1.3.1.1 Observed effects due to changes in the cryosphere

Effects of changes in the cryosphere have been documented in relation to virtually all of the cryospheric components, with robust evidence that it is, in general, a response to reduction of snow and ice masses due to enhanced warming.

Mountain glaciers and ice caps, ice sheets and ice shelves

Effects of changes in mountain glaciers and ice caps have been documented in runoff, changing hazard conditions (Haeberli and Burn, 2002) and ocean freshening (Bindoff et al., 2007). There is also emerging evidence of present crustal uplift in response to recent glacier melting in Alaska (Larsen et al., 2005). The enhanced melting of glaciers leads at first to increased river runoff and discharge peaks and an increased melt season (Boon et al., 2003; Hock, 2005; Hock et al., 2005; Juen et al., 2007), while in the longer time-frame (decadal to century scale), glacier wasting should be amplified by positive feedback mechanisms and glacier runoff is expected to decrease (Jansson et al., 2003). Evidence for increased runoff in recent decades due to enhanced glacier melt has already been detected in the tropical Andes and in the Alps. As glaciers disappear, the records preserved in the firn^[3] and ice layers are destroyed and disappear due to percolation of melt water and mixing of chemical species and stable isotopes (Table 1.2).

Table 1.2. Selected observed effects due to changes in the cryosphere produced by warming.

The formation of large lakes is occurring as glaciers retreat from prominent Little Ice Age (LIA) moraines in several steep mountain ranges, including the Himalayas (Yamada, 1998; Mool et al., 2001; Richardsonand Reynolds, 2000), the Andes (Ames et al., 1989; Kaser and Osmaston, 2002) and the Alps (Haeberli et al., 2001; Huggel et al., 2004; Kaab et al., 2005) (Table 1.2). Thawing of buried ice also threatens to destabilise the LIA moraines (e.g., Kaser and Osmaston, 2002). These lakes thus have a high potential for glacial lake outburst floods (GLOFs). Governmental institutions in the respective countries have undertaken extensive safety work, and several of the lakes are now either solidly dammed or drained, but continued vigilance is needed since many tens of potentially dangerous glacial lakes still exist in the Himalayas (Yamada, 1998) and the Andes (Ames, 1998), together with several more in other mountain ranges of the world. The temporary increase in glacier melt can also produce enhanced GLOFs, as has been reported in Chile (Peña and Escobar, 1985), although these have not been linked with any long-term climate trends.

Enhanced colonisation of plants and animals in deglaciated terrain is a direct effect of glacier and snow retreat (e.g., Jones and Henry, 2003). Although changes due to other causes such as introduction by human activities, increased UV radiation, contaminants and habitat loss might be important (e.g., Frenot et al., 2005), ‘greening’ has been reported in relation to warming in the Arctic and also in the Antarctic Peninsula. Tundra areas in the northern circumpolar high latitudes derived from a 22-year satellite record show greening trends, while forest areas show declines in photosynthetic activity (Bunn and Goetz, 2006). Ice-water microbial habitats have contracted in the Canadian High Arctic (Vincent et al., 2001).

Glacier retreat causes striking changes in the landscape, which has affected living conditions and local tourism in many mountain regions around the world (Watson and Haeberli, 2004; Mölg et al., 2005). Warming produces an enhanced spring-summer melting of glaciers, particularly in areas of ablation, with a corresponding loss of seasonal snow cover that results in an increased exposure of surface crevasses, which can in turn affect, for example, snow runway operations, as has been reported in the Antarctic Peninsula (Rivera et al., 2005). The retreat, enhanced flow and collapse of glaciers, ice streams and ice shelves can lead to increased production of iceberg calving, which can in turn affect sea navigation, although no evidence for this exists as yet.

Snow cover

Spring peak river flows have been occurring 1-2 weeks earlier during the last 65 years in North America and northern Eurasia. There is also evidence for an increase in winter base flow in northern Eurasia and North America. These changes in river runoff are described in detail in Section 1.3.2 and Table 1.3. There is also a measured trend towards less snow at low altitudes, which is affecting skiing areas (Table 1.2).

