Geoscience Reference
In-Depth Information
Plate 13.4
The record-breaking minimum Arctic sea ice extent of September 2007 compared with the
previous minimum in 2005. The mean September limit for 1979-2007 is shown.
Source:Courtesy NSIDC.
its ice shelves. These include breakup of the Wordie
shelf on the west side in the 1980s, the northern
Larsen shelf on the east side between 1995 and
March 2002, and the Wilkins ice shelf in 2008.
Snow cover extent also shows clear indica-
tion of a response to recent temperature trends.
Northern Hemisphere snow cover has been
mapped by visible satellite images since 1966.
Compared with the 1970s to mid-1980s, annual
snow cover since 1988 has shrunk by about 10
percent. The decrease is pronounced in spring
and is well correlated with springtime warming
(
Figure 13.24
). Winter snow extent shows little
or no change. Nevertheless, annual snowfall in
North America north of 55
4 Vegetation
An increase in CO
2
up to around 1000ppmv is
expected to enhance global plant growth, beyond
which a saturation limit may be reached. How-
ever, deforestation could decrease the biosphere's
capacity to act as a carbon sink. A sustained
increase of only 1°C can cause considerable
change in tree growth, regeneration and species
extent. Species migrate only slowly but, eventually,
extensive forested areas may change to new
vegetation types, and it has been estimated that 33
percent of the present forest area could be affected,
with up to 65 percent of the boreal zone being
subject to change. Alpine tree lines appear to be
quite resistant to climatic fluctuations. However,
surveys of plant species on peaks in the European
Alps indicate an upward migration of alpine
plants by 1-4m per decade during the past
century.
N increased during
1950-1990. Scenarios for the mid-twenty-first
century point to a shorter snow cover period in
North America, with a decrease of 70 percent over
the Great Plains. In alpine areas, snowlines will rise
by 100-400m, depending on precipitation.
°
