Item

Abstract

The surface mass balance for Greenland and Antarctica has been
calculated using model data from an AMIP-type experiment for the period
1979-2001 using the ECHAM5 spectral transform model at different
triangular truncations. There is a significant reduction in the
calculated ablation for the highest model resolution, T319 with an
equivalent grid distance of ca 40 km. As a consequence the T319 model
has a positive surface mass balance for both ice sheets during the
period. For Greenland, the models at lower resolution, T106 and T63, on
the other hand, have a much stronger ablation leading to a negative
surface mass balance. Calculations have also been undertaken for a
climate change experiment using the IPCC scenario A1B, with a T213
resolution (corresponding to a grid distance of some 60 km) and
comparing two 30-year periods from the end of the twentieth century and
the end of the twenty-first century, respectively. For Greenland there
is change of 495 km(3)/year, going from a positive to a negative surface
mass balance corresponding to a sea level rise of 1.4 mm/year. For
Antarctica there is an increase in the positive surface mass balance of
285 km(3)/year corresponding to a sea level fall by 0.8 mm/year. The
surface mass balance changes of the two ice sheets lead to a sea level
rise of 7 cm at the end of this century compared to end of the twentieth
century. Other possible mass losses such as due to changes in the
calving of icebergs are not considered. It appears that such changes
must increase significantly, and several times more than the surface
mass balance changes, if the ice sheets are to make a major contribution
to sea level rise this century. The model calculations indicate large
inter-annual variations in all relevant parameters making it impossible
to identify robust trends from the examined periods at the end of the
twentieth century. The calculated inter-annual variations are similar in
magnitude to observations. The 30-year trend in SMB at the end of the
twenty-first century is significant. The increase in precipitation on
the ice sheets follows closely the Clausius-Clapeyron relation and is
the main reason for the increase in the surface mass balance of
Antarctica. On Greenland precipitation in the form of snow is gradually
starting to decrease and cannot compensate for the increase in ablation.
Another factor is the proportionally higher temperature increase on
Greenland leading to a larger ablation. It follows that a modest
increase in temperature will not be sufficient to compensate for the
increase in accumulation, but this will change when temperature
increases go beyond any critical limit. Calculations show that such a
limit for Greenland might well be passed during this century. For
Antarctica this will take much longer and probably well into following
centuries.