The observed climate and glacier change in the Caucasus Mountains

There are about 2,000 glaciers in the Greater Caucasus located predominantly in its central part (Fig. 1). Glacier melt in the Caucasus is particularly strong in June, July and August (JJA) and variations in JJA temperatures are of particular importance. The long-term observations confirm that these are rising (Fig. 2). A strong increase in JJA temperatures has been observed at Terskol and Aragats since the late 1960s. At both sites, in the last 40 years, JJA temperatures have been increasing at a rate of 0.05°C per year and there is strong positive linear trend in the time series explaining 36% of the total variance. The Aragats record, dating back to 1929, confirms that the last 2 decades were the warmest in almost 80 years of observations. This warming is making a profound impact on glaciers. Glacier melt has intensified in the 1990s and its highest values have been recorded in the last two decades (Fig. 3). Thus, in the summers of 1998, 2000, and 2007 ablation exceeded the long-term average by two standard deviations from the long-term mean, reaching its highest value on record in 2007.

Figure 1. Landsat ETM+ satellite image of Main Caucasus Ridge (A) aligned north-west to south-east. This band combination (RGB, 5, 4 and 3) colours glaciers as bright blue and vegetation as green. Note the large glaciated area of Mt Elbrus in the top left. Clouds appear as white. The inset map in (B) shows six glaciers in the Adylsu Valley and their terminus positions in 1985 (red) and 2000 (yellow). All the glaciers have retreated and the maximum retreat rate is reported for the Skhelda Glacier (~350 m). Also note the large unvegetated proglacial area in front of the glaciers that delimits their most recent maximum at the LIA

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Figure 1. Landsat ETM+ satellite image of Main Caucasus Ridge (A) aligned north-west to south-east. This band combination (RGB, 5, 4 and 3) colours glaciers as bright blue and vegetation as green. Note the large glaciated area of Mt Elbrus in the top left. Clouds appear as white. The inset map in (B) shows six glaciers in the Adylsu Valley and their terminus positions in 1985 (red) and 2000 (yellow). All the glaciers have retreated and the maximum retreat rate is reported for the Skhelda Glacier (~350 m). Also note the large unvegetated proglacial area in front of the glaciers that delimits their most recent maximum at the LIA

Changes in the cold-season precipitation (Fig. 2) do not offset impacts of the observed summer warming. At Terskol, positive anomalies observed in the individual years do not compensate for a rapidly increasing melt.

As a result, the cumulative mass balance of Djankuat is declining and has reached its lowest value on record in the mass balance year of 2006/2007 (Fig. 3). At Aragats, the cold-season precipitation has been declining from

JJA air temperature

Terskol Aragats

1950 1975 2000

1925

October-April precipitation -Terskol

October-April precipitation -Terskol

1925 1950 1975 2000

Figure 2. JJA air temperature and October-April precipitation at Terskol and Aragats

Figure 2. JJA air temperature and October-April precipitation at Terskol and Aragats

Figure 3. Seasonal ablation totals (a) and cumulative mass balance (b) of Djankuat glacier (m w. e.) between 1968 and 2007

its maximum recorded in the 1960s (Fig. 2). This trend has been especially strong in the last 2 decades possibly in response to the strongly positive phase of the North Atlantic Oscillation (NAO).

Analysis of changes in the extent of 113 glaciers in the central Greater Caucasus (Fig. 1) between 1985 and 2000 has shown that 94% of the glaciers have retreated (Stokes et al., 2006). The mean retreat rate was 8 m year-1 and maximum rates approached ~38 m year-1. The termini of most glaciers have retreated by 50-150 m but the maximum retreat exceeded 500 m. Glaciers whose snouts are positioned at lower elevations exhibited stronger retreat; those at higher elevations were less sensitive to the warming climate. Fig. 1b illustrates changes in the extent of six neighbouring glaciers including Djankuat. The Skhelda glacier retreated by 350 m in just 15 years. The satellite imagery also reveals the inferred maximum limit of the Shkhelda Glacier at the end of the Little Ice Age (LIA). This limit is revealed by the 'fresh' unvegetated proglacial zone in front of the snout. Although the timing of the LIA limit is somewhat complex in the Caucasus Mountains (two possible maximal positions at AD 1300 and AD 1700) it can be seen that for the Skhelda, the recent retreat represents a significant proportion (~25%) of retreat since the most recent maximum. This is common for many other glaciers (Stokes et al., 2006) and suggests that the rate of retreat has increased in recent years.

Glacier retreat in the Caucasus is accompanied by the formation and expansion of glacial lakes (Stokes et al., 2007). Figure 1b shows that the Bashkara glacier terminates in a lake. It formed in 1995. Currently, it measures about 200 m across and its depth exceeds 15 m. The total number of lakes and their surface areas have increased dramatically in the Greater Caucasus from 16 in 1985 to 24 in 2000, representing a 57% increase in total lake area coverage (Stokes et al., 2007). Given that many of the glacial catchments feed directly into larger valleys, which house settlements and towns, this is a concern. Indeed, in August 2006 (the second warmest summer since 1951 in the central Greater Caucasus; Fig. 2), an outburst of a glacial lake, Birdjaly-Charan, located on the north-eastern slope of Mt. Elbrus, occurred in response to the enhance melting of the Chungurchat-Chiran glacier. It released 400,000 m3 of water over 2 days (Chernomorets et al., 2007) leading to a substantial loss of property and infrastructure. This event emphasizes the vulnerability of human activities to glacier-related hazards, which are likely to intensify in response to the climatic warming.

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