This indicator shows changes in the surface area and volume of the Triglav glacier in the period 1992–2018 (volume measurements are periodical) and the average decade temperature of the melting season (May–October) at Kredarica in the period 1961–2018. The cumulative specific mass balance of selected European glaciers in the period 1946 – 2018 and in 2015 is presented as well. The measured surface area of the Triglav glacier includes glacier ice as well as snow directly above or adjacent to the glacier unless indicated otherwise. Glacier ice is entirely exposed only on rare occasions and only exceptionally for the period of several consecutive years.

Mass balance is the difference between accumulation and ablation of ice or snow covering the glacier. Due to variable density of snow and ice, it is expressed in millimeters of water equivalent. Specific mass balance means an average value per unit of surface area.

A glacier is a perennial mass of ice on the Earth’s surface that moves downslope under its own weight in response to gravitational force. A glacier forms above the snow line in locations where the mass accumulation of snow and ice exceeds ablation over many years. The snow is gradually transformed into glacier ice, which moves downslope reaching below the snow line until the glacier ice disappears due to prevailing ablation. The Triglav glacier because of his smallness doesnʹt have any more all the characteristics of the glacier; for this kind of very small glaciers (smaller than 25 ha) enforce the term glacieret (Cogley and the others, 2011). The key factors of ablation are: sun radiation (intensity, duration), air temperature, precipitation and wind (Gabrovec, Zakšek, 2007).

Changes in glacier volume and extent are an illustrative indicator of climate change. During the last decade, the trend of rapid glacier retreat has been characteristic of all Alpine glaciers. In Slovenia, there are two glaciers: the Triglav glacier and the Skuta glacier. Due to their extreme south-eastern position within the Alps and low altitude, both are exceptionally sensitive to climate change. Due to the small size of Slovenian glaciers, their relative retreat in respect to their present extent and volume is even greater than in other Alpine glaciers.

Similar oscillations within the last 400 years are typical of all Alpine glaciers. Following their peak at the beginning of the 17^th century, glaciers remained at their maximum extent for the next 250 years, undergoing relatively insignificant changes. Most glaciers in the eastern Alps reached their second peak between 1770 and 1780, and in the mid-19^th century. However, the post-1920 period records a continuous retreat of glaciers; the only variations occurring between individual years and decades were those concerning the rate of glacier retreat.

The melting of the Triglav glacier intensified during the 1990’s. The increasingly rapid thinning of the glacier ice caused individual rock formations to appear in the middle of the glacier, finally cutting it into two completely separate parts in 1992. The melting and disintegration of the Triglav glacier is still continuing, with occasional halts in the process occurring in years with exceptionally high snow cover during late spring. This occurred e.g. in 2004, when, at the beginning of July, snow measurement rods below the glacier revealed more than 2 m of snow. The snow cover remained in place until the end of the ablation season, so the glacier remained covered by snow and the measurements made no sense. The snow remained at the bottom of the glacier until the end of summer 2005, which is why the glacier’s surface area in this year was greater than in 2003. The smallest glacier area before 2018 was recorded in 2007, when it measured 0.6 ha. At the end of the melting season in 2008, the glacier was still mainly covered with snow from the previous winter season, and its measured surface area therefore amounted to 1,1 ha. The most significant glacier recovery occurred in 2009 and 2014, mostly thanks to above-average snow cover during two consecutive winters of 2008/09–2009/10 and 2012/13–2013/14, during which numerous avalanches were triggered as well. They are also among the important factors that make a glacier or its existence possible. At the end of the melting season in 2010, when most of the glacier was covered with thick layers of old snow, the glacier’s surface area was 2.5 hectares. It was similar at the end of the melting season in 2014, when it measured 3,6 hectares.

It should be borne in mind that the increase at that time was the result of several months of glacial firing or the well-reclaimed old snow of the last snow season. Due to the specific geographic location of the Triglav glacier, the latter has been only at the beginning of the years or even decades long process of the formation of the greenish firn ice, which gave the glacier the almost forgotten name "Green snow".

In September 2013, georadar measurements were carried out (13 cross sections with a length of 40–82 m in the NNE–SSW direction). They revealed that the surface area of glacier ice was 0.38 hectares (2.5 hectares including the snow on top of and adjacent to the glacier), while its volume amounted to 7,400 m³. The greatest thickness in some places was 5 m and the average thickness was 1.95 m. Including the snow on top of and adjacent to the glacier, the greatest thickness was 8 metres, while the average thickness was no more than 3 metres.

The trend of ice build-up stopped during the melting season in 2015, when the glacier's surface area was reduced by more than 50 %, to 1.7 hectares. Its thickness, and consequently its volume, was reduced even more (several metres in places). All of the firm that accumulated at previous years (more exactly from the beginning of the snow season 2005/06 with some spaces until the end of snow season 2013/14) disappeared due to intensive melting. Two seasons with snow volumes below average which were followed by very warm summers, caused further intense melting of the firn originating from previous winters, leading to contraction and thinning of the glacier. Even the snow season of 2017/18, which was pretty above average and which largely coincides with "the growing age" of the glacier (November-April) didn't stop this trend, as we subsequently recorded the warmest average melting season temperature since 1955, i.e. since the Krearica data are available. All of the 10 hottest melting seasons fall in the period after 2000. Out of 5 hottest, 4 happened after the year 2012.

