scandinavian floods: from past observations to future trends
TRANSCRIPT
Global and Planetary Change 113 (2014) 34–43
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Global and Planetary Change
j ourna l homepage: www.e lsev ie r .com/ locate /g lop lacha
Scandinavian floods: From past observations to future trends
Eivind N. Støren a,b,⁎, Øyvind Paasche c
a Department of Geography, University of Bergen (UoB), Fosswinckelsgate 6, Bergen, Norwayb Bjerknes Centre for Climate Research, Bergen, Norwayc Bergen Marine Research Cluster, Prof. Keysersgate 8, Bergen, Norway
⁎ Corresponding author.E-mail address: [email protected] (E.N. Støren
0921-8181/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.gloplacha.2013.12.002
a b s t r a c t
a r t i c l e i n f oArticle history:Received 15 March 2013Received in revised form 29 November 2013Accepted 2 December 2013Available online 11 December 2013
Keywords:FloodsClimate changePrecipitation variabilityScandinaviaHolocene
Although most climate models agree on a general increase in future precipitation in the Northern Hemispheredue to higher temperatures, no consensus has yet been reached on how this warming will perturb floodingrates. Here we examine the potential co-variability between winter precipitation (Pw) and floods on millennialtime scales. This is accomplished by analyzing reconstructed Pw from five records in Scandinavia, which is, com-pared to data from two high-resolution flood records from southern Norway. These Holocene records reveal apositive correlation (R2 = 0.41, p N 0.01) between the number of floods and Pw on centennial time scalesover the last 6000 years. Future projections for Pw over central Scandinavia for the next 100 years suggest a con-tinued increase in Pw that approximates maximum Holocene precipitation values. Despite an anticipated in-crease in Pw, the paleodata, nevertheless, suggest that we are likely to witness a decrease in future floods50–100 years from now because the accompanying warming will cancel that net effect of a wetter regime.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
The IPCC special report on Managing the Risks of Extreme Eventsand Disasters to Advance Climate Change Adaptation (SREX) recentlyconcluded that: “… there is limited to medium evidence available toassess climate-driven observed changes in the magnitude and frequen-cy offloods at regional scales because the available instrumental recordsof floods at gauge stations are limited in space and time…” (IPCC, 2012,Chapter 3.5.2, page 178).
The lack of credible flood data represents a serious problem not onlyfor the science community, but also for stakeholders and policy makersbecause it undermines any informed decision-making given the largeuncertainty at hand. Longer time series are, in other words, required ifone is to improve present assessments of how future climatic perturba-tions, both natural and anthropogenic, are likely to impact flood fre-quency in the decades and centuries to come.
Regardless of what set of criteria that are employed for categorizingextreme weather events, which can be demanding for empirical andmodel data alike (Hegerl et al., 2011), rich historical records exist forfloods across Europe (e.g. Glaser et al., 2010 and refs. therein). This isprecisely because of the devastating impacts floods can have onsocieties due to the sudden increase in erosional capacity and subse-quent remobilization of sediments and vegetation. Historical recordscarry valuable information about flood variability (magnitude, frequen-cy as well as temporal patterns), but only rarely do they extend beyondthe last 500 years which implies that for floods with recurrenceintervals of N100 years such records have restricted value when it
).
ghts reserved.
comes to exploring potential co-variability between changing climateconditions and flood frequencies.
Paleoflood reconstructions from both Europe and the US (e.g. Baker,1987; Knox, 1993; Ely, 1997; Brown et al., 2000; Knox, 2000;Nesje et al.,2001a; Gilli et al., 2003; Bøe et al., 2006; Thorndycraft and Benito, 2006;Benito et al., 2008; Moreno et al., 2008; Dasgupta et al., 2010; Størenet al., 2010, 2012; Wilhelm et al., 2012; Glur et al., 2013; Swierczynskiet al., 2013; Vannière et al., 2013;Wirth et al., 2013a,b) afford additionalinsight into the nature of floods, only on longer time scales. The spec-trum of natural variability is, however, still poorly constrained becausereconstructions are few in number and usually cover intermittent anddiscontinuous periods of time, in addition to often being based on singlesites with poor or unknown spatial representations. Any intercompari-sons between different flood records are further complicated becausedifferent, and usually qualitative, methods are employed in the recon-structions. Recent methodological progress has, however, shown truepotential in using lake sediments as a source for past flood variability.
Støren et al. (2010) presents, for instance, an objective method foridentifyingflood deposits in lakes that allow for direct intercomparisonsbetween contrasting sites. This approach, which is vetted by historicaldata, has produced high-resolution time series of floods throughout theHolocene. Similar progress has also been made from the Swiss (e.g. Gluret al., 2013; Wirth et al., 2013a) and French Alps (e.g. Wilhelm et al.,2012) showing comparable lake sediment time series in terms of Holo-cene flood frequencies, which have opened for new perspectives onhow flood patterns can be connected to climate.
