Averaged over their entire areas, within the period 1960– 2003 glaciers in Patagonia and Alaska have thinned by ap- proximately 35 m and 25 m, respectively, whereas high mountain glaciers in Asia have thinned by over 10 m. Data for Patagonia and Alaska are computed from glacier surface elevations for dozens of glaciers. In many other high moun- tain environments such as the Himalayas and the high Andes, where data are limited due to both difficult access to the high- altitude regions and for political reasons, the exact amount of regional mass loss remains subject to some uncertainty (Fig. 2). This makes it difficult to compare rates of change with other regions. Recent satellite observations, however, have con- firmed that glaciers in many mountain regions are thinning (e.g. Berthier et al., 2007; Paul et al., 2007; Bolch et al., 2008a, b), conclusively showing that the majority of mountain glaciers are losing mass in response to climate changes.
In a few areas, mountain glaciers have undergone periods of growth in recent decades. For example, glaciers in western Norway and the South Island of New Zealand advanced in the 1990s, glaciers on the southwestern sector of Cordillera Darwin, Tierra del Fuego, are presently advancing (Chinn et al., 2005), while in the Karakoram mountains some glaciers have over-ridden areas that have been ice-free for 50 years or more (Hewitt, 2005), opposed to further north and east where glaciers are declining. In northern Karakoram in Chi- na, glaciers are receding and have resulted in increased num- ber of glacial lake outburst floods (Chen et al., 2010). These exceptions to the global trend of recession in ice mass can be partly explained in terms of regionally increased precipitation in a more energetic climate system, which locally offset the effects of temperature increase. In Norway and New Zealand, however, the recent glacier advances appear to be short-lived events superimposed on a longer-term trend of glacier reces- sion. Frequently also the actual number of glaciers increase locally as larger glaciers melt down into several smaller gla- ciers, while the total ice mass obviously declines.
Figure 2: Glacier recession and expansion in Hindu Kush- Himalayas (HKH) and Central Asia. Notice that while some glaciers, especially in the Karakoram, have increased, the ma- jority of the glaciers in the HKH region and on the Tibetan plateau are receding.
Predicting the future response of mountain glaciers is fraught with difficulty. The world’s mountain ranges encompass a huge range of topographic and climatic environments, and each glacier has a unique relationship with local terrain and microclimate. Indeed, local variation is so great that it is impos- sible to make general statements about glacier response even within single mountain ranges. For example, glaciers in the Himalayan region include large, winter-accumulation type gla- ciers in the high-altitude Karakoram, steep, summer-accumu- lation type glaciers on monsoonal southern slope of the moun- tains in Nepal, and small, cold-based cirque glaciers on the arid northern slope, each of which will exhibit different responses to local climate changes, demonstrated by the increase in some glaciers in western and southern Karakoram and recession in some in north-eastern (Chen et al., 2010). Within each of these regions, glaciers occur across a considerable range of eleva- tions. In general, the most vulnerable glaciers are at relatively low elevations, whereas glaciers at high altitudes are more robust.
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