Assessment of the Greenland ice sheet–atmosphere feedbacks for the next century with a regional atmospheric model coupled to an ice sheet model
Authors: Le clec'h Sébastien , Charbit Sylvie , Quiquet Aurélien , Fettweis Xavier , Dumas Christophe , Kageyama Masa , Wyard Coraline , Ritz Catherine 
Published: February 2019

Abstract. In the context of global warming, growing attention is paid to
the evolution of the Greenland ice sheet (GrIS) and its contribution to
sea-level rise at the centennial timescale. Atmosphere–GrIS interactions,
such as the temperature–elevation and the albedo feedbacks, have the potential
to modify the surface energy balance and thus to impact the GrIS surface mass
balance (SMB). In turn, changes in the geometrical features of the ice sheet
may alter both the climate and the ice dynamics governing the ice sheet
evolution. However, changes in ice sheet geometry are generally not
explicitly accounted for when simulating atmospheric changes over the
Greenland ice sheet in the future. To account for ice sheet–climate
interactions, we developed the first two-way synchronously coupled model
between a regional atmospheric model (MAR) and a 3-D ice sheet model (GRISLI).
Using this novel model, we simulate the ice sheet evolution from 2000 to 2150
under a prolonged representative concentration pathway scenario, RCP8.5. Changes in surface elevation and ice
sheet extent simulated by GRISLI have a direct impact on the climate
simulated by MAR. They are fed to MAR from 2020 onwards, i.e. when changes in
SMB produce significant topography changes in GRISLI. We further assess the
importance of the atmosphere–ice sheet feedbacks through the comparison of
the two-way coupled experiment with two other simulations based on simpler
coupling strategies: (i) a one-way coupling with no consideration of any
change in ice sheet geometry; (ii) an alternative one-way coupling in which
the elevation change feedbacks are parameterized in the ice sheet model
(from 2020 onwards) without taking into account the changes in ice sheet
topography in the atmospheric model. The two-way coupled experiment simulates
an important increase in surface melt below 2000 m of elevation, resulting in
an important SMB reduction in 2150 and a shift of the equilibrium line
towards elevations as high as 2500 m, despite a slight increase in SMB over
the central plateau due to enhanced snowfall. In relation with these SMB
changes, modifications of ice sheet geometry favour ice flux convergence
towards the margins, with an increase in ice velocities in the GrIS interior
due to increased surface slopes and a decrease in ice velocities at the
margins due to decreasing ice thickness. This convergence counteracts the SMB
signal in these areas. In the two-way coupling, the SMB is also influenced by
changes in fine-scale atmospheric dynamical processes, such as the increase
in katabatic winds from central to marginal regions induced by increased
surface slopes. Altogether, the GrIS contribution to sea-level rise, inferred
from variations in ice volume above floatation, is equal to 20.4 cm in 2150.
The comparison between the coupled and the two uncoupled experiments suggests
that the effect of the different feedbacks is amplified over time with the
most important feedbacks being the SMB–elevation feedbacks. As a result, the
experiment with parameterized SMB–elevation feedback provides a sea-level
contribution from GrIS in 2150 only 2.5 % lower than the two-way coupled
experiment, while the experiment with no feedback is 9.3 % lower. The
change in the ablation area in the two-way coupled experiment is much larger
than those provided by the two simplest methods, with an underestimation of
11.7 % (14 %) with parameterized feedbacks (no feedback).
In addition, we quantify that computing the GrIS contribution to sea-level
rise from SMB changes only over a fixed ice sheet mask leads to an
overestimation of ice loss of at least 6 % compared to the use of a time
variable ice sheet mask. Finally, our results suggest that ice-loss
estimations diverge when using the different coupling strategies, with
differences from the two-way method becoming significant at the end of the
21st century. In particular, even if averaged over the whole GrIS the
climatic and ice sheet fields are relatively similar; at the local and
regional scale there are important differences, highlighting the importance
of correctly representing the interactions when interested in basin scale
changes.