1.3.1 Cryosphere
The cryosphere reacts sensitively to present and past climate changes. The main components of the cryosphere are mountain 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).
Box 1.1. Retreat of Chacaltaya and its effects: case study of a small disappearing glacier in Bolivia
The observed general glacier retreat in the warming tropical Andes has increased significantly in recent decades (Francou et al., 2005). Small-sized glaciers are particularly vulnerable in warmer climates, with many of them having already disappeared in several parts of the world during the last century. The Chacaltaya Glacier in Bolivia (16°S) is a typical example of a disappearing small glacier, whose area in 1940 was 0.22 km2, and which has currently reduced (in 2005) to less than 0.01 km2 (Figure 1.1) (Ramirez et al., 2001; Francou et al., 2003; Berger et al., 2005), with current estimates showing that it may disappear completely before 2010. In the period 1992 to 2005, the glacier suffered a loss of 90% of its surface area, and 97% of its volume of ice (Berger et al., 2005). Although, in the tropics, glacier mass balance responds sensitively to changes in precipitation and humidity (see Lemke et al., 2007, Section 4.5.3), the fast glacier shrinkage of Chacaltaya is consistent with an ascent of the 0°C isotherm of about 50 m/decade in the tropical Andes since the 1980s (Vuille et al., 2003), resulting in a corresponding rise in the equilibrium line of glaciers in the region (Coudrain et al., 2005).
Ice melt from Chacaltaya Glacier, located in Choqueyapu Basin, provides part of the water resources for the nearby city of La Paz, allowing the release of water stored as ice throughout the long, dry winter season (April-September). Many basins in the tropical Andes have experienced an increase in runoff in recent decades, while precipitation has remained almost constant or has shown a tendency to decrease (Coudrain et al., 2005). This short-term increase in runoff is interpreted as the consequence of glacier retreat, but in the long term there will be a reduction in water supply as the glaciers shrink beyond a critical limit (Jansson et al., 2003).
Chacaltaya Glacier, with a mean altitude of 5,260 m above sea level, was the highest skiing station in the world until a very few years ago. After the accelerated shrinkage of the glacier during the 1990s, enhanced by the warm 1997/98 El Niño, Bolivia lost its only ski area (Figure 1.1), directly affecting the development of snow sports and recreation in this part of the Andes, where glaciers are an important part of the cultural heritage.