1.Introduction up in the Antarctic ice sheet, equivalent to

1.Introduction

Approximately
25.7×106 km3 of ice is currently locked up in the
Antarctic ice sheet, equivalent to 61.1 m of global sea level (msl), while the
Greenland ice sheet harbours 2.85×106 km3 of ice, or 7.2
msl (Marshall, 2005). This makes the Antarctic ice sheet 10 times the size of the Greenland
ice sheet. Bedrock beneath the central part of the Greenland ice sheet is
remarkably flat and close to sea level, but the ice sheet is fringed almost
completely by coastal mountains, through which it is drained by many glaciers (Lytle, 2008). Antarctica, on
the other hand, is so large that is greatly influences a climate of its own,
which also affects the surrounding oceans.

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Mass is added to the ice sheets in the form of precipitation (rain and snow),
and removed by processes such as iceberg calving, and basal and surface
melting. These processes are known as accumulation and ablation, respectively,
and the difference between the two is known as mass balance. Previous records show that ice sheets can decrease in
size at a much faster rate than that at which they grow. Surface melting rates
can be much larger than snowfall rates because ice discharge may be accelerated
by processes such as enhanced basal lubrication, or by the removal of restraint
to flow when marginal ice shelves disintegrate (Allison
et al., 2009). Flow within a glacier is
caused mainly by an increase in the surface slope, which arises from an
imbalance between accumulation and ablation. An imbalance results in an
increase in shear stress on the glacier, causing it to flow. The velocity at
which it flows, and deformation increase as the line of equilibrium between the
two is approached.

 

 

2.Results and discussion

 

2.1. Increasing air
temperatures

 

Because of the low mass turnover— due to the
extremely cold surface temperatures, low accumulation rates and large size—the
Antarctic ice sheet is considered to remain relatively stable for 100- year
time scales under warming scenarios of up to 20°C (Pattyn, 2006). The results
of the model show that, for the Antarctic ice sheet, it would take more than a
25K increase in air temperature for the ice sheet to be completely melted.

In Figure 1, we can see that at an increase in
global mean air temperature of 25K causes a significant reduction in the size
of the ice sheet. The velocity has decreased from around 550m/yr to only
100m/yr. This could be caused by a decrease in the slope angle of the ice
sheet. The rate of ablation is significantly higher than the rate of
accumulation. A potential cause of this could be that, since the ice sheet has
become so small, it can no longer regulate its own local climate, and so is
being affected by the 25K global temperature increase, and perhaps even warming
sea temperatures.

 

 

 

 

 

 

 

 

 

 

Figure 1: Affects a 25K increase in global mean
temperature has on the Antarctic ice sheet.

 

Air temperatures over the ice sheets can be
extremely cold, meaning little moisture can be held resulting in very low
precipitation rates. In terms of their precipitation, the ice sheets are
deserts, but because of their vast size, there is still a large annual addition
of snow mass (Allison et al., 2009). An increase in air temperature enables the air mass to have a higher
moisture content, causing an increase in the rate of precipitation, increasing
the accumulation rate. Figure 2 shows that the Antarctic ice sheet volume
doesn’t start to decrease until about 9K, in fact it increases slightly before
this. Since the ice sheet is so large, and able to regulate its own localised climate,
it takes far longer for increasing global temperatures to have an effect on its
mass balance.

The main cause of ice loss from the Antarctic ice
sheet is basal melting and iceberg calving. Glaciers and streams of ice from
the ice sheet flow into ice shelves, which thin toward their seaward ice
fronts. Since Antarctica is at a very high latitude, incoming shortwave
radiation has a much larger area that it must be dispersed over, meaning less
energy is available to melt the snow and ice.

 

 

 

 

 

 

 

 

 

 

Figure 2: The affects an increasing global mean air
temperature (TFOR) has on the ice volume percentage of the Greenland and
Antarctic ice sheets.

 

Observations of the Greenland ice sheet over the last two decades show
an increase in ice loss rate, associated with speeding up of glaciers and
enhanced melting (Khan et al., 2015).
Greenland climate is strongly affected by its proximity to other land masses
and to the North Atlantic, with the Gulf Stream to the south and regions of
North Atlantic deep-water production to the east and west (Bamber and Payne, 2004).

 

 

 

 

 

 

 

Figure 3: Affects a 6K increase in global mean
temperature has on the Greenland ice sheet.