The pioneering work by Ely (1997) and Knox (1993, 2000) showedthat even modest changes in climate, which affect the large-scaleatmospheric circulation and distribution of precipitation over theUnited States, caused abrupt changes in Holocene flood frequency in
35E.N. Støren, Ø. Paasche / Global and Planetary Change 113 (2014) 34–43
the Upper Mississippi Valley and the Colorado River. Marked changes inHolocene flood frequency are also documented in layered stalagmites intheMidwest, which show significant changes in rainfall-induced floods,indicating an increase in frequency during warm conditions (Dasguptaet al., 2010). Recentwork from the European Alps concludes that shiftingatmospheric circulation caused an increase in flood frequency that coin-cides with cool summer temperatures (Glur et al., 2013; Wirth et al.,2013a), whereas Wilhelm el. al. (2013) relate increased flood frequencyand magnitude on multi-decadal time scale to climate warming and oncentennial scale to regional precipitation patterns. By integrating numer-ical climate analysis with intercomparison of two paleoflood recordsStøren et al. (2012) suggest that observed differences in part are attribut-ed to mesoscale shifts in weather patterns over southern Scandinavia.The new and improved paleoflood datasets referred here points to po-tential links between changing climate conditions and flood frequency,but towhat extent past variabilitymight serve as an analog for anticipat-ed future shifts is commonly not addressed.
In this paper, we quantify for the first time the relationship or evencausality between solidwinter precipitation and corresponding changesin flood frequencies in Scandinavia during the Holocene. We show thatthe reoccurrence offloods is significantly impacted by changes inwinterprecipitation, and in the context of this observationwe discuss how thisrelationship may shed light on future flood trends in Scandinavia giventhe anticipated increase in temperature and winter precipitation.
2. Study area and data
Southern Norway (60–63°N) is located at the eastern flank of theNorth Atlantic (Fig. 1) and has, due to the westerly storm tracks, a
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JOSTEDALSBREENJOSTEDALSBREEN
SPØRTEGGBREENSPØRTEGGBREEN
HARDANGEHARDANGE
FOLGEFONNAFOLGEFONNA
0
LAKELAKE
GLACIERGLACIER
50500 KmKm
Fig. 1.Map of Southern Norway, showing the locations of glaciers and lakes included in this stumidity from Blektjärnen in Western Sweden. The Pw reconstructions are compared to flood reButjønna). A complete list of data and references is found in Table 1.
climate that is relatively warm and humid. Winter precipitation inwestern Norway is highly correlated with the North Atlantic Oscillation(NAO) during the instrumental period (Hurrel, 1995; Uvo, 2003), aconnectivity that decreases as one moves towards the interior ofScandinavia (Nesje et al., 2000; Nordli et al., 2005) where the climateregime is more continental with cold dry winters and precipitationmaximum during the summer.
Due to the maritime climate, mass balance budgets of cirque andplateau glaciers located in western Norway are mainly determinedby winter precipitation, which counterbalances the melting duringsummer. This dependency, and strategic location, has over the last20 years made the glaciers a target for systematic paleoclimatic studiesaimed at reconstructing solid winter precipitation. Consequently, aseries of glacier-basedwinter precipitation reconstructions are availablefrom a number of sites in western Norway. In the following discoursewe review how these, and the flood records (Fig. 1), have beenconstructed and then we proceed with analysis and comparison of thedifferent datasets. Characteristics of all lake sediment records are sum-marized in Table 1.
2.1. Precipitation data
Except for certain glacier types (Yde and Paasche, 2010; Post et al.,2011), glacier growth and shrinkage are generally the products of theaccumulation of solid precipitation during the winter season, and abla-tion bymelting during the summer season. Records of mountain glaciervariability, commonly expressed by fluctuations in the Equilibrium-Line-Altitude (ELA), therefore contain a potential for investigatingboth ablation-season temperature and winter precipitation into the
0 400Km
10 12 14
MERINGSDALSVATNETMERINGSDALSVATNET
BUTJØNNABUTJØNNA
JØKULENJØKULEN
BLEKTJBLEKTJÄRNENÄRNEN
dy. Winter precipitation (Pw) has been derived from four glacier sites in Norway and hu-constructions obtained from two lakes located in central Norway (Meringdalsvatnet and
Table 1Lake sediment records.
Record Coordinate Type Data Present glacier size (km2)/ELA (m asl)
Catchment area(km2)
Lake altitude(m asl)
Period(cal. yr BP)
Source
Spørteggbreen 61° 36′ N7° 3′ E
Proglacial lake ELA/Pw 28/1540 5 1045 c. 11000 Nesje et al. (2006)A. Nesje (pers. com.)
Hardangerjøkulen 60° 36′ N7° 25′ E
Proglacial lakes/peatbogs
ELA/Pw 73/1630 180 1270/1204/1124/1104
c. 10000 Dahl and Nesje (1996) Bjuneet al. (2005)
Jostedalsbreen/Flatebreen
61° 45′ N7° 2′ E
Proglacial lake ELA/Pw 1.5/1420 4 1001 c. 10600 Nesje et al. (2001) Bjune et al.(2005)
Folgefonna 60° 14′ N6° 28′ E
Proglacial lake ELA/Pw 23/1465 27 938 c. 11500 Bjune et al. (2005)Bakke et al. (2005)
Blektjärnen 62° 59′ N14° 39′ E
Non-glacial lake Isotopes/Humidity
– 0.5 330 c. 4400 Andersson et al. (2010)
Meringsdalsvatnet 61° 41′ N9° 11′ E
Non-glacial lake Floodfreq.
– 171 634 c. 10000 Støren et al. (2010)
Butjønna 62° 08′ N10° 10′ E
Non-glacial lake Floodfreq.
