Data from satellite monitoring (see box on measur-
Snow covered
ing snow cover extent) from 1966 to 2005 show that area (million km
2
)
mean monthly snow-cover extent in the Northern
41
Hemisphere is decreasing at a rate of 1.3 per cent per
decade (Figure 4.2). For the calendar year of 2006 av-
38
erage snow-cover extent was 24.9 million km
2
, which
is 0.6 million km
2
less than the 37-year average
2
. In
the Northern Hemisphere, spring and summer show
35
the strongest decreases in snow-cover extent. Satellite
observations of snow-cover extent show a decreasing
32
trend in the Northern Hemisphere for every month
1930 1940 1950 1960 1970 1980 1990 2000
except November and December, with the most sig-
Figure 4.3: Northern Hemisphere snow-covered area (SCA)
nificant decreasing trends during May to August
3
. The
for the spring (March–April) from 1922–2005. The linear trend
average Northern Hemisphere snow-cover extent for shows a decrease in SCA of 2.7 ± 1.5 × 106 km
2
or 7.5 ± 3.5 %.
March and April decreased by 7.5 ± 3.5 per cent from
The shaded area represents the 5 to 95% range of the data.
1922–2005
4
(Figure 4.3). Source: Based on IPCC 2007
4
, updated from Brown 2000
5
Snow cover
anomaly (million km
2
)
12-month running mean of monthly
anomalies
4
Trend
Monthly snow-cover
anomalies
2
0
-2
-4
10
1970
1975 1980 1985 1990 1995 2000 2005
Figure 4.2: Northern Hemisphere snow-cover extent departures from monthly means from 1966 to 2005, with the 12-month run-
ning mean also shown. The decreasing trend of –1.3% per decade is significant at the 90% level.
Source: M.J. Brodzik; data from NOAA snow charts revised by D. Robinson, Rutgers University
42 GLOBAL OUTLOOK FOR ICE AND SNOW