Climate Change in the Arctic - The Changing Arctic Environment: The Arctic Messenger - page 268

through some freeze-thaw cycles) transforms into ice. There is evidence (see Harper and
colleagues) that the percolation zone's ability to “absorb” water may act as a sort of buffer
by delaying the loss of meltwater from the glacier. However, if this is the case, its capacity
to soak up melt will vanish once all the pore space is occupied. It is also thought that melt-
water falling through moulins (downwardly directed meltwater channels) in glaciers may
reduce the drag of the glacier on its bedrock (basal drag), but this proposed effect has so far
been less conclusive than it is for mountain glaciers.
We earlier noticed that in addition to surface mass balance, there is a second com-
ponent to the total mass balance of the Greenland ice sheet. This is solid mass discharge
from marine terminating glaciers. Anyone who has seen a glacier calving icebergs into the
ocean will not forget the sight, particularly if large bergs are being born. It is spellbinding.
However, what you see is only a part of the story because the terminus of the glacier is
floating in the sea and is therefore melting from the bottom throughout the year as well as
from the top in summer. Calving of icebergs and sub-ice melting are the major mass loss
mechanisms for these glaciers. It has been estimated that the floating portion of the Peter-
man glacier in north-west Greenland loses 80% of its ice mass from undersea melting be-
fore it calves. This reinforces the importance of knowing the temperature of the seawater
that is sitting below tidewater glaciers.
The big problem with quantifying solid mass discharge is that the basic mechanics
(known as dynamical forces ) for ice behaviour under the enormous pressures of the ice
sheetandintheconstrictingconditionsofanoutletglacierareverypoorlyunderstood.This
is compounded by large variations in flow that can be seen in individual glaciers. It is an
enormous headache for glaciologists trying to relate glacial observations to climate warm-
ingandevenmoresowhenthinkingaboutprojectionsintothefuture.Alittleearlier,Imen-
tioned the Peterman glacier. The floating part of this glacier was about 70 kilometres long
and15kilometreswide.InAugust2010,anicebergmeasuring260squarekilometresbroke
free, reducing its area and volume by about 25% and 10%, respectively. This was followed
in July 2012 by the calving of a 130-square-kilometre berg. How can trends be quantified
in these types of discharge? You can see that small errors when dealing with such things