– 22 667 c. 9900 Bøe et al., 2006Støren et al., 2012
36 E.N. Støren, Ø. Paasche / Global and Planetary Change 113 (2014) 34–43
past. Several studies have attempted to quantify the relationshipbetween the two, and most demonstrate a non-linear relationship be-tweenmean ablation-season temperature (Ts) (1May to 30 September)and winter precipitation (Pw) (1 October to 30 April) at the ELA. Fromthis relationship, it follows that whenever the Pw or Ts is known closeto the ELA, the other may be calculated (Sissons, 1979; Leonard, 1984;Sutherland, 1984; Ballantyne, 1989; Ohmura et al., 1992; Lie et al.,2003). Studying modern Norwegian glaciers, Olav Liestøl (in Sissons,1979) and later Sutherland (1984) and Lie et al. (2003) demonstrateda robust exponential relationship between Pw and Ts:
Pw ¼ 0:915 e0:339Ts: ð1Þ
Continuous sedimentary successions in downstream glacier-fedlakes can record past glacier fluctuations over thousands of years(Karlén, 1976). Connecting historical (e.g. photos) and geomorphologi-cal (e.g. moraine ridges) evidence with downstream lake sedimentarchives allows for the construction of so-called ‘tie-points’ i.e., fixedaltitudes for ELAs in time that are regressed with correspondingphysical lake sediment parameters. Bakke et al. (2010) show, forexample, that the remnant magnetization values and bulk density oflake sediments are positively correlated with altitudinal changes inELA of the Okstandinan glacier in Norway, which in due turn opens fora continuous reconstruction of the ELA based on the regression equationin question.
By combining reconstructed ELA variability with independentrecords of Ts, Dahl and Nesje (1996) were the first to use the “Liestøl-equation” (Eq. (1)) to reconstruct continuous Holocene Pw variability.Although uncertainties do emerge at every step of theprocess, includingsedimentary interpretation, dating uncertainties, ELA estimation and Tsreconstruction before arriving at the final Pw reconstruction, this ap-proach was later successfully followed by numerous studies, in particu-lar in western Scandinavia (e.g. Nesje et al., 2001b; Bjune et al., 2005;Matthews et al., 2005). The main advantage of the approach is that itcan provide continuous high-resolution records covering thousands ofyears. The main limitation, besides those associated with interpreta-tions of the sediments forming the basis for the glacier reconstructions(e.g., Paasche et al., 2007), is that whenever glaciers are absent, the re-constructed Pw is solely determined by Ts.
The Holocene records of Pw included in this study are expressed in %of the 1961–1990 normal period and are based on pollen-derived Tsreconstructions and reconstructed ELA at Jostedalsbreen/Flatebreen(Nesje et al., 2001b; Bjune et al., 2005) (61° 45′N, 7° 2′E),Hardangerjøkulen (Dahl and Nesje, 1996; Bjune et al., 2005) (60° 36′N, 7° 25′ E), Spørteggbreen (Nesje et al., 2006; Nesje, 2012) (61° 36′N, 7° 3′ E) and Northern Folgefonna (Bakke et al., 2005; Bjune et al.,2005) (60° 14′ N, 6° 28′ E). These four glaciers are all located along
the maritime west coast of Norway, where the distribution of Pw ismainly dominated by the strength of the NAO (Nordli et al., 2005). Towiden the geographical area, and for the sake of comparison with amore continental climatic regime in central Scandinavia, one additionalrecord from central Sweden was included in the study. Since noHolocene glacier record exists from this part of Scandinavia, thereconstructed precipitation record is based on analysis of stableisotopes (δ18O and δ13C) of lacustrine carbonates (Chara spp. algae andPisidium spp. mollusks) from Lake Blektjärnen (62° 59′ N, 14° 39′ E) incentral Sweden (Andersson et al., 2010). The isotopic composition ofthis lake sediment record shows similar temporal trends in δ18O andδ13C and these reflect changes in the ratio between summer season evap-oration and precipitation. This is because increased evaporation andenhanced vapor exchange cause loss of light isotopes from the lakewater. Elevated values of δ18O and δ13C are thus related to periods withrelatively dry climate, whereas negative values are related to humid con-ditions (Talbot, 1990; Li and Ku, 1997).
Estimations of future Pw over Scandinavia are based on an ensembleof 22 dynamically downscaled Atmosphere–Ocean General CirculationModels (AOGCMs) and global atmosphere models (AGCMs), scaled toestimate changes over southern Norway for the periods AD2071–2100 and AD 2021–2050 relative to the meteorological referenceperiod (AD 1961–1990) (Hanssen-Bauer et al., 2009). The models useIPCC emission scenarios SRES A2, A1B, B2, and IS92a, and therefore rep-resent amean of awide range of emission scenarios (seeHanssen-Baueret al., 2009 for details on modeling and boundary conditions).
2.2. Flood data
Flood data included here have been obtained from lakes in non-glaciated catchments in mountainous southern Norway, where hun-dreds of snowmelt driven floods have been recorded (Bøe et al., 2006;Støren et al., 2010, 2012). Floods in these areas occur mainly due tospringtime melting of snow accumulated in high-altitude reservoirsduring the winter. This enhances the competence and capacity of theriver, which in turn enables erosion and subsequent transport of rela-tively coarse-grained, non-organic flood-sediments. Upon enteringdownstream lakes, sediments are deposited due to the sudden loss ofcarrying capacity. The material freighted with the volatile rivers duringspring is significantly different from the background sediment produc-tion in the respective lakes; these typically are organic-rich and consistof phytoplankton and plant remains. By analyzing variations in sedi-ment composition, using a suite of different physical sediment parame-ters and quantitativemethods, identification of individualflood layers ispossible, as is construction of time series of flood frequencies over thou-sands of years (see Støren et al., 2010 for methodology).
