Elimination of the Greenland Ice Sheet in a High CO2 Climate

Abstract

Projections of future global sea level depend on reliable estimates of changes in the size of polar ice sheets. Calculating this directly from global general circulation models (GCMs) is unreliable because the coarse resolution of 100 km or more is unable to capture narrow ablation zones, and ice dynamics is not usually taken into account in GCMs. To overcome these problems a high-resolution (20 km) dynamic ice sheet model has been coupled to the third Hadley Centre Coupled Ocean-Atmosphere GCM (HadCM3). A novel feature is the use of two-way coupling, so that climate changes in the GCM drive ice mass changes in the ice sheet model that, in turn, can alter the future climate through changes in orography, surface albedo, and freshwater input to the model ocean. At the start of the main experiment the atmospheric carbon dioxide concentration was increased to 4 times the preindustrial level and held constant for 3000 yr. By the end of this period the Greenland ice sheet is almost completely ablated and has made a direct contribution of approximately 7 m to global average sea level, causing a peak rate of sea level rise of 5 mm yr Ϫ1 early in the simulation. The effect of ice sheet depletion on global and regional climate has been examined and it was found that apart from the sea level rise, the long-term effect on global climate is small. However, there are some significant regional climate changes that appear to have reduced the rate at which the ice sheet ablates.

Rapid early Holocene deglaciation of the Laurentide ice sheet

“…Net melt is transported NGS-2008-04-00345A 10 through a 9 kyr BP-appropriate river routing scheme based on LIS topography and North American isostasy 7 . Ideally, a higher resolution model with ice dynamics would be employed, but these usually use simplified predictions of summer melt and are asynchronously coupled to GCMs 3,4 , not allowing the ice sheet hydrology to directly communicate with the atmosphere and ocean systems. Within the model, there is an imposed global water balance, which modulates the ice calving term to prevent long-term salinity drift in the ocean, with the Southern and Northern Hemispheres partitioned separately.…”

“…Currently, ablation, ice streaming and calving control GIS mass loss 2 . However, ice streaming and calving will decrease or cease if the GIS retreats inland 4 , making it more analogous to the LIS 7 . Nevertheless, predictions of the rate of sea level rise from the GIS by the end of this century in the A1B scenario 1 are 6 to 40 times smaller than the estimated rate of LIS mass loss in the early Holocene.…”

“…This uncertainty arises in part because the period of direct observations available to create empirical predictions and to test the current generation of ice sheet models is extremely short [1][2][3][4][5] . Examination of past ice sheet-climate interactions provides a record of ice sheet growth and decay longer than modern observations.…”

An efficient regional energy-moisture balance model for simulation of the Greenland Ice Sheet response to climate change

“…Over recent decades, significant GIS mass losses have been diagnosed by on-site measurements (Abdalati and Steffen, 2001), InSAR velocity measurements (Rignot and Kanagaratnam, 2006;Rignot et al, 2008), GRACE satellite measurements of gravity changes (Velicogna and Wahr, 2005;Ramillien et al, 2006;Velicogna, 2009) and regional modeling (Box et al, 2006). While it is expected that only a rather small portion of the GIS can melt over the 21st century (Lemke et al, 2007), modeling studies ( Van de Wal and Oerlemans, 1994;Huybrechts and de Wolde, 1999;Ridley et al, 2005;Charbit et al, 2008) show that on the millennial time scale, the GIS can melt completely if temperatures stay above a certain threshold.…”

“…This requires bi-directional coupling between climate and ice sheet models, which makes these models even more computationally expensive. So far, only a few experiments of this sort have been performed, using rather coarse resolution GCMs (e.g., Ridley et al, 2005;Mikolajewicz et al, 2007;Vizcaíno et al, 2008).…”