The margin of the Greenland ice sheet moves back and forth
depending on the climate. During cold times, like the ice ages,
the ice margin advances and covers the land and may even form ice
shelves extending into the ocean. In a warming climate, the ice
margin retreats, revealing freshly exposed land.
The most
sensitive part of the ice sheet margin is the outlet glaciers
extending into the deep fjords. These may respond on an annual
basis to changes in ocean temperature and circulation. When warm
ocean water reaches the fjords and get in direct connection with
the outlet glacier tongue, rapid changes may occur as has been
seen for a large number of outlet glaciers in Greenland over the
last decade.
The ice margin terminating on land responds
more slowly to climate change, but is an area of considerable
interest to the Greenland society. This part of the ice margin is
utilized for generating hydro-electric power and also covers
mineral deposits or poses challenges to neighbouring natural
resource exploration. For these activities, local glaciers and ice
caps separated from the ice sheet proper are just as relevant.
Monitoring the retreat of the ice margin requires knowledge of
where the ice margin is in the first place. This was not
well-defined at the onset of PROMICE and consequently, an effort
was initiated to produce a map of all the Greenland ice masses,
including both the ice sheet and the localized ice masses
surrounding it, based on aerophotogrammetic surveys conducted
1978-1987 (see Figure 1). Airborne surveys are more detailed than
satellite images and given the necessary attention, the error in
determining the margin position is less than 10 metres.
The PROMICE map was recently published and the result can be seen
in Figure 2. The total glacierized area including the ice sheet
and all local glaciers and ice caps was calculated to be 1,804,638
± 2,178 km2 (Citterio & Ahlstrøm, 2013). Of this area,
1,716,555 ± 947 km2 was the Greenland ice sheet and 88,083 ± 1,240
km2 consisted of local glaciers and ice caps.
As each
major region in Greenland was surveyed in different years, the
resulting map is only strictly synchronous within each large
sub-region as shown in Figure 1. Yet, the PROMICE map has already
been used for area change assessment yielding a loss of 2560 ± 260
km2 between the mid 1980s and 2011 (Kargel et al., 2012).
This number was obtained by updating the PROMICE map to summer
2011 using NASA LANCE Rapid Response MODIS imagery at 128 sites,
while excluding known glacier surges. The observed area shrinkage
from this analysis was c. 92 km2 per year from the 1980s to 2011.
This estimate is lower than recent estimates focused on outlet
glacier fluctuations over the last decade, partly because outlet
glaciers changed more slowly prior to the last decade and partly
because land-terminating parts of the ice margin reacts more
slowly to climate change.
An ice mask based on the PROMICE
map is available here
