Climate-driven variability in the occurrence of major floods across North America and Europe - Journal of Hydrology Review

An assessment of major flood trends was conducted by a group of researchers including Environment and Climate Change Canada and University of Waterloo Civil and Environmental Engineering. They found no evidence that climate change has to date increased the occurrence of floods.

Concern over the potential impact of anthropogenic climate change on flooding has led to a proliferation of studies examining past flood trends. Many studies have analysed annual-maximum flow trends but few have quantified changes in major (25–100 year return period) floods, i.e. those that have the greatest societal impacts. Existing major-flood studies used a limited number of very large catchments affected to varying degrees by alterations such as reservoirs and urbanisation. In the current study, trends in major-flood occurrence from 1961 to 2010 and from 1931 to 2010 were assessed using a very large dataset (>1200 gauges) of diverse catchments from North America and Europe; only minimally altered catchments were used, to focus on climate-driven changes rather than changes due to catchment alterations. Trend testing of major floods was based on counting the number of exceedances of a given flood threshold within a group of gauges. Evidence for significant trends varied between groups of gauges that were defined by catchment size, location, climate, flood threshold and period of record, indicating that generalizations about flood trends across large domains or a diversity of catchment types are ungrounded. Overall, the number of significant trends in major-flood occurrence across North America and Europe was approximately the number expected due to chance alone. Changes over time in the occurrence of major floods were dominated by multidecadal variability rather than by long-term trends. There were more than three times as many significant relationships between major-flood occurrence and the Atlantic Multidecadal Oscillation than significant long-term trends."

The paper is available from the Journal of Hydrology. The authors are from across the world.

Glenn A.HodgkinsaPaul H.WhitfieldbDonald H.BurncJamieHannaforddBenjaminRenardeKerstinStahlfAnne K.FleiggHenrikMadsenhLuisMedieroiJohannaKorhonenjConorMurphykDonnaWilsong
U.S. Geological Survey, 196 Whitten Road, Augusta, ME 04330, United States
Environment and Climate Change Canada, 401 Burrard Street, Vancouver, BC V6C 3S5, Canada
University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada
Centre for Ecology and Hydrology, Maclean Building, Benson Lane, Wallingford, Oxfordshire OX10 8BB, United Kingdom
Irstea Lyon, Hydrology-Hydraulics, 5 rue de la Doua BP32108, 69616 Villeurbanne cedex, France
Albert-Ludwigs-Universität Freiburg, Fahnenbergplatz, 79098 Freiburg, Germany
Norwegian Water Resources and Energy Directorate, P.O. Box 5091, Majorstua, 0301 Oslo, Norway
DHI, Agern Allé 5, DK-2970 Hørsholm, Denmark
Technical University of Madrid, ETSI Caminos, Canales y Puertos, c/ Profesor Aranguren, 3 28040 Madrid, Spain
Finnish Environment Institute, SYKE, Freshwater Centre, P.O. Box 140, 00251 Helsinki, Finland
Irish Climate Analysis and Research UnitS (ICARUS), Department of Geography, Maynooth University, Maynooth, Co. Kildare, Ireland

Since rainfall intensities are not increasing dramatically across Canada, as demonstrated by Environment and Climate Change Canada (ECCC) in the Atmosphere-Ocean in 2014, saying rainfall intensities are stationary, it makes sense that major floods are not increasing either. Of course there may be local regional trends - ECCC found that there are decreasing rainfall intensities in regions such as the St Lawrence basin of southern Quebec and the Maritimes. Their Engineering Climate Datasets (version 2.3) also show twice as many statistically significant decreasing trends as increasing ones in southern Ontario.

The paper Climate-driven variability in the occurrence of major floods across North America and Europe focused on minimally altered catchments in order to isolate climatic as opposed to hydrologic drivers. It focused on large watersheds. As noted on this blog, urbanization is a key driver of flood risk in small urban catchments and is expected to have increased of the past  50-100 years in many Ontario cities. This is link to GIS mapping of changes: Ontario city urbanization affecting runoff and flood risk since from 1966 to 2000.