In den Medien liest man des öfteren, dass der fortschreitende Klimawandel pauschal zu verstärktem Extremwetter führen würde. Wenn man dann im Detail nachschaut, inwieweit sich das Extremwetter in den letzten 100 Jahren im Zuge der Erwärmung verändert hat, erlebt man regelmäßige Überraschungen. Jüngstes Beispiel ist eine Studie von Archfield und Kollegen, die im Oktober 2016 in den Geophysical Research Letters erschien. Die Forscher untersuchten Hochwassertrends in den gesamten USA und fanden eine sehr unregelmäßige Entwicklung. In einigen Flüssen gab es weniger Hochwasser, in anderen mehr, in wiederum anderen konnte das Archfield-Team keine Veränderungen feststellen. Zudem lassen sich die Entwicklungen nicht in geographische Muster aufgliedern, so dass nicht einmal regionale Trends ersichtlich sind. Die Wissenschaftler kommen zu dem nüchternen Schluss, dass sich für Hochwasser und Überschwemmungen in den USA keine allgemeinen Aussagen tätigen lassen. Im Folgenden die Kurzfassung der Arbeit:
Fragmented patterns of flood change across the United States
Trends in the peak magnitude, frequency, duration, and volume of frequent floods (floods occurring at an average of two events per year relative to a base period) across the United States show large changes; however, few trends are found to be statistically significant. The multidimensional behavior of flood change across the United States can be described by four distinct groups, with streamgages experiencing (1) minimal change, (2) increasing frequency, (3) decreasing frequency, or (4) increases in all flood properties. Yet group membership shows only weak geographic cohesion. Lack of geographic cohesion is further demonstrated by weak correlations between the temporal patterns of flood change and large-scale climate indices. These findings reveal a complex, fragmented pattern of flood change that, therefore, clouds the ability to make meaningful generalizations about flood change across the United States.
Wo wir gerade beim Thema sind. Roger Pielke Jr. erläuterte, dass die Hochwasserschäden in den USA in den letzten 70 Jahren gesunken sind, wenn man gegen die Entwicklung des Brutosozialprodukts normiert. Insgesamt aber ist das Überflutungsrisiko in den US-amerikanischen Städten gestiegen. Ursache ist nicht etwa der Klimawandel, sondern hat vor allem sozioökonomische Gründe, insbesondere das ungebremste Wachstum der Städte in überschwemmungsbedrohte Gebiete. Was früher bewusst unbebaut blieb, ist heute Siedlungsfläche geworden. Hierauf wies die Texas A&M University am 3. März 2015 in einer Pressemitteilung explizit hin:
Watch Out NY, Houston: Floods Could Be Headed Your Way
A heads-up to New York, Baltimore, Houston and Miami: a new study suggests that these metropolitan areas and others will increase their exposure to floods even in the absence of climate change, according to researchers from Texas A&M University. Their work is published in Global Environmental Change.
The study presents first-ever global forecasts of how the exposure of urban land to floods and droughts may change due to urban expansion in the near future. In 2000, about 30 percent of the global urban land (over 75,000 square miles) was located in the high-frequency flood zones; by 2030, this will reach nearly 40 percent (280,000 square miles) as the global urban land grows from 250,000 square miles to 720,000 square miles, the authors say. The researchers also predict that by 2030 the urban extent in drylands will nearly double, reaching over 190,000 square miles, and that even without climate change, extent of urban areas exposed to both floods and droughts would more than triple by 2030, according to the study.
“Urban areas exposed to flood and drought hazards will increase considerably due to the sheer increase in their extents driven primarily by socio-economic forces,” says Burak Güneralp, lead author of the study and a research assistant professor in geography at Texas A&M. “In particular, coastal megacities will house a majority of the urban populations and they will increasingly be hubs of significant economic activity in the coming decades. Yet, potential future changes in the extent and layout of the urban areas are typically ignored in resilience planning for these cities.”
The study points to significant variation in the geographical patterns of how the exposure of cities, especially to flood hazards, will change in the first three decades of the 21st century. The largest increases in urban exposure are expected in high-frequency flood zones and arid lands of Asia and Africa. In particular, emerging coastal metropolitan regions in these two continents are expected to have larger areas exposed to flooding than those in developed countries. Although relatively small compared to what we can expect in Asia and Africa, in the U.S. the expansion of Houston and Miami metropolitan areas as well as the metropolitan area between Baltimore and New York is expected to increase their exposure to flooding. In addition, the expansion of many urban areas in the Southwest U.S., in particular central and southern California, is expected to increase their exposure to drought.
The researchers used existing urban land cover maps and urban expansion forecasts and employed GIS analysis to identify the amount of urban expansion in flood and drought prone zones around the world in 2000 and 2030. The infrastructure requirements of all this forecasted growth would be phenomenal. For example, elsewhere, it is estimated that a staggering $57 trillion will be required to meet the global infrastructure demand worldwide by 2030. These are huge investments on infrastructure, some of which will be located in flood or drought prone areas. But the authors say there are initiatives that can be taken to adapt to and mitigate floods and droughts. These include individual cities preventing development in flood-prone zones and thus, protecting natural habitats that would reduce the likelihood of being flooded. Large-scale adoption of green infrastructure, which includes reducing impervious surface cover, can decrease the exposure of cities to high-frequency floods and droughts.
Although the researchers’ focus was on the increase in urban areas, “the impact of urban land cover on river flooding is not simply a matter of increasing impervious surface cover, which causes an increase in the intensity and duration of peak river discharges,” says İnci Güneralp, second author of the study and assistant professor at Texas A&M. “Through land change, bank protection, channelization, and other means, urbanization can also alter the geomorphology of river channels and floodplains, which in turn may contribute to increased risk of flooding.” “Our findings suggest that future urban expansion in flood and drought prone zones will at least be as important as population growth and economic development in increasing their exposure,” the researchers add. “With climatic changes, this exposure is only expected to increase in the future. Thus, proper planning and financing in fast growing cities today will be critical in mitigating future losses due to floods and droughts.”
The study can be viewed at http://authors.elsevier.com/a/1Qc0c3Q8oPoXNl.