Sonne steuert Überschwemmungen in Zentralchina: Forscher entdecken 500-Jahreszyklus in Höhlentropfsteinen

Immer hören wir in den Nachrichten vor schlimmen Überflutungen in Asien mit vielen Opfern. Im Jahr 2008 ereignete sich in Myanmar eine besonders schlimme Katastrophe, die Brakenridge und Kollegen 2017 untersuchten. Dabei stellte sie fest, dass die meteorologische Situation zwar selten war, aber leider immer wieder in der Region in der Vergangenheit aufgetreten ist. Die Autoren mahnten besseren Flutschutz im Land an und empfahlen, dass die Bevölkerung Ansiedlungen in besonders flutanfälligen Bereichen vermeiden sollte. Abstract:

Design with nature: Causation and avoidance of catastrophic flooding, Myanmar 
Myanmar is among 15 nations that account for 80% of global population exposed to flooding. In 2008, the country suffered exceptional damage and human mortalities (> 138,000) from tropical storm Nargis, which followed an unusual but not unprecedented storm track. In 2015, heavy monsoonal rains related to the tropical Madden–Julian Oscillation plus a slow-moving tropical storm (Komen) together caused major flooding, ~ 130 fatalities, and very severe damage and losses. Both events triggered international food, medical, and other assistance, including efforts to design rebuilding with greater resilience to floods. Orbital remote sensing can be employed to characterize such damaging floods and quantify future flood risk; advanced lead-time flood prediction is also increasingly accurate and available. These capabilities must, however, be applied in a context of environmental variables (climate, sea level, dams, and land cover) that are changing the hazard. In addition to the hydrometeorology, distal causes for flood disasters include: high sediment loads carried by Myanmar rivers, locally rapid rates (> 50–100 m/y) of channel migration, expansion of population into vulnerable locations, and anthropogenic modifications to floodplains, watersheds, and the coastal zone. Engineering projects can protect local communities, but flood control structures will fail again unless the environmental changes that increase exposure to flood damage are also mitigated. Earth Science-based methods for long term reduction of societal exposure include floodplain reconnection, levee removal, controlled avulsions, and redirecting new housing and other economic development onto lands with less severe flood risk.

Im September 2014 kam es in der Kaschmir-Region zu Überschwemmungen. Leider nicht das erste mal, worauf Madhav Khandekar in einem Bericht für die GWPF hinwies (pdf hier):

Floods and Droughts in the Indian Monsoon: Natural variability trumps human impact
The floods and unfortunate deaths of several dozen people in the Kashmir region of India in September 2014 reignited the debate about increasing human emissions of carbon dioxide and their putative linkage to extreme weather events such as floods, droughts and heat waves. What is missing from many of the media reports and scientific publications on this subject is critical analysis of past weather extremes to determine if there has been an increase in recent years. In this brief report, past floods and droughts in the Indian monsoon are examined carefully and it is shown that such events have occurred throughout the excellent 200-year-long summer monsoon rainfall dataset. It is further documented that such floods and droughts are caused by natural variability of regional and global climate, and not by human carbon dioxide emissions. Improving our understanding of the inter-annual variability of the monsoon and the associated extremes may help reduce damage to infrastructure and loss of life in the future.

Historische Weitsicht könnte so einige Fehlinterpretationen vermeiden. Wasson et al. 2013 haben eine 1000-jährige Flutgeschichte im Zentralhimalaya rekonstruiert. Überflutungen waren häufig während der Mittelalterlichen und Modernen Wärmeperiode. In der Kleinen Eiszeit waren die Fluten seltener. Ursache scheinen Veränderungen im Monsun zu sein. Abstract:

A 1000-year history of large floods in the Upper Ganga catchment, central Himalaya, India
Determining the frequency, magnitude and causes of large floods over long periods in the flood-prone Himalaya is important for estimating the likelihood of future floods. A thousand year record (with some information from 2600 years ago) of the frequency and some estimates of velocities and discharges of large floods has been reconstructed in the Upper Ganga catchment, India, using written reports, litho-stratigraphy and sedimentology, and dated by optical and radiocarbon methods. In the Upper Ganga catchment rainfall triggers large landslides that dam rivers and release large amounts of water when they burst, thereby amplifying the effects of rainfall. The large floods in the catchment may be the result of landslide dam bursts rather than glacial lake bursts, and these are likely to continue and possibly worsen as the monsoon intensifies over the next century. However preliminary information suggests that the recent devastating flood of June 2013 was the result of heavy rainfall not landslide dam bursts. The frequency record is non-random and shows a high frequency between AD 1000 and AD 1300 (omitting uncertainties), then a low frequency until a cluster of floods occurred about 200 years ago, then increased frequency. This temporal pattern is like but not identical with that in Peninsular India, and both appear to be the result of variations in the monsoon.

Kim et al. 2017 brachten eine 1500-jährige Flutgeschichte aus der Mongolei.