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Records of flood frequencies used here are expressed as interarrivaltimes and originate from lake sediment cores from Meringsdalsvatnet(61° 41′ N, 9° 11′ E, 630 m asl) (Støren et al., 2010) and Butjønna(62° 08′ N, 10° 10′ E, 667 m asl) (Bøe et al., 2006; Støren et al., 2012) insouthern Norway (Fig. 1). The high-resolution flood records from thesetwo lakes have been obtained using the same methodology, originatefrom similar catchments and are argued to be inter-comparable onmulti-decadal to centennial scale (Støren et al., 2012). The records spanclose to 10 000 years (Table 1), and reveal around 100 flood events ineach of the lakes (n = 92 in Meringsdalsvatnet and n = 112 inButjønna).
3. Results and discussion
3.1. Long-term variability
Histograms of available Holocene records of Pw expressed as a per-centage of the present (AD 1961–1990) reveal normal distributions(Fig. 2). Moreover, all records have mean values that are just slightlyabove present values (100%), indicating that the present conditionsare close to Holocene average. Typical maximum values are around160–180%, whereasminimumvalues are ca. 60% of present (Fig. 2). His-tograms of the river flood data indicate lognormal distributions, whichare often also found in instrumental records (e.g. Arnell, 2002) whereshort interarrival times are exponentially more frequent than longerinterarrival times (Fig. 3). The median interarrival time over the last100 years is 43 years in the Meringsdalsvatnet record and 34 years in
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Pw (% of 1961-1990)
E
A
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Fig. 2. Histograms with fitted normal distribution curves of the Holocene (10 000 years) winte100% for each site. (A) Spørteggbreen, (B) Hardangerjøkulen, (C) Jostedalsbreen, (D) Folgefonnreferences). (For interpretation of the references to color in this figure legend, the reader is ref
the Butjønna record, and are thus similar to the Holocene medianinterarrival times of 40.1 years in the Meringsdalsvatnet record(Fig. 3A) and 30.1 years in the Butjønna record (Fig. 3).
During the early-/mid-Holocene the reconstructed Norwegian gla-ciers were small or possibly absent until the start of the Neoglacialglacier advance between 6000 and 4000 years ago (Nesje, 2009), coin-ciding with a general increase in flood frequency in Southern Norway(Vasskog et al., 2011). In order to minimize the possible errors associat-edwith small or absent glaciers aswell as low flood frequencies forminga sparse pool of earlyHolocenedata the following comparison of Pwandflood frequencies are based on the timeperiod ranging from6000 cal. yrBP and up to present day. By averaging the datasets on centennial timescale for the correlation analysis, the uncertainties attached to the prox-ies themselves are reduced as well as the accompanying dating uncer-tainties. Also, the fact that snow reservoirs in these high-altitude areasvary in size over multi-decadal time scales suggests that comparisonof results on centennial time scales, rather than annual or decadal, aremore likely to reveal any potential trends.
Cross correlation coefficients (Pearson r and Spearman rho) of Pwand flood frequency records over the period are listed in Table 2. Corre-lating point-to-point, we find no significant relationship between Pwand either of the flood frequency records. In contrast, when using an av-erage Pw from the four glacier records (Fig. 2E) and comparing withflood data from Meringsdalsvatnet (Fig. 3A), we find that the generallong-term trends co-vary in concert over the last 6000 years (Fig. 4A).Using a 500-year moving average we observe a significant positive cor-relation and an explained variance of R2 = 0.41 (p b 0.01) between
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Pw (% of 1961-1990)
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r precipitation (Pw) expressed in % of the 1961–1990 reference period. Red lines indicatea, and (E) average of records A–D over the last 6000 years (see main text and Table 1 forerred to the web version of this article.)
10001001010.0
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Fig. 3.Histogramswith fitted normal distribution curve of theHoloceneflood interarrival times reconstructed fromMeringsdalsvatnet (A) and Butjønna (B) (seemain text and Table 1 forreferences). Red lines indicate median interarrival times over the last 100 years. Note the logarithmic x-axis indicating a lognormal distribution. (For interpretation of the references tocolor in this figure legend, the reader is referred to the web version of this article.)
38 E.N. Støren, Ø. Paasche / Global and Planetary Change 113 (2014) 34–43
mean annual interarrival time and Pw (Fig. 5). This non-linear relation(note log axis in Fig. 5) indicates a general long-term tendency towardshigher flood frequency during snow rich winters.
Comparing the average glacier-based Pw record with flood datafrom Butjønna we find no relationship on either short or long timescale(Table 2). This prompts questions about the regional variability of bothprecipitation and flood frequency. To test this the eastern flood record(Butjønna) was compared to the paleorecord from central Sweden,representing a continental central Scandinavian climate. This area hasno glaciers, and a humidity record derived from stable isotopes(Andersson et al., 2010) has been used. Cross correlation between the500-year moving average flood interarrival times from Butjønna andthe humidity record from central Sweden over the last 4400 years(Figs. 4B and 5B) reveal a non-linear relation and a significant positivecorrelation (R2 = 0.41, p b 0.01). The humidity record from Swedenthus has a similar explanatory power on flood frequency in Butjønnaas glacier derived Pw from western Norway has on flood frequency inMeringsdalsvatnet, whereas no significant correlations were foundbetween the humidity record from Sweden and the flood record fromMeringdalsvatnet or Pw records from western Norway (Table 2). Thisindicates that the relation between Pw and flood frequency is robustfor the continental central Scandinavia aswell for themoremaritime in-fluenced southern Norway.