Frozen ground

Degradation of seasonally frozen ground and permafrost, and an increase in active-layer thickness, should result in an increased importance of surface water (McNamara et al., 1999), with an initial but temporary phase of lake expansion due to melting, followed by their disappearance due to draining within the permafrost, as has been detected in Alaska (Yoshikawa and Hinzman, 2003) and in Siberia (Smith et al., 2005).

Permafrost and frozen ground degradation are resulting in an increased areal extent of wetlands in the Arctic, with an associated ‘greening’, i.e., plant colonisation (see above). Wetland changes also affect the fauna. Permafrost degradation and wetland increase might produce an increased release of carbon in the form of methane to the atmosphere in the future (e.g., Lawrence and Slater, 2005; Zimov et al., 2006), but this has not been documented.

The observed permafrost warming and degradation, together with an increasing depth of the active layer, should result in mechanical weakening of the ground, and ground subsidence and formation of thermokarst will have a weakening effect on existing infrastructure such as buildings, roads, airfields and pipelines (Couture et al., 2000; Nelson, 2003), but there is no solid evidence for this yet. There is evidence for a decrease in potential travel days of vehicles over frozen roads in Alaska (Table 1.2). Permafrost melting has produced increased coastal erosion in the Arctic (e.g., Beaulieu and Allard, 2003); this is detailed in Section 1.3.3.

Thawing and deepening of the active layer in high-mountain areas can produce slope instability and rock falls (Watson and Haeberli, 2004), which in turn can trigger outburst floods (Casassa and Marangunic, 1993; Carey, 2005), but there is no evidence for trends. A reported case linked to warming is the exceptional rock-fall activity in the Alps during the 2003 summer heatwave (Table 1.2).

Sea ice

Nutritional stresses related to longer ice-free seasons in the Beaufort Sea may be inducing declining survival rates, smaller size, and cannibalism among polar bears (Amstrup et al., 2006; Regehr et al., 2006). Polar bears are entirely dependent on sea ice as a platform to access the marine mammals that provide their nutritional needs (Amstrup, 2003). Reduced sea ice in the Arctic will probably result in increased navigation, partial evidence of which has already been found (Eagles, 2004), and possibly also a rise in offshore oil operations, with positive effects such as enhanced trade, and negative ones such as increased pollution (Chapter 15; ACIA, 2005), but there are no quantitative data to support this.

Increased navigability in the Arctic should also raise issues of water sovereignty versus international access for shipping through the North-west and North-east Passages. Previously uncharted islands and seamounts have been discovered due to a reduction in sea ice cover (Mohr and Forsberg, 2002), which can be relevant for territorial and ocean claims.

Ocean freshening, circulation and ecosystems

There is evidence for freshening in the North Atlantic and in the Ross Sea, which is probably linked to glacier melt (Bindoff et al., 2007). There is no significant evidence of changes in the Meridional Overturning Circulation at high latitudes in the North Atlantic Ocean or in the Southern Ocean, although important changes in interannual to decadal scales have been observed in the North Atlantic (Bindoff et al., 2007). Ocean ecosystem impacts such as a reduction of krill biomass and an increase in salps in Antarctica, decline of marine algae in the Arctic due to their replacement by freshwater species, and impacts on Arctic mammals, are described in Section 1.3.4.2.

Lake and river ice

Seasonal and multi-annual variations in lake and river ice are relevant in terms of freshwater hydrology and for human activities such as winter transportation, bridge and pipeline crossings, but no quantitative evidence of observed effects exists yet. Shortening of the freezing period of lake and river ice by an average of 12 days during the last 150 years (Lemke et al., 2007) results in a corresponding reduction in skating activities in the Northern Hemisphere. In Europe there is some evidence for a reduction in ice-jam floods due to reduced freshwater freezing during the last century (Svensson et al., 2006). Enhanced melt conditions could also result in significant ice jamming due to increased break-up events, which can, in turn, result in severe flooding (Prowse and Beltaos, 2002), although there is a lack of scientific evidence that this is already happening.

Changes in lake thermal structure and quality/quantity of under-ice habitation in lakes have been reported, as well as changes in suspended particles and chemical composition (see Section 1.3.2). Earlier ice-out dates can have relevant effects on lake and river ecology, while changes in river-ice dynamics may also have ecological effects (see Section 1.3.4).

Table 1.3. Observed changes in runoff/streamflow, lake levels and floods/droughts.