The history of palaeoflood and palaeoclimate recorded in the flood deposits of the Kherlen River, Mongolia
This study examines the 1500-year history of massive floods as recorded in the slackwater deposits of the Kherlen River basin in Mongolia. The study area is located along the Kherlen River in Baganuur district, Ulaanbaatar. Site HL1 has a flood frequency of 89 years and an accumulation rate of 1.2 mm/y over approximately 1500 years. Site HL2 has a flood frequency of 72.2 years and an accumulation rate of 1.46 mm/y during about 700 years. The range of calculated value for flood frequency and annual accumulation rate during the period of the 10th century to the early 20th century at site HL1 is entirely different from that in other periods. It is considered that the palaeohydrological environment of the study site during that time might have been influenced by climatic change as well as geomorphological and hydrological change. Based on the results of identification of discrete flooding and age dating (137Cs and 14C), the sedimentary layers of HL1 and HL2 were divided into 4 periods (period 1: 1960–2012, period 2: 970–1960, period 3: 533–970, period 4: 427–533) and 2 periods (period 1: 1960–2012, period 2: 1290–1960), respectively. The authors suggest that the past climate of the region was greatly influenced by the East Asia summer monsoon. It is suggested that the occurrence of the large-scale floods in eastern Mongolia was influenced by the strengthening and weakening of the summer monsoon due to climate change. 

Aus China berichteten Liu et al. 2014 und Chen et al. 2015 über vorindustrielle Überflutungsphasen. Zhu und Kollegen publizierten im Januar 2017 in PNAS eine Überflutungsstudie auf Basis von Höhlentropfsteinen aus Zentralchina. Die Forscher fanden einen 500 Jahreszyklus, der eng an die El Nino/La Nina-Oszillation und solare Schwankungen gekoppelt ist. Hier die Pressemitteilung zum Paper von der University of Minnesota:

Caves in Central China show history of natural flood patterns

Researchers at the University of Minnesota have found that major flooding and large amounts of precipitation occur on 500-year cycles in central China. These findings shed light on the forecasting of future floods and improve understanding of climate change over time and the potential mechanism of strong precipitation in monsoon regions. The research is published in the Proceedings of the U.S. National Academy of Sciences (PNAS), a leading scientific journal.

“To predict how climate change will impact the future, it’s important to know what has happened in the past,” said Joshua Feinberg, a University of Minnesota associate professor of Earth Sciences and associate director of the Institute for Rock Magnetism, who collaborated on the research with his Chinese colleagues. “As the variability and intensity of storms increase in the world, we need to reevaluate what the frequency of these major storms could be,” Feinberg said. “We didn’t have the potential to develop these kinds of precipitation records for most of the world, until now. These speleothems provide more than 8,000 years of data that led us to identify with strong confidence the presence of a 500-year cycle,” he added.

The research used stalagmites collected from Heshang Cave in central China within the Yangtze River drainage. Researchers measured the magnetic properties of layered stalagmites, or columnar mineral deposits formed in caves by the growth of carbonate minerals from dripping groundwater. As they form over time, stalagmites develop annual layers of the mineral calcite, which are broadly similar to the rings of a tree. They also collect iron-rich magnetic materials within these layers, which originated in overlying soil and are transported into the cave during precipitation and flooding events. These iron-rich minerals are far less than the width of human hair in size, but produce a strong magnetic signal that can be easily measured by modern magnetometers.

Feinberg and his team analyzed the magnetic properties of the layered stalagmites and discovered more than 8,000 years of data within the materials. The magnetic data varied in such a way as to trace out a 500-year cycle of storm variation, where wetter intervals showed an increased concentration of magnetic minerals. This correlates well with the cycles of El Niño Southern Oscillation pattern and measured changes in the amount of the energy from the sun. The cycle can be used to anticipate broad precipitation patterns in the future, and provide insight on climate change in the region over time. Feinberg and his team hope to expand this work wherever possible around the globe.

With the help of the Institute for Rock Magnetism (IRM), based at the University of Minnesota, the group was able to measure the magnetic materials within the speleothems to at a higher resolution and sensitivity than previously possible. Many rocks record the direction and strength of the Earth’s magnetic field at the time of their formation. By measuring these magnetizations, researchers are able to show how tectonic plates have moved around the globe through time, as well as how the Earth’s magnetic field has varies over timescales ranging from millions of years to decades. The short-term behavior of the Earth’s magnetic field has important ramifications for satellites and satellite communication.

Postdoctoral research associates Zongmin Zhu and Mark Bourne, IRM and Department of Earth Sciences; research scientist Hai Cheng, Xi’an Jiatong University; and Shucheng Xie, Chunju Huang and Chaoyong Hu, researchers from the China University of Geosciences are co-authors of the study. The research was financially supported by the National Natural Science Foundation of China, State Key R&D program of China, and the 111 program (National Bureau for Foreign Experts and the Ministry of Education of China). The Institute for Rock Magnetism is funded by the U.S. National Science Foundation (NSF) Division of Earth Sciences Instruments and Facilities Program and the University of Minnesota. Feinberg’s research is funded by NSF-EAR1316385. To read the complete study entitled “Holocene ENSO-related cyclic storms recorded by magnetic minerals in speleothems of central China,” visit the PNAS website

Wang und Kollegen publizierten 2014 eine Flutrekonstruktion aus Taiwan und fanden einen Rückgang der Überflutungen während der Kleinen Eiszeit. Überflutungen hängen hier mit dem Taifun-Geschehen zusammen.

Wir wünschen allen dkS-Lesern ein frohes Neues Jahr 2018!