IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentChapter 3rising air and rock temperatures (Gruber and Haeberli, 2007) or duringor following brief, anomalously warm events (Huggel et al., 2010) dueto meltwater infiltration and shear strength reduction. Debuttressingeffects due to glacier retreat can also destabilize or over-steepen slopes(Tuffen, 2010). Furthermore, on volcanoes, geothermal heat flow canenhance ice melting and thus create weak zones at the ice-bedrockinterface; and hydrothermal alteration of rocks can decrease the slopestability (Huggel, 2009). On unglaciated high volcanoes in the Caribbean,Central America, Europe, Indonesia, the Philippines, and Japan, anincrease in total rainfall or an increase in the frequency or magnitude ofsevere rainstorms (see Section 3.3.2) could cause more frequent debrisflows by mobilizing unconsolidated, volcanic regolith and by raising porewaterpressures, which could lead to deep-seated slope failure. Heavyrainfall events could also influence the behavior of active volcanoes. Forexample, Mastin (1994) attributes the violent venting of volcanic gasesat Mount St. Helens between 1989 and 1991 to slope instability oraccelerated growth of cooling fractures within the lava dome followingrainstorms, and Matthews et al. (2002) link episodes of intense tropicalrainfall with collapses of the Soufriere Hills lava dome on Montserrat inthe Caribbean. It is well established that ice mass wastage following theend of the last glaciations led to increased levels of seismicity associatedwith post-glacial rebound of the lithosphere (e.g., Muir-Wood, 2000;Stewart et al., 2000). There has been a large reduction in glacier coverin southern Alaska. Sauber and Molnia (2004) reported several hundredmeters vertical reduction. This ice reduction may be responsible for anincrease in seismicity in the region where earthquake faults are at thethreshold of failure (Sauber and Molnia, 2004; Doser et al., 2007). Anincrease in the frequency of small earthquakes in the Icy Bay area, alsoin southeast Alaska, is interpreted to be a crustal response to glacierwastage between 2002 and 2006 (Sauber and Ruppert, 2008). Largescaleice mass loss in glaciated volcanic terrain reduces the load on thecrust and uppermost mantle, facilitating magma formation and itsascent into the crust (Jull and McKenzie, 1996) and allowing magma toreach the surface more easily (Sigmundsson et al., 2010). At the end ofthe last glaciation, this mechanism resulted in a more than 10-foldincrease in the frequency of volcanic eruptions in Iceland (Sinton et al.,2005).The AR4 projected that glaciers in mountains will lose additional massover this century because more ice will be lost due to summer meltingthan is replenished by winter precipitation (Meehl et al., 2007b). Thetotal area of glaciers in the European Alps may decrease by 20 to morethan 50% by 2050 (Zemp et al., 2006; Huss et al., 2008). The projectedglacier retreat in the 21st century may form new potentially unstablelakes. Probable sites of new lakes have been identified for some alpineglaciers (Frey et al., 2010). Rock slope and moraine failures may triggerdamaging surge waves and outburst floods from these lakes. Thetemperature rise also will result in gradual degradation of mountainpermafrost (Haeberli and Burn, 2002; Harris et al., 2009). The zone of warmpermafrost (mean annual rock temperature approximately -2 to 0°C),which is more susceptible to slope failures than cold permafrost, mayrise in elevation a few hundred meters during the next 100 years(Noetzli and Gruber, 2009). This in turn may shift the zone of enhancedinstability and landslide initiation toward higher-elevation slopes that inmany regions are steeper, and therefore predisposed to failure. Theresponse of bedrock temperatures to surface warming through thermalconduction will be slow, but warming will eventually penetrate toconsiderable depths in steep rock slopes (Noetzli et al., 2007). Other heattransport processes such as advection, however, may induce warming ofbedrock at much faster rates (Gruber and Haeberli, 2007). The responseof firn and ice temperatures to an increase in air temperature is fasterand nonlinear (Haeberli and Funk, 1991; Suter et al., 2001; Vincent et al.,2007). Latent heat effects from refreezing meltwater can amplify theincrease in air temperature in firn and ice (Huggel, 2009; Hoelzle et al.,2010). At higher temperatures, more ice melts and the strength of theremaining ice is lower; as a result, the frequency and perhaps size of iceavalanches may increase (Huggel et al., 2004; Caplan-Auerbach andHuggel, 2007). Warm extremes can trigger large rock and ice avalanches(Huggel et al., 2010).Current low levels of seismicity in Antarctica and Greenland may be aconsequence of ice-sheet loading, and isostatic rebound associatedwith accelerated deglaciation of these regions may result in an increasein earthquake activity, perhaps on time scales as short as 10 to 100years (Turpeinen et al., 2008; Hampel et al., 2010). Future ice mass losson glaciated volcanoes, notably in Iceland, Alaska, Kamchatka, theCascade Range in the northwest United States, and the Andes, couldlead to eruptions, either as a consequence of reduced load pressures onmagma chambers or through increased magma-water interaction.Reduced ice load arising from future thinning of Iceland’s VatnajökullIce Cap is projected to result in an additional 1.4 km 3 of magmaproduced in the underlying mantle every century (Pagli and Sigmundsson,2008). Ice unloading may also promote failure of shallow magmareservoirs with a potential consequence of a small perturbation of thenatural eruptive cycle (Sigmundsson et al., 2010). Initially, ice thinningof 100 m or more on volcanoes with glaciers more than 150-m thick,such as Sollipulli in Chile, may cause more explosive eruptions, withincreased tephra hazards (Tuffen, 2010). Additionally, the potential foredifice lateral collapse could be enhanced by loss of support previouslyprovided by ice (Tuffen, 2010) or to elevated pore-water pressuresarising from meltwater (Capra, 2006; Deeming et al., 2010). Ultimatelythe loss of ice cover on glaciated volcanoes may reduce opportunitiesfor explosions arising from magma-ice interaction. The incidence of icesourcedlahars may also eventually fall, although exposure of newsurfaces of volcanic debris due to ice wastage may provide the rawmaterial for precipitation-related lahars. The likelihood of both volcanicand non-volcanic landslides may also increase due to greater availabilityof water, which could destabilize slopes. Many volcanoes provide aready source of unconsolidated debris that can be rapidly transformedinto potentially hazardous lahars by extreme precipitation events.Volcanoes in coastal, near-coastal, or island locations in the tropics areparticularly susceptible to torrential rainfall associated with tropicalcyclones, and the rainfall rate associated with tropical cyclones isprojected to increase though the number of tropical cyclones is projectedto decrease or stay essentially unchanged (see Section 3.4.4). The impactof future large explosive volcanic eruptions may also be exacerbated by an188