Geoscience Reference
In-Depth Information
Figure 8.6.
Cumulative annual mass balance
(Gitatonnes and equivalent sea level rise)
over the 1992-2011 period for Greenland,
Antarctica, and Greenland and Antarctica
combined (adapted from Shepherd et al.,
2012
, by permission of AAAS). (See plate
section for color version.)
accelerated flow, increasing iceberg calving rates. Jakobshavn Glacier, which drains
6.5 percent of the ice sheet and produces around 10 percent of all Greenland ice-
bergs, has been the focus of considerable research. One explanation for the accel-
eration, termed the “Zwally effect” (named after Jay Zwally), is that (1) there has
been an increase in seasonal surface melt and runoff; (2) the extra meltwater reaches
the glacier base via moulins and crevasses; and (3) the flow is large enough and is
added quickly enough to reduced friction at ice/bedrock interface, meaning more
basal sliding of the ice (Zwally et al.,
2002
). Although the Zwally effect has been
observed at Swiss Camp, it is seasonal in nature, and it not viewed as the primary
contributor. Attention has shifted to thinning of the calving tongues of the outlet
glaciers, which reduces backpressure (Thomas,
2004
). This thinning can arise from
both surface melt and melt at the base of the calving tongues by ocean heat. If there
is thinning, the glacier becomes more buoyant, exerting less backpressure on the
upstream ice. Thinning also reduces backpressure through reduced friction on the
fiord walls. The reduced resistive force at the calving front is then propagated up
the glacier via longitudinal extension. This results in both a retreat of the floating
tongue and a more rapid flow of the glacier.
8.2
Polar Desert
8.2.1
Basic Characteristics
Recall from
Chapter 2
that the term tundra applies to a wide range of Arctic vegeta-
tion, although its basic meaning is treeless. Tundra accounts for about 20 percent of
the land surface within the Arctic Circle. At the southern end of tundra, we encoun-
ter the forest-tundra ecotone, some 50-100 km wide, which in turn transitions to
boreal forest. The range of climatic conditions in tundra areas is large (Barry et al.,
1981
), not only between major tundra types (
Chapter 2
), but over short distances
within a tundra type. The latter issue is highlighted in energy balance studies from
the LAII program (
Table 5.4
) Polar desert is the most extreme of tundra types. Polar
desert is particularly fascinating, and warrants more discussion.
Polar desert is widespread in the Canadian Arctic Archipelago, northern
Greenland, and the islands of the Siberian Arctic (Korotkevich,
1972
). In the New
