UK flooding and climate change: a long term perspective. Mark Macklin, Paul Brewer and Anna Jones IGES

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1 UK flooding and climate change: a long term perspective Mark Macklin, Paul Brewer and Anna Jones IGES

2 York: Millennium Floods, Autumn 2000 Boscastle: August 2004 Tewkesbury: July 2007 Cumbria: November 2009 Over the last decade a series of major floods in the UK have lead many environmental protection agencies, politicians and members of the general public to believe that floods are becoming more frequent and larger. Recent floods are viewed as unprecedented and are attributed by many to anthropogenically caused climate change. Is this really true?

3 The problem: gauged river flow records in the UK (as elsewhere in the world see below) are remarkably short (average length 45 yrs) and many catchments in upland and sparsely populated areas are un gauged. Estimating the magnitude of the 1 in 100 year flood with 45 yrs of record is therefore problematic! Country Most recent data submitted to GRDC in Average length of daily flow records (years) Country Most recent data submitted to GRDC in Average length of daily flow records (years) Algeria Malaysia Australia Mali Austria Mexico Belarus Namibia Benin Netherlands Botswana New Zealand Canada Niger Central African Republic Nigeria China Oman Cote D'Ivoire Panama Cyprus Puerto Rico Czech Republic Romania Denmark Russia Ecuador Serbia Finland Slovakia Germany Slovenia Ghana South Africa Guinea Sweden Iceland Switzerland Ireland Thailand Japan United Kingdom Latvia United States of America Lithuania Zambia

4 The longest gauged records (e.g. Wye & Thames) do not show consistent patterns. Statistical analysis employed is usually simplistic and fits trends in terms of a straight line fit or smoothed sinusoidal curve.

5 But variations in flow regime are more usually in the form of a step shift, related to abrupt changes in atmospheric circulation (North Atlantic Oscillation, NAO) that control storm frequency and type.

6 The primary challenge for river scientists is to find alternative methods to extend the flood series, to establish whether or not floods are becoming more frequent and/or larger. This is an area that AU have been researching over the last decade; they now lead the UK in this field with methodologies developed being adopted worldwide. Two principal ways of extending the flood record: 1. Documentary sources 2. Flood sediments

7 Documentary sources (A) Documented extreme floods in Yorkshire Dales (Longfield, 1998, unpub. PhD) (B) River Ouse flood record at York (gauge record in black, documentary record in grey: Longfield, 1998, unpub. PhD; McDonald, 2003, unpub. PhD)

8 Flood sediments: boulder berms 300+ lichendated boulder berm flood units in upland England and Wales (AD 1630 present)

9 When boulder berm records are plotted for separate upland areas some differences emerge. Storms and floods vary regionally Notable periods of reduced upland flooding during the last decade of the 19 th century & particularly since the 1950s

10 Lichen dated flood units can be allocated to a single year and season using documentary records. 15 out of 21 floods occur when NAO is in negative phase. Those events that don t fit pre date 1850, which is when instrumental based records of NAO begin. Not all negative NAO winters produced extreme floods Flood magnitude significantly decreased at the turn of the 20 th century

11 Flood sediments: meta analysis of 14 C dated fluvial units 14 C dates in fluvial sequences that coincide with a modification in sedimentation rate, or style, identify significant changes in river activity associated with large floods 2010: C dates from 174 sites Overbank gravel splay, lower Thames c cal BP. Largest flood in the last 11,500 yrs!

12 17 episodes ( yr) of increased flooding in the last 11,800 yrs

13 Relative probability curves of 14 C dates associated with major Holocene flood events in northern Britain plotted against the stacked palaeo water table record (100 year moving average) derived from analysis of testate amoebae in northern British peatlands (Charman et al., 2006a; 2006b)

14 Relative probability dtsi (Watts per m 2 ) Solar irradiance Onset of incision Age (cal. BP) A solar climate link to extreme flood events? Cockermouth November 2009

15 Conclusions Are floods in the UK becoming larger and more frequent? Short answer, in the context of the long view, is clearly no. BUT homes and infrastructure are more vulnerable because of construction and encroachment on floodplains. The near future? Recent cooling of the North Atlantic (weakening sun?), southerly shift of the jet stream, switch to predominant NAO, could be greater frequency of rain-on-snow generated events and summer flooding typical of the Little Ice Age? These are not the flooding regimes predicted by GCM models where climate is forced solely by changing greenhouse gas emissions!