Table 2Pearson r and Spearman rho correlation coefficients of Pw records andflood interarrival times. S(two tailed).
Pearson r/Spearman rho
Jostedals-breen Pw
Hardanger-jøkulen Pw
SpørteggbreenPw
FolgefonPw
Jostedalsbreen Pw 1/1Hardangerjøkulen Pw .67*/.68* 1/1Spørteggbreen Pw .85*/.80* .76*/.80* 1/1Folgefonna Pw .35*/.41* .44*/.50* .49*/.54* 1/Mean average Pw(500 yr mov. avg)
.72*/.71* .61*/.65* .67*/.70* .62*/.
Flood interarrival timeMeringsdalsvatnet
− .28**/− .36* − .17/− .30** − .20/− .30** − .30**/−
Flood interarrival time Butjønna − .12/.− .13 .16/.−.15 − .09/− .06 −.04/.500 yr mean int.arr. timeMeringsdals-vatnet (log)
− .52*/−47* − .39*/− .38* − .38*/− .37* − .32**/−
500 yr mean int.arr. timeButjønna (log)
− .11/− .14 − .16/− .18 − .05/− .07 .02/−
Blektjärnen δ18O(500 yr mov. avg)
− .04/− .08 .06/.09 .09/.08 .14/
3.2. Future in context of the past
Partly because of greater predicted warming, but also because ofhigh influence of snowmelt in the water balance, the largest changesin the future hydrological cycle are predicted for areas withsnowmelt-dominated discharge at mid- to higher latitudes (Nijssenet al., 2001). The observed natural centennial scale variability of Pw inSouthern Norway over the Holocene period has a mean value close topresent conditions and a variability that exceeds the instrumental re-cord by roughly 50% (Fig. 2E). This is slightly higher than the estimatesof projected winter precipitation increase over the coming century(Hanssen-Bauer et al., 2009) and suggests that the Holocene Pw andflood frequency records can be relevant when it comes to assessingthe full range of precipitation changes that we have in store towardsthe end of the century (cf. Trenberth, 2011).
Instrumental and historical data (e.g. IPCC, 2012) show that humanactivities such as land use and deforestation can potentially impactflood frequencies, and it is also documented that anomalous summerrainstorms occasionally have caused major floods during the historicalrecord (Roald, 2002). The fact that non-related processes can impactflood frequencies can be viewed as noise in the records presentedhere since there currently do not exists a method that enables the po-tential discrimination between material deposited by floods triggered
ignificant correlations aremarkedwith * and ** indicating significance level at 0.01 and0.05
na Mean Pw(500 yr mov.average)
Floodinter-arrivaltimeMerings-dalsvatnet
Floodinter-arrivaltimeButjønna
500 yr meanint.arr. timeMeringsdals-vatnet (log)
500 yrmeanint.arr.timeButjønna(log)
Blektjärnenδ18O(500 yrmov. avg)
167* 1/1
.34* − .36*/− .47* 1/1
02 − .27/− .31** .64*/.68* 1/1.35* − .64*/− .60* .84*/.90* .63*/.64* 1/1
.03 − .17/− .26 .62*/.63* .64*/.74* .63*/.65* 1/1
.11 .04/−.02 .13/.08 .31**/.40* .09/.08 .64*/57* 1/1
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Fig. 4. A.Winter precipitation (Pw) as percent of 1961–1990 based on glacier reconstructions from Hardangerjøkulen, Jostedalsbreen, Folgefonna, and Spørteggbreen (seemain text andTable 1 for references). The solid black line is a 500-year moving average of all sites. Flood data from Meringsdalsvatnet are plotted as mean interarrival time per 100 years (gray bars)smoothed by a 500-year moving average (solid blue line). B. Chara δ18O (higher values indicating humid conditions) from Blektjärnen (Andersson et al., 2010), smoothed by a 500-yearmoving average. Flood data fromButjønna are plotted asmean interarrival time per 100 years (gray bars), smoothed by a 500-yearmoving average (solid blue line). (For interpretation ofthe references to color in this figure legend, the reader is referred to the web version of this article.)
39E.N. Støren, Ø. Paasche / Global and Planetary Change 113 (2014) 34–43
e.g. solely by rain storms versus snowmelting. Taking non-climatic fac-tors such as changes in drainage patterns due to land-use changes orisostatic rebound into consideration, it is no reason to expect a 1:1 rela-tionship between Pw and the reconstructed flood frequency. Having
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Pw (% of 1961-1990)
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Fig. 5. 500 yearmean flood interarrival time as recorded inMeringsdalsvatnet (A) and Butjønnasouthern Norway (error bars indicate 10th and 90th percentile of range) (A) and humidity (Chumidity values. Regression lines with 90% confidence intervals indicate positive correlations (
said that, instrumental data show that discharge and flood frequencyin eastern, northern and the mountainous central parts of Scandinaviaat present primarily are driven by snowmelt (Gottschalk et al., 1979;Wilson et al., 2010), which in turn is largely determined by the
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(B), plotted against 500 years running averagewinter precipitation (Pw) from four sites inhara δ18O) from central Sweden (B). Dotted lines across the charts indicate present Pw/R2 = 0.41, p b 0.01) between flood interarrival times and Pw/humidity for both regions.
