Past trends

Satellite observations on the monthly snow cover extent in the Northern Hemisphere are available since 1967. Snow covered on average 45 % of the land area of the Northern Hemisphere in winter (December to February), varying from less than 41 % in 1980–1981 to over 48 % in 1977–1978. A detailed analysis of Northern hemisphere snow cover based on multiple sources shows significant decreases in snow cover extent during the spring melting season (March to June); changes in other seasons are not significant. The decrease in snow cover extent in the Northern Hemisphere during the period 1967 to 2012 was 7% on average in March and April and 53% in June (Figure 1) [i]. A separate analysis for Europe (EEA39 region) shows even larger reductions of 13% for March and April and 87% for June (Figure 2).

Decreases in snow cover extent are caused by an earlier onset and a shorter duration of the snow-melting season. Since 1972, the duration of the snow season averaged over the Northern Hemisphere declined by 5 days per decade, but with substantial regional variation. The duration of the snow season has decreased by up to 25 days in western, northern and eastern Europe due to earlier spring melt whereas it has increased by up to 15 days in southeastern Europe due to an earlier onset [ii].

Trends in snow depth vary in different parts of Europe. In some mountain regions, such as the Alps and the Norwegian mountains, snow depth has decreased at low elevations where the temperature increased over the freezing point whereas it has increased at high elevations where both precipitation and temperature have increased but the temperature has remained below the freezing point for extended periods [iii]. In other mountain regions such as the Carpathians, Tatra, Pyrenees, and Caucasus, there have been either decreasing or variable trends [iv]. In the lowlands of western Europe, snow is not a permanent winter phenomenon — it may come and go several times during the cold months. Decreasing snow cover trends have been observed in this region, but snow conditions correlate strongly with large-scale circulation patterns as indicated by the NAO [v].

The snow mass (i.e. the amount of water that the snow contains) is another important variable describing seasonal snow. For the whole Northern Hemisphere, a 7 % decrease in March snow mass has been observed between 1982 and 2009 [vi]. An extension of this data focusing on EEA member countries demonstrates a stronger average decline of more than 20 % for the period 1980-2012, although the year-to-year variation is large (Figure 3).

Projections

Northern Hemisphere snow cover will continue to shrink as temperatures rise [vii]. The multi-model mean from the CMIP5 modelling exercise projects changes in March/April snow cover extent in the Northern Hemisphere during the 21st century from 7 % for a low emission scenario (RCP 2.6) to 25 % for a high emission scenario (RCP 8.5) (Figure 4). Projected reductions in snow mass range from about 10 % for RCP2.6 to about 30 % for RCP8.5; projected reductions in the duration of the snow season range from about 10 days for RCP2.6 to about 40 days for RCP8.5 [viii]. Significant reductions in snow mass in Europe are likely to occur in Switzerland [ix], the alpine range of Italy [x], the Pyrenees [xi], the Turkish mountains [xii] and Balkan mountains [xiii]. In these areas the change can have dramatic down-stream effects as melt water contributes up to 60–70 % of annual river flows. Despite the projected decrease in long-term mean snow mass in the Northern Hemisphere, model simulations indicate occasional winters of heavy snowfall, but these become increasingly uncommon towards the end of the 21st century.



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