Cryosphere

The cryosphere reacts sensitively to present and past climate changes. The main components of the cryosphere are mountain

2 Spring green-up is a measure of the transition from winter dormancy to active spring growth.

glaciers and ice caps, floating ice shelves and continental ice sheets, seasonal snow cover on land, frozen ground, sea ice and lake and river ice. In Chapter 4 of WGI, the changes in the cryosphere since the TAR are described in detail, including the description of climate and non-climate forcing factors and mechanisms (Lemke et al., 2007). Chapter 6 of WGI describes glacier changes in the geological past, including Holocene glacier variability (Jansen et al., 2007, Box 6.3). Here we describe the observed effects on the environment and on human activities due to these recent cryospheric changes.

There is abundant evidence that the vast majority of the cryospheric components are undergoing generalised shrinkage in response to warming, with a few cases of growth which have been mainly linked to increased snowfall. The observed recession of glaciers (Box 1.1) during the last century is larger than at any time over at least the last 5,000 years, is outside of the range of normal climate variability, and is probably induced by anthropogenic warming (Jansen et al., 2007). In the Arctic and the Antarctic, ice shelves several thousand years old have started to collapse due to warming (Lemke et al., 2007). In many cases the cryospheric shrinkage shows an increased trend in recent decades, consistent with the enhanced observed warming. Cryospheric changes are described by Lemke et al. (2007), including the contribution of the cryosphere to sea-level rise. Sea-level rise is treated in Section 1.3.3, in the regional chapters of WGII, and in WGI, Chapters 4 and 5 (Bindoff et al., 2007; Lemke et al., 2007).

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 firn3 and ice layers are destroyed and disappear due to percolation of melt water and mixing of chemical species and stable isotopes (Table 1.2).

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

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

Environmental factor

Observed changes

Time period

Location

Selected references

Glacial lake size

Increase from 0.23 km2 to 1.65 km2

0 0

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