40 E.N. Støren, Ø. Paasche / Global and Planetary Change 113 (2014) 34–43
distribution and amount of solid Pw (Shorthouse and Arnell, 1999;Tveito and Roald, 2005). An increased probability of flooding as a re-sponse to increased amount of snow is therefore to be anticipated andhence a plausible explanation for the correlation between reconstructedPw and flood frequency in Meringsdalsvatnet over the Holocene(Fig. 5).
During the Neoglacial period (b4000 years), and especially betweenca. 2000 and 1000 cal. yr BP, the Pw and flood frequency were signifi-cantly higher than at present (Fig. 4A). The reconstructed average Pwreached a Holocene maximum of 155% at 1000 cal. yr BP, with a meanvalue of 129% over the millennium (2000–1000 cal. yr BP). The floodrecord from Meringsdalsvatnet shows a dramatic increase during thesame period, indicating a non-linear response in flood frequency tothe increase in Pw (Figs. 4A and 5) on a centennial scale, the Holocenemaximum in flood frequency is recorded at 1400 cal. yr BP, when themean interarrival time was 9 years in the Meringsdalsvatnet record(Fig. 4A).
Fig. 6 shows cumulative distribution functions of theHolocene Pw asreconstructed from the glaciers Hardangerjøkulen, Spørteggbreen,Jostedalsbreen, and Folgefonna, as well as an average of these (Fig. 2),showing the full spectrum of the Holocene Pw expressed as percent ofthe present (AD 1961–1990). This long term natural variability is com-pared with instrumentally observed values and future estimates byplotting the Pw records from the period AD 1861–2010 from Bergen,western Norway (http://eklima.no) as well as the modeled future Pwfor next 50 and 100 years (Hanssen-Bauer et al., 2009) over the sameregion against the average cumulative distribution (all values are in %of AD 1961–1990) (Fig. 6). Projections for the Pw in Norway indicatean average increase of 11.1% for the period AD 2021–2050, and 21.4%for the period AD 2071–2100, with a “high-estimate” increase of 45%(Hanssen-Bauer et al., 2009). This increase of 45% exceeds the 97th per-centile of the reconstructed Holocene Pw (Fig. 6) and suggests that thepredicted Pw for the coming century is in the range of themost extremevalues observed for the last 10 000 years.
Recent modeling of regional precipitation changes performed byMahlstein et al. (2012) shows that a 1.4 °C warming is required inorder to get statistically significant results with respect to correspond-ing precipitation, which might suggest that the upper end of theprojected Pw is on the high side. Nevertheless, over the last 30-year
192
1861-18901951-19
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Fig. 6. Instrumental (black dots) and future (open circles) Pw values plotted on cumulative de(DJF) over consecutive 30-year periods AD 1861–2010 from Bergen, western Norway (http2071–2100 with bars indicating low (10th percentile) and high (90th percentile) estimates (Spørteggbreen, Jostedalsbreen, Folgefonna, and an average of these (see references in themain(cf. Fig. 2).
period (AD 1981–2010), the average Pw in Bergen Western Norwayreached 128% of the normal period (AD 1961–1990), which is alreadyat the 85th percentile of the average Holocene Pw (Fig. 6). The amountof Pwwill in all likelihood increase in the near future and following thediscussion above and analogous to the period at 2000–1000 cal. yr BP,this will cause a relative non-linear increase in flood frequency (i.e., adecrease in interarrival time).
There are, however, several complicating factors that maymodify orcounterbalance this response, and above all the increasing temperature.In maritime regions such as Scandinavia the snow cover, and thus alsothe snow melt induced floods, is highly sensitive to changing tempera-tures (Brown and Mote, 2009). Winter temperatures in the NorthernHemisphere are predicted to increase during the coming century(IPCC, 2007), and observed andmodeled trends over the last decades al-ready indicate a general reduction in snow cover (Brown and Mote,2009; Derksen and Brown, 2012). An earlier onset of spring over thelast 50 years has extended the liquid-precipitation (rain) season by upto three weeks in some regions of the boreal high latitudes (Trenberthet al., 2007), causing a relatively larger degree of the Pw to fall as rainrather than snow.
In mountainous catchments the hypsometry is typically skewed to-wards larger areas at low elevation, and a temperature induced increasein the elevation of the snowline is thus likely to cause a non-lineardecrease in snow cover. Fig. 7 shows the cumulative hypsometriccurve of the Meringsdalsvatnet catchment (red line), and how thetotal amount of received Pw change with altitude in the catchment(blue lines). The total amount of Pw (expressed in m3) at present(AD 1961–1990) and for the next century (AD 2071–2010) is calculatedby multiplying the area at different altitudes with estimated Pw(using 8%/100 m gradient) at corresponding altitudes (blue line). Ap-proximately 80% of the Meringsdalsvatnet catchment lies below1200 mand although the amount of precipitation increasewith altitudethe total received precipitation reaches a maximum at an altitudearound 1000 m. Consequently, the predicted increase in Pw of 21.4%over the next century (Hanssen-Bauer et al., 2009)will bemost effectiveat around 1000 m and less effective at high altitude simply because theprecipitation has less area to fall on. This shows that the distribution ofarea at different elevations is decisive for the potential snow accumula-tion. Moreover, the area that receives the most precipitation is also the
2021-2050
1891-1920
1-1950
80
1981-20102071-2100
110 120 130 140 150 160 170 180
of 1961-1990)
Future Pw
Instrumental Pw
nsity functions of the Holocene Pw (lines) from Norway. Instrumental data are mean Pw://eklima.no). Estimates for future Pw are ensemble means of AD 2021–2050 and AD
Hanssen-Bauer et al., 2009). The Holocene Pw are reconstructed from Hardangerjøkulen,text and Table 1) and plotted as cumulative density functions of fitted normal distributions
May snowline at present
0 20 40 60 80 100
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May snowline 2100 (+3.4o C)
Fig. 7. Cumulative hypsometric curve of the Meringsdalsvatnet catchment (red line), andtotal received Pw (m3) at different altitudes in the catchment estimated for AD1961–1990(dark blue line) and AD 2071–2010 (light blue line). Dotted horizontal lines shows Maysnowline at present (median of AD 1971–2010) and AD 2071–2100 (estimatedusing + 3.4 °C, and 0.6 °C−100m lapse rate). (For interpretation of the references tocolor in this figure legend, the reader is referred to the web version of this article.)
