Snow cover

Northern hemisphere snow-cover extent variation 1966-2005

Note: The figure shows the snow-cover extent variation 1966 - 2005

Data source:

Brodzik, M. J.; Armstrong, R. I.; Weatherhead, E. C.; Savoie, M. H.; Knowles, W. K. and Robinson, D. A., 2006. Regional trend analysis of satellite derived snow extent and global temperature anomalies, American Geophysical Union, Fall 2006. San Francisco, USA

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Observed change in spring snow-cover duration 1970-2004

Note: The map shows the observed change in spring snow-cover duration 1970 to 2004 covering the Northern Hemisphere

Data source:

Grid Arendal

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Annual number of days with snow cover over European land areas 1961-1990 and projected change for 2071-2100

Note: Note: Results are based on seven regional climate-model simulations.

Data source:

Jylhä, K.; Fronzek, S.; Tuomenvirta, H.; Carter, T. R. and Ruosteenoja, K., 2007. Changes in frost and snow in Europe and Baltic sea ice by the end of the 21st century. Climatic Change, DOI 10.1007/s10584-007-9310-z

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Past trends

Data from satellite monitoring (NESDIS-database at NOAA) from 1966 to 2005 show that monthly snow-cover extent in the northern hemisphere is decreasing by 1.3 % per decade (Figure 1), with the strongest retreat in spring and summer (UNEP, 2007). Snow cover fell in all months except November and December, with the most significant decrease during May to August (Brodzik et al., 2006). This was accompanied by lower springtime water content, earlier disappearance of continuous snow cover in spring (Figure 2) by almost two weeks in the 1972-2000 period (Dye, 2002), less frequent frost days (days with minimum temperature below 0 ^oC) and shorter frost seasons (period of consecutive frost days).
The trends in duration and depth of northern hemispheric snow cover at higher latitudes differ between regions. In contrast to a reduced duration of snow cover over North America, a long-term increase in depth and duration has been observed over most of northern Eurasia (Kitaev et al., 2005).
Snow-cover trends in the mountain regions of Europe vary considerably with region and altitude. Recent declines in snow cover have been documented in the mountains of Switzerland (e.g. Scherrer et al., 2004), Slovakia (Vojtek et al., 2003), and in the Spanish ski-resorts in the Sierra Nevada and the Pyrenees (Rodriguez et al., 2005), but no change was observed in Bulgaria over the period 1931-2000 (Petkova et al., 2004). Declines, when observed, were largest at lower elevations, and Scherrer et al. (2004) statistically attributed the declines in the Swiss Alps to warming. Lowland areas of central Europe are characterised by recent reductions in annual snow-cover duration of about 1 day/year (Falarz, 2002). At Abisko in sub-Arctic Sweden, increases in snow depth have been recorded since 1913 (Kohler et al., 2006), and trends towards greater maximum snow depth but shorter snow season have been noted in Finland (Hyvarinen, 2003).

Projections

Model simulations project widespread reductions in snow cover over the 21st century (IPCC, 2007a). Decreases of between 9 and 17 % in the annual mean northern hemisphere snow cover by the end of 21st century are projected by individual models (ACIA, 2004). Although winter precipitation is projected to increase in northern and central Europe (Christensen and Christensen, 2007), less frequent frost occurrences associated with higher temperatures are projected to reduce the number of days with snow cover (Figure 3). Decreases of more than 60 snow-cover days are projected to occur (for the period 2071-2100 compared with 1961-1990) around the northern Baltic Sea, on the west slopes of the Scandinavian mountains and in the Alps (Jylha et al., 2007). The beginning of the snow accumulation season is projected to be later and the end earlier, and snow coverage during the snow season is projected to decrease (Hosaka et al., 2005).
For every 1 ^oC increase in average winter temperature, the snowline in the European Alps rises by about 150 metres (Beniston, 2003). Regional climate model runs, following the SRES emission scenarios A1B, B1 and A2, project milder winters with more precipitation in this region, increasingly falling as rain (Jacob et al., 2007). A recently-published study on the sensitivity of the Alpine snow cover to temperature by Hantel and Hirtl-Wielke (2007) reported a distinctive and strong variation of snow-cover sensitivity to temperature change with altitude. The study estimated that a 1 ^oC increase in temperature over central Europe (5-25^oE and 42.5-52.5^oN) would result in a reduction of about 30 days in snow duration (snow cover of at least 5 cm) in winter at the height of maximum sensitivity (about 700 m).
Snowfall in lower mountain areas is likely to become increasingly unpredictable and unreliable over the coming decades (Elsasser and Burki, 2002), with consequences for natural snow reliability and therefore difficulties in attracting tourists and winter sports enthusiasts (OECD, 2007).