf die es von offizieller Seite bislang nur eine Antwort gab: dass die bestehenden Schutzmaßnahmen eine Überprüfung erforderten.
Weiterlesen in der FAZ.
Auch der Sunday Express nahm einen Tag später kein Blatt vor den Mund:
Historic weather records rubbish Cameron’s attempts to blame floods on global warming
David Cameron has sparked outrage by blaming Britain’s flood crisis on global warming while admitting defences are not fit for purpose.The Prime Minister said more frequent ‘extreme weather events’ driven by climate change were the main driver as Cumbria and the north braces for more hell. However experts branded his comments “ludicrous excuses” blaming lack of investment on flood defences for the disaster and pointed to historic flooding which pre-dated global warming. They accused the Prime Minister of deflecting attention away from accusations Britain is woefully unprepared for severe weather. Climatologists say although devastating, the floods have nothing to do with global warming but are part of a natural weather cycle.Weiterlesen im Sunday Express. Weitere Artikel hier, hier und hier.
Das Argument höherer Überschwemmungsversicherungsschäden sticht nicht. Der Schadensanstieg ist schlichtweg eine Folge eines Zuwachses an versicherten Werten, wie die University of Southampton am 20. August 2014 mitteilte:
Increase in reported flooding a result of higher exposure
A rise in the number of reported floods in the UK over the past 129 years can be related to increased exposure, resulting from urban expansion and population growth, according to new research by the University of Southampton.
In one of the most comprehensive studies of its kind, scientists have discovered that although the number of reported floods has gone up during the 20th and 21st Century, this trend disappears when the figures are adjusted to reflect population growth and increased building numbers over the same period. Published in the journal Hydrological Sciences, the study looks at data sets from 1884 to 2013 and found an upward trend in reported flooding, with flood events appearing more frequently towards the end of the 20th century, peaking in 2012 when annual rainfall was the second highest in over 100 years.
The rise in UK flood reports over the 20th Century coincides with population growth from 38.2 million to 59.1 million and a tripling in the number of houses, from 7.7 million to 24.8 million. “As a result there were more properties exposed to flooding and more people to report flooding,” says lead author Andrew Stevens. “A higher exposure to flooding will result in more reported flood events and larger potential damages.” The study found significant variation between decades in both the raw and adjusted data, with the years between 1908 -1934, 1977 – 1988 and 1998 – 2013 featuring a relatively high numbers of reported floods.
The effect of increasing and improving flood defences is unclear. While upgrades to artificial defences, like the Thames Barrier, have reduced the effect of extreme sea level events, natural flood defences may have declined over the study period. “Attributing periods of reduced flood damage simply to the effects of improved management is difficult and must be done with care,” says co-author Derek Clarke. Professor Robert Nicholls adds “These observations should not stop concern about future flood impacts, especially in coastal areas where faster sea-level rises are expected and areas potentially exposed to higher rainfall intensities. Future flood risk may be very sensitive to changes in funding or management approaches and this has important implications for decision makers.”
Eine interessante Übersicht zu britischen Wettertrends allgemein gibt es von Neil Catto.