41E.N. Støren, Ø. Paasche / Global and Planetary Change 113 (2014) 34–43
area that ismost susceptible to changes in temperature. Increasing tem-peratureswill push the 0 °C isothermupward,which is bound to impactthe spatiotemporal distribution of the corresponding snow cover.
The annual precipitation in the area (met. station Skåbu, stnr.13670,890 m asl) reaches amaximum in July (74 mm) and aminimum in Feb-ruary (21 mm), and has a marked increase in precipitation over thespring season (22 mm in April, 44 mm in May) and decrease duringfall (53 mm in Okt., 45 mm in Nov.) The implications of this is that in-creasing temperatures over the next century, shifting both the startand end of the snow cover season towards the winter months, willsignificantly reduce the area above the snowline during the high-precipitation months May and October. The median May snowline,which at present (AD 1970–2010) lies at c. 900 m, will with a tempera-ture increase of 3.4 °C over the next century (AD 2070–2010, Hanssen-Bauer et al., 2009) lie at c. 1500 m (using a lapse rate of 0.6 °C−100m),reducing the area suitable for snow accumulation to 2% of the total(Fig. 7).
Flood records used in this study, aswell as other records from South-ern Norway (Vasskog et al., 2011) and studies from the European Alps(e.g. Swierczynski et al., 2013; Vannière et al., 2013) show significantlylonger interarrival times during the relatively warm early and mid-Holocene (Fig. 4). During the Holocene thermal Maximum (HTM)(c. 8000–6000 cal. yr BP) much of Europe experienced mean annualtemperatures similar to those projected for the next century. Recon-structions of mean annual temperatures in Southern Scandinaviaindicate approximately 2.0–2.5 °C higher temperatures than at presentduring the HTM (Seppä et al., 2009), and future mean annualtemperatures for Norway are estimated to increase by 1.9 °C and3.4 °C for the periods AD 2021–2050 and AD 2071–2100, respectively(Hanssen-Bauer et al., 2009). Although theHTMdiffers from thepresentand predicted future warming by being primarily warmer during thesummer season (e.g. Davis et al., 2003), the mid-Holocene warming
may still serve as an analog for the predicted temperature increase inthe region (cf., Renssen et al., 2012). The average flood interarrivaltime in the Meringsdalsvatnet flood record was 1400 years over theperiod 7000–6000 cal. yr BP, compared to 33 years over the period2000–1000 cal. yr BP. Mechanisms causing this change may includetemperature driven changes in the snow line, but also changes in vege-tation cover and ground frost during the HTM. Pollen reconstructionsshow that the early Holocene warming enabled the formation ofdense pine forest (Pinus sylvestris) up to an altitude of c. 1200 m inthe region (Barnett et al., 2001). Such an increase, from the presenttree line at c. 800 m, would cause an increase in the forest-covered areain the Meringsdalsvatnet catchment from c. 15% to 80% (cf. hypsometryin Fig. 7). Similarly the HTM-warming shifted the permafrost limitin Southern Norway above c. 1600–1700 m (Lilleøren et al., 2012), i.e.above the entire Meringsdalsvatnet catchment. In sum this may have in-creased the amount of water stored and buffered in the soil and vegeta-tion and this dampened runoff peaks during the HTM.
Hence, applying the mid-Holocene as an analog for future flood fre-quency indicates that increasing temperatures are likely to counterbal-ance the effect of increased Pw, and cause a reduction in snowmelttriggered floods. This is in agreement with modeled snow cover in themountains of Southern Norway predicting little or no change for theAD 2071–2100 period compared to the present, despite an increase inPw (Schuler et al., 2006). In the Alpes, the amount of Pw at elevationslower than c. 1500–2000 m is already insufficient to compensate forthe observed rise in winter temperatures over the past century(Beniston, 2012). Such a scenario alsofitswellwith a predicted decreasein flood magnitude in the snowmelt dominated areas as well as an shiftin the timingoffloods fromspring to fall due to a change from snowmeltto rainfall as the main flood-inducing mechanism in Scandinavia(Bergström et al., 2001; Andreasson et al., 2004; Wilson et al., 2010;Lawrece and Hisdal, 2011).
