Influence of the Greenland ice sheet melting on the Atlantic meridional overturning circulation

E. Driesschaert(1), T. Fichefet(1), H. Goosse(1), P. Huybrechts(2), I. Janssens(2), A. Mouchet(3), G.Munhoven(3), V. Brovkin(4),S.L. Weber(5) (1) Institut d’Astronomie et de Géophysique Georges Lemaître, Université Catholique de Louvain, 1348 Louvain-la-Neuve, Belgium, driess@astr.ucl.ac.be(2) Vrije Universiteit Brussel, Departement Geografie, B-1050 Brussel, Belgium(3) Université de Liège, Institut d’Astrophysique et de Géophysique, Laboratoire de Physique Atmosphérique et Planétaire, B-4000 Liège, Belgium(4) Potsdam Institute for Climate Impact Research, D-14412, Potsdam, Germany(5) Royal Netherlands Meteorological Institute, 3730 AE De Bilt, The Netherlands

A three-dimensional Earth system model of intermediate complexity including a dynamic ice sheet component has been used to investigate the long-term evolution of the Greenland ice sheet and its effects on the Atlantic meridional overturning circulation (AMOC) in response to a range of stabilized anthropogenic forcings. Our results suggest that the Greenland ice sheet volume should experience a significant decrease in the future. For a radiative forcing exceeding 7.5 W m-2, the modeled ice sheet melts away within 3000 years. A number of feedbacks operate during this deglaciation, implying a strong non-linear relationship between the radiative forcing and the melting rate. In the most extreme scenario considered, the freshwater flux from Greenland into the surrounding oceans is higher than 0.1 Sv during a few centuries. This is however insufficient to induce a shutdown of the AMOC in the model.

• AGISM: Huybrechts, P., 2002, Sea-level changes at the LGM from ice-dynamic reconstructions of the Greenland and Antarctic ice sheets during the glacial cycles, Quaternary Science Reviews, 21, 1-3, 203-231.

•ECBILT-CLIO: Goosse, H., Selten, F.M., Haarsma, R.J., Opsteegh, J.D., 2001, Decadal variability in high northern latitudes as simulated by an intermediate-complexity climate model, Annals of Glaciology, pp 525,532.

• LOCH: Mouchet, A. and L. François, 1996: Sensitivity of a global oceanic carbon cycle model to the circulation and to the fate of organic matter: Preliminary results. Phys. Chem. Earth, 21, 511-516.

• VECODE: Brovkin V., A. Ganopolski, Y. Svirezhev, 1997, A continuous climate-vegetation classification for use in climate-biosphere studies, Ecological modelling, 101, 251-261.

• LOVECLIM: E. Driesschaert, T. Fichefet, H. Goosse, P. Huybrechts, I. Janssens, A. Mouchet, G. Munhoven, V. Brovkin, and S.L. Weber, 2007, Modeling the influence of Greenland ice sheet melting on the Atlantic meridional overturning circulation during the next millennia, Geophys. Res. Lett., 34,L10707, doi:10.1029/2007/GL029516.

Model references:

SP simulations • Initial conditions: yr 1500 simulated by an equilibrium simulation (called CTL hereafter). • Length: 1000 years. • Anthropogenic forcing: changes in concentration of CO

2 following

the stabilization profiles intended for the IPCC AR4.IS simulations• Initial conditions: yr 1500 simulated by an equilibrium simulation (called CTL hereafter).• Anthropogenic forcing: CO

2 concentration increased by 1% per

year until it reaches twice (four times) its initial value and remained unchanged afterwards.• Length: 3000 years.

Model description

Numerical experiments

SRESA2F A sensitivity experiment has been performed in order to quantify the influence of the interactions between the ice sheets and climate. SRESA2F has the same forcing as SRESA2 but neither the geometry nor the freshwater flux are allowed to evolve

Figure 4: (a) Annual mean freshwater flux from Greenland averaged over integration years 950–1000 as a function of the radiative forcing. (b) Change in annual mean NADW export at 30° S in the Atlantic basin averaged over integration years 950–1000 as a function of the radiative forcing. The plotted changes are relative to the CTL value. In both panels, each lozenge corresponds to one particular simulation (see Figure 1), the empty lozenges correspond to SRESA2F.

Impact of the Greenland deglaciation on the THC

Figure 3: time evolution of (a) the freshwater flux from Greenland (Sv) in the experiment SRESA2 and (b) the exportation of NADW at 30°S (Sv) in experiments SRESA2 and in SREA2F.

a

b

Time (Years)

SRESA2SRESA2F

Results

. LOVECLIM

ECBILT: ECBILT: atmosphereatmosphere

AGISM: AGISM: ice sheetsice sheets

VECODE: VECODE: vegetationvegetation

CLIO: ocean - CLIO: ocean - sea icesea ice

LOCH: oceanic carbon LOCH: oceanic carbon cyclecycle

AlbedoTemperaturePrecipitationatm. CO

2 concentration

ocean and sea ice state

Carbon fluxes

atmospheric CO2

concentration

Surface wind and pressure

Temperature Precipitation

Heat flux at the ice shelves base

Fresh water and latent heat fluxes

TopographySoil type

Time (Years)

SRES simulations• Initial conditions: yr 2000 simulated by a transient simulation over the period 1500-2000.• Length: 1000 years for SRESA1B and SRESB1, 2000 years for SRESA2. • Natural forcings: solar irradiance fixed to its yr-2000 value, no volcanic activity.• Anthropogenic forcings: changes in concentration of 20 greenhouse gases and sulphate aerosols. CO

2 atmospheric

concentration is prescribed. The forcing follows SRES scenarios until 2100 and is held constant thereafter.

Figure 2: time series of the annual mean Greenland ice sheet volume for all experiments. Integration year 0 corresponds to the year 2000 AD for SRES and SP experiments and to the year 1500 AD for IS experiments. In the simulations SRESA2 and IS4XCO2, the Greenland ice sheet has almost disappeared in 2000 years.

Figure 1: time series of the radiative forcing for all experiments. Integration year 0 corresponds to the year 2000 AD for SRES and SP experiments and to the year 1500 AD for IS experiments. The equivalent CO2 concentrations are also given following the IPCC [2001] formula: F = 5.35 ln(C/C0), where C and C0 (= 280 ppmv) are the current and reference CO2 concentrations, respectively, and F is the radiative forcing.

Evolution of the Greenland ice sheet volume

Our study suggests that, for a wide range of greenhouse gas stabilization profiles, the Greenland ice sheet volume should significantly decrease in the future. In the most extreme case considered here, which corresponds to a sustained radiative forcing of 8.5 W m-2, Greenland becomes ice-free in about 2000 years.

We also found that the freshwater flow from the melting Greenland ice sheet into the neighbouring oceans, which, for the most extreme forcing scenario selected, peaks at 0.11 Sv and remains above 0.1 Sv for three centuries, is not large enough to trigger a shutdown of the AMOC in our model. Only a slight decline of this circulation is simulated. Some models are, however, more responsive to freshwater perturbations than ours.

Finally, we showed that climate feedbacks considerably enhance the greenhouse gas-induced warming over Greenland and induce a highly non-linear response of the melting rate to the radiative forcing. This stresses the importance of incorporating the two-way interactions between the Greenland ice sheet and climate in climate and sea level change projections at the millennial time scale.

Conclusion

Relationship between the freshwater flux from Greenland and the THC decrease