Since 2017, the surface of the glacier is again smaller than one hectare. The maximal thickness of the glacial layer nowhere exceeds 5 meters at the end of the melting period. Only a patch of glacial ice caught in a karst depression remains, which is by the specific volume density increasingly similar to water ice.

The precise time determination of disappearance the glacier is impossible, because it isn't possible to be accurate enough in the prediction of the local climate change on  the Triglav glacier area. The latter is a result of the global climate change. With the continued occurrence of unfavourable climatic conditions in the coming years, there is a strong likelihood that the glacier will disintegrate into several smaller parts and disappear gradually.

When measurements began in the mid-1950s, the melting period was slightly shorter than the accumulation period, while in recent decades it has usually been the opposite. The glacier balance is very fragile, as current temperature conditions and other factors threaten the preservation of the Triglav glacier (table Average temperature (° C) melting season at Kredarica in decades). If atmosphere warming continues at the rate witnessed in the last two decades, the glacier will eventually disappear. More snowfall in the area of the glacier, which is a possible result of global climate change, will only temporarily delay the glacier's gradual disappearance.

Data from a nearby meteorological station in Kredarica show that the average melting season temperature has a rather large year-on-year variability, with a marked upward trend, which is consistent with observed patterns of temperature change at other stations in the Alps. After a fairly stable average in the 1950s and 1960s, the minimum was in the 1970s. At the beginning of the 1980s temperatures began to rise during the melting season and this trend continued into this millennium. A comparison of 10-year series of average melting season temperatures (May-October) shows the first two major temperature jumps in the second half of the 70's and the second half of the 80's of the 20^th century (0.3 and 0.4 respectively) and a remarkable temperature jump (0.7 ° C) at the turn of the 20^th to the 21^st century. The average melting temperature of the next, that is, the last ten-year series (2007-2016), hasnʹt increased further than the previous one.

The highest average melting season temperatures between 1955 and 2016 were recorded at 5.5 ° C in 2018 (0.3 ° C higher than in the extremely warm year and the summer of 2003) and the lowest (1.7 ° C) in years 1972 and 1974. The average temperature of the last melting season (May-October 2016) was with 4.0 ° C almost equal to the long-term average (3.9 ° C) of the last climatological period (1981-2010). The rather cold October or the last month of the melting season stands out. The melting season in 2015 was also outstandingly warm (for a whole degree of Celsius too hot) relative to the comparative data for the climatological period. Compared to the reference period 1961-1990, the average temperature of the last melting season (2016) was 0.8 ° C above the average. A possible turnaround for glacier growth would require a sequence of sub-average melting seasons or a series of very snowy winters. However, it canʹt be overlooked, that since the beginning of the yearlong measurements at Kredarica (1955), as many as 9 warmest melting seasons fall in the period after 2000. In the first half of this 62-year observation series, almost all the melting seasons are from 2000 onwards, with the exception of 2010 and 2014. In the last period the glacier initially increased significantly-in 2013 and 2014. But in the next two years it reduced again and reached the surface decades ago. A significant decrease in glacier thickness or volume in 2016 was aided by relatively late (March) snowfall in the accumulation period and the above-average warming period, since four (June-September) of all six months were well above long-term average.

If the favorable conditions observed in the period 1992-2007 and in 2011, 2012, 2015 and 2016-2018 recur in coming years the glacier will continue to shrink. In this case the glacier may again disappear completely, which almost already happened at the beginning of this millennium. This is why the Triglav glacier will continue to be extremely important and one of the direct indicators of climate change in the region. Similar movements are typical of all Alpine glaciers. The largest mass losses were also recorded in the Sarennes (France), Careser (Italy) and the Hintereis (Austria) glaciers in 1946-2014 and 2015. Simultaneously mass increased in some Norwegian glaciers, also on one of the glaciers in the Pyrenees of Spain. Variations in the rate of change are caused by varying altitude, location and the size of glaciers in addition to the variable annual weather factors.

From 1900 until today, according to the European Environment Agency data, the Alps have lost about 50 % of their ice mass, mainly due to elevated  summer temperatures. The continued retreat of glaciers is predicted during the 21^st century. According to the moderately optimistic scenario of RCP4.5 releases, the extent of European glaciers will decrease from 20 to 84 % until the year 2100 (compared to the year 2006). The pessimistic PCP 8.5 emission scenario forecasts the reduction of 38-89 %. The largest relative volume loss is expected in central Europe, while in Norway the glacier areas are expected to decrease by about third till the end of the century, according to the SRES B2 greenhouse gas emissions scenario.