Holocene data reveals that in addition to catchment changes, thespatial distribution as well as the amount of Pw needs to be factoredin if changes inflood frequency in Scandinavia are to be explained. Shiftsin atmospheric circulation patterns are suggested to impact floodfrequency in Europe and Scandinavia alike, and to have caused non-synchronous variability over relatively short distances. Glur et al.(2013) recently showed, based on paleorecords from ten lakes in theSwiss Alps, that flood frequency over the last 2500 years has beenmod-ulated by the strength and location of the Azores high-pressure systemcausing shifts in thewesterly storm tracks. Similarly, Benito et al. (2008)deduced from 13 sites across Spain (a distance of ca. 700 km) that long-term changes in flood frequency are linked to the NAO where fewerfloods correspond to NAO+and vice versa. A close connection betweenprevailing changes in precipitation patterns over both Spain and theFrench Alps is shown to explain non-synchronous changes in floodfrequency in the region over the last 1400 years (Wilhelm et al.,2012).Whereas flood frequencies in the southern French Alps and east-ern Spain (c. 700 km apart) both increase during AD 1600–1660,1750–1900 and 1950–2000 corresponding to periods of southwesterlywinds and prevailing NAO-, they are inversely correlated to flood fre-quency in the Central Massif of southern France (c. 200 km from theFrench Alps) that are triggered during south-easterly winds. Suchconditions were prevailing over the periods AD 1680–1750 and AD1900–1950 (Wilhelm et al., 2012).
In southern Norway, significant differences in flood frequencybetween the two sites Meringsdalsvatnet and Butjønna (Fig. 1) areattributed to shifting atmospheric circulation patterns, where a suggestedre-routing of themainmoisture transport from the North-Atlantic causesanomalous high Pw at Meringsdalsvatnet when southerly windsdominate, whereas Butjønna receives more Pw during northerly winds(Støren et al., 2012). The Pw records presented in Fig. 2 are basedpredominantly on maritime glaciers (Fig. 1) having mass balances thatare highly influenced by the southwesterly storm tracks (Nordli et al.,2005). It is therefore suggested that the western flood record
42 E.N. Støren, Ø. Paasche / Global and Planetary Change 113 (2014) 34–43
(Meringsdalsvatnet) is influenced by south-southwesterly winds thatalso bring Pw to the glaciers, whereas Butjønna, receiving Pw primarilyduring northerly winds, shows no correlation (Table 2).
The paleoevidence is supported by documentary sources showingthat shifting winter season atmospheric circulation patterns have mod-ulated the flood frequency in Central Europe on multi-decadal to cen-tennial timescales since AD 1500 (Jacobeit et al., 2003a). Hence,future dominating atmospheric circulation patterns, whatever theyare, will likely play an important role in distributing the Pw and willtherefore also impact the regional flood frequency. Presently, theNAO+ is the most frequently occurring North Atlantic wintertimeweather regime (e.g. Cassou, 2008), and several models agree on an in-tensification of the NAO+ in response to future warming (Gillett et al.,2002; Bader et al., 2011). The relative occurrence rates of differentmodes of North Atlantic atmospheric circulation are, however, not nec-essarily constant (Jacobeit et al., 2003b) and predictions of futurechanges in Pw and runoff in Scandinavia, obtained using differentmodels and emission scenarios, reveal large regional differences andmodel uncertainties regarding the future dominating atmosphericcirculation patterns (Andreasson et al., 2004; Hanssen-Bauer et al.,2009). By increasing the number of high-resolution flood frequency re-constructions from areas that, based on model projections, are likely toexperiencemore floods in future, improved assessmentmight be possi-ble regarding the spectrum of natural climate variability with respect tothe climate evolution of the last 10 000 years.
4. Conclusions
In this paper, we have examined the potential long-term relation-ship between the winter precipitation and flood frequency in SouthernScandinavia over the last 6000 years. Applying Holocene variability asanalogs for future change the main results are:
• A positive non-linear correlation is observed between the amount ofwinter precipitation and interarrival times of floods on centennialtimescales where winter precipitation explains around 40% of chang-es in the flood frequency. This tends to imply that the number offloods will increase in response to increased winter precipitation inthe time to come. Increasing temperatures towards the end of thecoming century may, however, counterbalance the effect of increasedprecipitation and cause a reduction in snowmelt-triggered floods.
• Poor correlations are evident at shorter timescales, which suggeststhat other factors such as human activities, the number of summerrainstorms and possibly various dynamic catchment responses havesome explanatory power. This highlights the complexity of climate-floodmechanisms and emphasizes the need for long-term (paleo) re-cords for assessing the climate-driven impact on flood frequencies.
• The spectrum of natural variability in winter precipitation exceedsthat of the instrumental record by approximately 50%. This is in theupper endof regional climate-model estimates ofwhat is expected to-wards the end of the century, and suggests that the Holocene winterprecipitation and flood frequency records are still relevant when itcomes to assessing future changes.
• Based on analysis of available flood and precipitation records, we sug-gest that anthropogenic forced climate changewill evolve towards thefull spectrum of the Holocene variability sometime during the comingcentury. For predictions exceeding the AD 2071–2100 estimates, theHolocene data may therefore no longer serve as suitable analogs.
Acknowledgment
We wish to thank Kerim Hestnes Nisacigolu for valuable commentson the manuscript, and Atle Nesje for making the precipitation recon-struction from Spørteggbreen available for the authors. The paper alsobenefited from the comments from two anonymous reviewers for
which we are grateful. This is publication no. A439 from the BjerknesCentre for Climate Research.
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