Hønefossen July 20. 2007. Photo: T.Bjørnstad/ Wikimedia Commons
Most climate models agree on a general increase in future precipitation in the Northern Hemisphere. Mainly due to short monitoring series (<50 years) and limited station networks, consensus has however not yet been reached on how this will perturb flooding rates.
In a recent paper published in Global and Planetary Change Eivind Støren and Øyvind Paasche examine the potential co-variability between winter precipitation and floods on millennial time scales, and the results are somewhat surprising.
The impact of climate change on flood frequency in the past
Reconstructed winter precipitation from five records in Scandinavia is compared to data from two high-resolution flood records from southern Norway over a period of 6000 years.
The variability in flood frequency is large and at times abrupt over this period, and although uncertainties emerge at every step in the process of transforming a sedimentary stratigraphy into climatic variables, results show a positive non-linear correlation between amount of winter precipitation and interarrival, times of floods on centennial timescale.
Winter precipitation (snow) explains around 40% of changes in the flood frequency indicating that the spring snowmelt flood peak that dominates the present discharge regime in the major rivers in Southern Norway is dominating on long timescales as well. Poor correlations are evident at shorter timescales, which suggests that factors such as human activities, the number of summer rainstorms and possibly various dynamic catchment responses have some explanatory power as well.
The impact of climate change on flood frequency in the future
Projections for future winter precipitation over central Scandinavia for the next 100 years suggest a continued increase. This tends to imply that the number of floods will increase in response to a wetter regime.
This is however, according to the present study, a passing effect as the accompanying warming, causing an increasing share of precipitation to fall as rain rather than snow, will cancel that net effect of a wetter regime and cause a rapid reduction in snowmelt-triggered floods.