 

Present-day conditions of the
Greenland ice sheet in the GRANTISM model, show that the net rate of ablation
is around -10mm/h at the edges of the ice sheet, increasing to 0 at the centre.
The accumulation decreases from about 0.5mm/h at the centre, to almost -1mm/h
at the ice sheet edges. Roughly, a 10°C warming doubles the vapor content of
saturated air. Once air saturates, further cooling drives condensation and the
formation of cloud droplets, which precipitate if they grow large enough (Cuffey and Paterson, 2011). This, and a present atmospheric
uplift causes a great amount of snowfall onto the ice sheet. In Figure 3 we can
see that the ablation and accumulation rates remain fairly similar after a 6K
temperature increase, even though the size of the ice sheet has reduced
significantly. The present temperature velocity is around 400 to 700m/yr at the
edges of the ice sheet, and almost 0m/yr at the centre. The reason for this is
that there is no slope angle at the centre of the ice sheet, meaning there is
no (or, very little) ice flow.

 

From the results above, it is
clear that the Greenland ice sheet has a far more sensitive response to increasing
temperatures than the Antarctic ice sheet does. Since the Greenland ice sheet
is about 10 times smaller, it has a faster response to environmental changes
than Antarctica. Looking at Figure 2, we can see that it only takes a
temperature of about 6K to completely melt away the Greenland ice sheet,
whereas the Antarctic ice sheet only starts to show a decrease in ice volume
percentage between 9-10K above present conditions.
Antarctica is at a higher latitude than Greenland (around 90° 00′ S and 72° 00′
N respectively, (Mapsofworld.com, 2002)). Incoming
shortwave radiation received at the Antarctic ice sheet is diffused over a
larger area, whereas in Greenland it is not spread out as much, meaning this
energy has more of an affect on the ice and snow.

Antarctica is a
large continent, and so the majority of its area is not affected by maritime
air masses. Further in land, there is an increase of net radiation due to
clearer skies. Greenland, however, is affected more by maritime air masses.
These air masses are very moist and condense easily to form cloud. Sea breezes
then blow this maritime air over ice sheets, leading to a transfer of sensible
heat. Murray et al. (2010) found
that during the 2000s, there was a speedup in the Greenland tidewater outlet
glaciers, followed by a slowdown. It is thought that the speedup was caused by
the contact of warm ocean water and the Greenland glaciers. The cold East
Greenland Coastal Current waters were weakened during the speedup, and regained
their strength during the slowdown. This is an example of a negative feedback,
similar to that suggested with the loss of ice from the Greenland ice sheet and
an increasingly warming climate. It was suggested that regional ocean forcing,
such as this, have a major role in the control of the Greenland glacier
dynamics, and that the water in the local glacier fjords will become a buffer
between the ocean water and the glaciers.

 

 

 

 

 

2.2. Decreasing air
temperatures

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4: The affects a decreasing global mean
air temperature (TFOR) has on the ice volume percentage of the Greenland and
Antarctic ice sheets.

 

 

Figure 4 shows that a decrease
in the global mean air temperature results in an increase of the ice volume of
both the Antarctic and Greenland ice sheets. In some areas, a low relief means
a small drop in temperature would cause a large southward shift of the
equilibrium line, allowing the area of accumulation to expand considerably (Cutler et al., 2003). This would cause the ice volume
to increase greatly. At the Antarctic ice sheet, a -10K reduction in global air
temperatures causes the ablation rate to become 0mm/h over the entire ice
sheet, while the accumulation rates are around 0.5mm/h at the edges of the ice
sheet. If the edges of the ice sheet are constantly growing, this will cause
there to be a drop in sea level.

A colder climate causes a
reduction in the net accumulation rates on the Antarctic ice sheet. The extremely cold temperature means that very little moisture can be
held in the Antarctic air mass resulting in very low precipitation rates.
However, this is balanced out due to the colder climate causing the ice to
become stiffer and less susceptible to deformation.
The
growth of the Greenland ice sheet is limited by ablation and ice calving into
the sea, found mainly at the edge of the ice sheet. The
present ice mass raises the surface to more than 3000 m above the bedrock,
which creates a climate very different from that which would have existed on
bare bedrock if the ice sheet were to have been removed (Letréguilly, Huybrechts and Reeh, 1991). Oerlemans and Van der Veen
(1984) suggested that the current climate would be so warm that the ice sheet
would not be able to re-form.

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