“The results of this study, for North America and Europe, provide a firmer foundation and support the conclusion of the IPCC that compelling evidence for increased flooding at a global scale is lacking.”
Fig. 2. Monthly distribution of floods with ≥25 year return periods for 1204 study gauges from 1961 to 2010, by major Köppen-Geiger climate for North America on the left in green and Europe on the right in blue. Monthly values are percent of total number of floods with > 25 year return periods for each Köppen-Geiger climate.
G.A. Hodgkins et al., Climate-driven variability in the occurrence of major floods across North America and Europe, Journal of Hydrology, Volume 552, September 2017, Pages 704-717
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. […]
Reference hydrologic networks isolate catchments where climate has been the principal driver of streamflow change by minimizing other drivers, such as regulation, diversions and urbanisation. The relationship between floods and climate change is more difficult to discern where catchments have been altered, making attribution to any single driver uncertain.
Trends over time in the occurrence of major floods (exceeding 25, 50, and 100 year return periods) in North America and Europe were evaluated for 1961–2010 and 1931–2010. All gauges drain catchments that are considered by local and national experts to be minimally affected by catchment alterations. Trend testing of major floods required the grouping of gauges. The 1204 gauges that met study criteria for 1961–2010 and the 322 gauges for 1931–2010 were grouped by continent, Köppen-Geiger climate and catchment size. The number of significant trends for 246 groups of gauges was approximately the same as would be expected by chance alone.
There were more than three times as many groups of gauges with significant relationships between the number of annual major floods and annual values of the Atlantic Multidecadal Oscillation than expected due to chance. Catchment size was important to the results; there were significant negative relations between floods and the AMO at large (>1000 km2) North American catchments and significant positive relations at medium (100–1000 km2) European catchments. The opposite relations between European and North American major flood occurrence and the AMO are consistent with previous work on general wetness and dryness related to the AMO. There were no significant relationships, for any group of catchments, between major flood occurrence and the Pacific Decadal Oscillation.
The results of this study, for North America and Europe, provide a firmer foundation and support the conclusion of the IPCC (Hartmann et al., 2013) that compelling evidence for increased flooding at a global scale is lacking. Generalizations about climate-driven changes in floods across large domains or diverse catchment types that are based upon small samples of catchments or short periods of record are ungrounded. Networks of streamflow data from minimally altered catchments will provide an essential foundation for future efforts to understand the complex temporal and spatial dynamics of major floods.