Friday, May 31, 2013

New paper finds warming decreases floods

A new paper published in Climate of the Past finds floods are more common during periods of cooling and less frequent during periods of warming, the opposite of the claims of climate alarmists. The paper shows that flooding was more common during the Little Ice Age than during the 20th century or the Medieval Warming Period. The paper adds to many other peer-reviewed publications finding that global warming leads to fewer floods. The authors also find flood frequency is "under orbital and possibly solar control."

Flood frequency was highest during the periods shaded grey, including the Dark Ages Cooling Period [DCAP] and the Little Ice Age [LIA]

Clim. Past, 9, 1193-1209, 2013

Orbital changes, variation in solar activity and increased anthropogenic activities: controls on the Holocene flood frequency in the Lake Ledro area, Northern Italy

B. Vannière1, M. Magny1, S. Joannin1,2, A. Simonneau3, S. B. Wirth4, Y. Hamann4, E. Chapron3, A. Gilli4, M. Desmet5, and F. S. Anselmetti6
1CNRS, UMR6249, Chrono-Environnement, Université de Franche-Comté, Besançon, France
2LGL TPE, Université Lyon 1, Villeurbanne, France
3ISTO, UMR 7327, CNRS, University of Orléans, BRGM, France
4Geological Institute, ETH Zurich, Switzerland
5GéHCO, UFR ST, Université Francois Rabelais, Tours, France
6Institute of Geological Sciences and Oeschger Centre for Climate Change Research, University of Bern, Switzerland

Abstract. Two lacustrine sediment cores from Lake Ledro in northern Italy were studied to produce chronologies of flood events for the past 10 000 yr. For this purpose, we have developed an automatic method that objectively identifies the sedimentary imprint of river floods in the downstream lake basin. The method was based on colour data extracted from processed core photographs, and the count data were analysed to capture the flood signal. Flood frequency and reconstructed sedimentary dynamics were compared with lake-level changes and pollen inferred vegetation dynamics. The results suggest a record marked by low flood frequency during the early and middle Holocene (10 000–4500 cal BP). Only modest increases during short intervals are recorded at ca. 8000, 7500, and 7100 cal BP. After 4500–4000 cal BP, the record shows a shift toward increased flood frequency. With the exception of two short intervals around 2900–2500 and 1800–1400 cal BP, which show a slightly reduced number of floods, the trend of increasing flood frequency prevailed until the 20th century, reaching a maximum between the 16th and the 19th centuries. Brief-flood frequency increases recorded during the early and middle Holocene can be attributed to cold climatic oscillations. On a centennial time scale, major changes in flood frequency, such as those observed after ca. 4500/4000 and 500 cal BP, can be attributed to large-scale climatic changes such as the Neo-glacial and Little Ice Age, which are under orbital and possibly solar control. However, in the Bronze Age and during the Middle Ages and modern times, forest clearing and land use probably partially control the flood activity.