IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Chapter 3Changes in Climate Extremes and their Impacts on the Natural Physical Environmenta rate close to that of the upper range of the AR4 projections. Whetherthe faster rate of increase during the latter period reflects decadalvariability or an increase in the longer-term trend is not clear. However,there is evidence that the contribution to sea level due to mass lossfrom Greenland and Antarctica is accelerating (Velicogna, 2009; Rignotet al., 2011; Sørensen et al., 2011). The AR4 also reported that the risein mean sea level and variations in regional climate led to a likelyincrease in the trend of extreme high water worldwide in the late 20thcentury (Bindoff et al., 2007), it was very likely that humans contributedto sea level rise during the latter half of the 20th century (Hegerl et al.,2007), and therefore that it was more likely than not that humanscontributed to the trend in extreme high sea levels (IPCC, 2007a). Sincethe AR4, Menendez and Woodworth (2010), using data from 258 tidegauges across the globe, have confirmed the earlier conclusions ofWoodworth and Blackman (2004) that there was an increasing trend inextreme sea levels globally, more pronounced since the 1970s, and thatthis trend was consistent with trends in mean sea level (see also Loweet al., 2010). Additional studies at particular locations support this finding(e.g., Marcos et al., 2009; Haigh et al., 2010).Various studies also highlight the additional influence of climatevariability on extreme sea level trends. Menendez and Woodworth(2010) report that ENSO (see Section 3.4.2) has a large influence oninterannual variations in extreme sea levels in the Pacific Ocean andthe monsoon regions based on sea level records since the 1970s. Insouthern Europe, Marcos et al. (2009) report that changes in extremesare also significantly negatively correlated with the NAO (see Section3.4.3). Ullmann et al. (2007) concluded that maximum annual sea levelsin the Camargue had risen twice as fast as mean sea level during the20th century due to an increase in southerly winds associated with ageneral rise in sea level pressure over central Europe (Ullmann et al.,2008). Sea level trends from two tide gauges on the north coast ofBritish Columbia from 1939 to 2003 were twice that of mean sea levelrise, the additional contribution being due to the strong positive phaseof the PDO (see Section 3.4.3), which has lasted since the mid-1970s(Abeysirigunawardena and Walker, 2008). Cayan et al. (2008) reportedan increase of 20-fold at San Francisco since 1915 and 30-fold at La Jollasince 1933 in the frequency of exceedance of the 99.99th percentile sealevel. They also noted that positive sea level anomalies of 10 to 20 cmthat often persisted for several months during El Niño events producedan increase in storm surge peaks over this time. The spatial extent ofthese oscillations and their influence on extreme sea levels across thePacific has been discussed by Merrifield et al. (2007). Church et al.(2006a) examined changes in extreme sea levels before and after 1950in two tide gauge records of approximately 100 years on the east andwest coasts of Australia, respectively. At both locations a strongerpositive trend was found in the sea level exceeded by 0.01% of theobservations than the median sea level, suggesting that in addition tomean sea level rise, other modes of variability or climate change arecontributing to the extremes. At Mar del Plata, Argentina, Fiore et al.(2009) noted an increase in the number and duration of positive stormsurges in the decade 1996 to 2005 compared to previous decades,which may be due to a combination of mean sea level rise and changesin wind climatology resulting from a southward shift in the SouthAtlantic high.Thus, studies since the AR4 conclude that trends in extreme sea level aregenerally consistent with changes in mean sea level (e.g., Marcos et al.,2009; Haigh et al., 2010; Menendez and Woodworth, 2010) althoughsome studies note that the trends in extremes are larger than theobserved trend in mean sea levels (e.g., Church et al., 2006a; Ullmann etal., 2007; Abeysirigunawardena and Walker, 2008) and may be influencedby modes of climate variability, such as the PDO on the Canadian westcoast (e.g., Abeysirigunawardena and Walker, 2008). These studies areconsistent with the conclusions from the AR4 that increases in extremesare related to trends in mean sea level and modes of variability in theregional climate.The AR4 (Meehl et al., 2007b) projected sea level rise for 2090-2099relative to 1980-1999 due to ocean thermal expansion, glaciers andice caps, and modeled ice sheet contributions of 18 to 59 cm, whichincorporates a 90% uncertainty range across all scenarios. An additionalcontribution to the sea level rise projections was taken into account fora possible rapid dynamic response of the Greenland and West Antarcticice sheets, which could result in an accelerating contribution to sea levelrise. This was estimated to be 10 to 20 cm of sea level rise by 2090-2099using a simple linear relationship with projected temperature. Becauseof insufficient understanding of the dynamic response of ice sheets,Meehl et al. (2007b) also noted that a larger contribution could not beruled out.Several studies since the AR4 have developed statistical models thatrelate 20th-century (e.g., Rahmstorf, 2007; Horton et al., 2008) or longer(e.g.,Vermeer and Rahmstorf, 2009; Grinsted et al., 2010) temperatureand sea level rise to extrapolate future global mean sea level. Thesealternative approaches yield projections of sea level rise under a rangeof SRES scenarios by 2100 of 0.47 to 1.00 m (B1 to A2 scenarios; Hortonet al., 2008), 0.50 to 1.40 m (B1 to A1FI scenarios; Rahmstorf, 2007),0.75 to 1.90 m (B1 to A1FI scenarios; Vermeer and Rahmstorf, 2009),and 0.90 to 1.30 m (A1B scenario only; Grinsted et al., 2010). However,future rates of sea level rise may be less closely associated with globalmean temperature if ice sheet dynamics play a larger role in the future(Cazenave and Llovel, 2010). Furthermore, Church et al. (2011) note thatthese models may overestimate future sea levels because non-climaterelated contributions to trends over the observational period such asgroundwater depletion may not have been removed, and non-lineareffects such as the reduction in glacier area as glaciers contract and thereduction in the efficiency of ocean heat uptake with global warming inthe future are not accounted for. Pfeffer et al. (2008), using a dynamicalmodel of glaciers, found that sea level rise of more than 2 m by 2100 isphysically implausible. An estimate of 0.8 m by 2100 that includedincreased ice dynamics was considered most plausible.New studies, whose focus is on quantifying the effect of storminesschanges on storm surge, have been carried out over northern Europesince the AR4. Debernard and Roed (2008) used hydrodynamic models179