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changes in its location and intensity through time and is
important to tropical and middle-latitude climate variability.
Rising temperatures are driven by water vapor feedback in
the climate system, which has potential rami
There are several well-documented examples of alpine
glacier retreat throughout the world from the tropics to the
subpolar latitudes, some of which are reviewed brie
y here
beginning with North American glaciers. A study from the
mid-1950s to the mid-1990s of 67 Alaskan glaciers shows
that all are thinning, and subsequent measurements on 28 of
these in the following years shows that the thinning rate has
increased [Arendt et al., 2002]. In the Brooks Range of
northern Alaska, 100% of the glaciers are in retreat, while
in southeastern Alaska, 98% are shrinking [Molnia, 2007].
When Glacier National Park in Montana was established in
cations for
Earth
'
is future climate, since it is linked to the circulation of
the Hadley cell. There is evidence that over the last 20 years,
this cell has expanded north and south by about 2° of lati-
tude, which may broaden the desert zones [Seidel and Ran-
del, 2007; Seidel et al., 2008] as the dry descending arms
move poleward. Under this scenario, droughts might become
more frequent and persistent, not only in the American
Southwest, but in the Mediterranean region and SH (Austra-
lia and parts of South America and Africa).
The world
'
s ice caps and glaciers are in retreat as tropo-
spheric temperatures warm. Today, ice covers about 10% of
the Earth
s surface area, compared to 30% coverage during
the coldest part of the last ice age. Because this warming is
ampli
'
ed at high altitudes [Bradley et al., 2006], the loss of
ice is most drastic in mountainous areas such as the Andes
and the Tibetan Plateau. Alpine glaciers at low latitudes are
located in the midtroposphere (~5000 to 7000 m above sea
level (asl)) where persistent warming is evident in instru-
mental records over the last few decades [Liu and Chen,
2000]. Mountain glaciers are smaller and thinner than the
massive polar ice sheets and thus respond more quickly to
abrupt temperature and precipitation changes as their surface
area to volume ratio is much greater. Although global ice
retreat at the beginning of the twenty-
rst century appears to
be driven mainly by increasing temperatures, regional factors
such as deforestation and precipitation de
cits can also im-
pact individual glaciers. Currently, 60% of the land-based ice
loss is occurring on the small glaciers and ice caps. Although
alpine glaciers constitute a small percentage of the world
s
ice cover, their rapid rate of melting in comparison to the
large polar ice sheets means they will contribute more to sea-
level rise in the short term; the ice loss from mountain
glaciers may raise sea level ~0.25 m by 2100 [Meier et al.,
2007].
A composite of
'
18
O records from high-altitude, low-
latitude, and midlatitude ice cores demonstrates that the most
isotopically enriched values of the last two millennia oc-
curred during the twentieth century (Figure 7a). This increase
in
δ
18
O is discussed by Thompson et al. [2006a] as being
representative of temperature, since it is in agreement with
NH temperature reconstructions [Mann and Jones, 2003]
and modern meteorological observations [Jones and Moberg,
2003] (Figure 7b). That a similar profile can be determined
independently by different climate recorders gives us confi-
δ
Figure 8.
Aerial photographs taken in 2000, 2006, and 2007 of the
Furtwängler Glacier in the center of the Kibo Crater, Mount Kili-
manjaro, Tanzania, showing the rapid deterioration of this ice
-
dence that the warming trend at the end of the twentieth
century and the beginning of the twenty-first century is
unprecedented in at least the last two millennia.
field.
The open circles indicate identical surface features in each image for
reference. Modi
ed after Thompson et al. [2009].
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