IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Chapter 3Changes in Climate Extremes and their Impacts on the Natural Physical Environmentincrease in extreme precipitation events (see Section 3.3.2) that providean effective means of transferring large volumes of unconsolidatedash and pyroclastic flow debris from the flanks of volcanoes intodownstream areas.Quantification of possible trends in the frequency of landslidesand ice avalanches in mountains is difficult due to incompletedocumentation of past events. There is high confidence thatchanges in heat waves, glacial retreat, and/or permafrostdegradation will affect high mountain phenomena such as slopeinstabilities, mass movements, and glacial lake outburst floods,and medium confidence that temperature-related changes willinfluence bedrock stability. There is also high confidence thatchanges in heavy precipitation will affect landslides in someregions. There is medium confidence that high-mountain debrisflows will begin earlier in the year because of earlier snowmelt,and that continued mountain permafrost degradation and glacierretreat will further decrease the stability of rock slopes. There islow confidence regarding future locations and timing of large rockavalanches, as these depend on local geological conditions andother non-climatic factors. There is low confidence in projectionsof an anthropogenic effect on phenomena such as shallowlandslides in temperate and tropical regions, because these arestrongly influenced by human activities such as poor land usepractices, deforestation, and overgrazing. It is well establishedthat ice mass wastage following the end of the last glaciationsled to increased levels of seismicity, but there is low confidencein the nature of recent and projected future seismic responses toanthropogenic climate change.3.5.7. High-latitude Changes Including PermafrostPermafrost is widespread in Arctic, in subarctic, in ice-free areas ofAntarctica, and in high-mountain regions, and permafrost regions occupyapproximately 23 million km 2 of land area in the Northern Hemisphere(Zhang et al., 1999). Melting of massive ground ice and thawing ofice-rich permafrost can lead to subsidence of the ground surface and tothe formation of uneven topography known as thermokarst, havingimplications for ecosystems, landscape stability, and infrastructureperformance (Walsh, 2005). See also Case Study 9.2.10 for discussion ofthe impacts of cold events in high latitudes. The active layer (nearsurfacelayer that thaws and freezes seasonally over permafrost) playsan important role in cold regions because most ecological, hydrological,biogeochemical, and pedogenic (soil-forming) activity takes place withinit (Hinzman et al., 2005).Observations show that permafrost temperatures have increased since the1980s (IPCC, 2007b). Temperatures in the colder permafrost of northernAlaska, the Canadian Arctic, and Russia have increased up to 3°C near thepermafrost table and up to 1 to 2°C at depths of 10 to 20 m (Osterkamp,2007; Romanovsky et al., 2010; S.L. Smith et al., 2010) since the late1970s/early 1980s. Temperature increases have generally been less than1°C in the warmer permafrost of the discontinuous permafrost zone ofthe polar regions (Osterkamp, 2007; Romanovsky et al., 2010; S.L. Smithet al., 2010), and also in the high-altitude permafrost of Mongolia andthe Tibetan Plateau (Zhao et al., 2010). When the other conditionsremain constant, active layer thickness is expected to increase inresponse to warming. Active layer thickness has increased by about20 cm in the Russian Arctic between the early 1960s and 2000 (T. Zhanget al., 2005) and by up to 1.0 m over the Qinghai-Tibetan Plateau sincethe early 1980s (Wu and Zhang, 2010), with no significant trend in theNorth American Arctic since the early 1990s (Shiklomanov et al., 2010).However, over extreme warm summers, active layer thickness mayincrease substantially (Smith et al., 2009), potentially triggering activelayerdetachment failures on slopes (Lewkowicz and Harris, 2005).Extensive thermokarst development has been found in Alaska (Jorgensonet al., 2006; Osterkamp et al., 2009), Canada (Vallée and Payette, 2007),and central Yakutia (Gavriliev and Efremov, 2003). Increased rates ofretrogressive thaw slump activities have been reported on slopes overthe Qinghai-Tibetan Plateau (Niu et al., 2005) and adjacent to tundralakes over the Mackenzie Delta region of Canada (Lantz and Kokelj,2008). Substantial expansion and deepening of thermokarst lakes wasobserved near Yakutsk with subsidence rates of 17 to 24 cm yr -1 from1992 to 2001 (Fedorov and Konstantinov, 2003). Satellite remote sensingdata show that thaw lake surface area has increased in continuouspermafrost regions and decreased in discontinuous permafrost regions(Smith et al., 2005). Coasts with ice-bearing permafrost that are exposedto the Arctic Ocean are very sensitive to permafrost degradation. SomeArctic coasts are retreating at a rapid rate of 2 to 3 m yr –1 and the rateof erosion along Alaska’s northeastern coastline has doubled over thepast 50 years, related to declining sea ice extent, increasing sea surfacetemperature, rising sea level, thawing coastal permafrost, and possiblyincreases in storminess and waves (Jones et al., 2009; Karl et al., 2009)Increases in air temperature are in part responsible for the observedincrease in permafrost temperature over the Arctic and subarctic, butchanges in snow cover also play a critical role (Osterkamp, 2005; Zhang,2005; T. Zhang et al., 2005; S.L. Smith et al., 2010). Trends toward earliersnowfall in autumn and thicker snow cover during winter have resultedin a stronger snow insulation effect, and as a result a much warmerpermafrost temperature than air temperature in the Arctic. On the otherhand, permafrost temperature may decrease even if air temperatureincreases, if there is also a decrease in the duration and thickness ofsnow cover (Taylor et al., 2006). The lengthening of the thaw season andincreases in summer air temperature have resulted in changes in activelayer thickness. Model simulations have projected thickening of theactive layer, a northward shift of the permafrost boundary, reductionsin permafrost area, and an increase in permafrost temperature in the21st century and beyond (Saito et al., 2007; Schaefer et al., 2011). Theprojected permafrost degradation may result in ancient carbon currentlyfrozen in permafrost being released into the atmosphere, providing apositive feedback to the climate system (Schaefer et al., 2011). Expansionof lakes in the continuous permafrost zone may be due to thawing ofice-rich permafrost and melting of massive ground ice, while decreasesin lake area in the discontinuous permafrost zone may be due to lake189