Modeling degree of the forcings expected to form a frost sheet on the Antarctica advise that ice progress responds nonlinearly to help you altering heat [ Oerlemans, 2004 ]. So it nonlinearity is a result of feedbacks such as for instance height–mass harmony views (the extra air conditioning caused by the temperature gradient from the surroundings since the a frost sheet grows vertically), rain viewpoints, and you can frost-albedo opinions [ Oerlemans, 2002 ; Notz, 2009 ]. The fresh new initiation from glaciation displays a limit effect, having growth performing as the descending accumulated snow line intercepts large topographic countries [ Oerlemans, 1982 ; Pollard, 1982 ; a ].
A study of give (inception) and contrary (deglaciation) design runs examined ice sheet hysteresis (recreated when you look at the Figure 5) [ Pollard and ]
In Figure 5, the model output from Pollard and ] for the formation and melting of the East Antarctic Ice Sheet is shown. The original CO2 axis is converted to a temperature (average global surface) axis using the climate sensitivity of their GCM (2.5°C per fcn chattips doubling of CO2 [ Thompson and Pollard, 1997 ]) and accounting for the logarithmic dependence of temperature to CO2. Ice volumes are converted to sea levels assuming an ice-free sea level of 64 m above present [ Lythe and V ] and adjusting for the change in volume with change in state from ice to seawater. The original CO2 forcing and model time is included but converted to a logarithmic scale.
The results of b] show the formation of ice on Antarctica in multiple stages under various atmospheric CO2 concentrations. With a high atmospheric CO2 concentration of 8 ? preindustrial CO2 (PIC), equal to 2240 ppmv, ice is limited to mountain glaciers in the Transantarctic Mountains and Dronning Maud Land. Isolated ice caps first form in the high-elevation regions of Dronning Maud Land and the Gamburtsev and Transantarctic Mountains as atmospheric CO2 v). In the model, as CO2 falls to ?2.7 ? PIC (?760 ppmv) a threshold is crossed and height-mass balance feedback leads to the three isolated ice caps coalescing into a continent sized ice sheet. Although there are multiple “steps” in their results, there are two major steps marking (1) the transition from no ice to isolated mountain ice caps and (2) the transition to a full ice sheet (see Figure 5a). The total sea level shift in this two-step model of Pollard and ] is on the order of 50 m (?33 m after accounting for hydroisostasy for comparison with the NJ record [ Pekar et al., 2002 ]); Figure 5 does not include other causes of sea level change, such as thermosteric sea level change.
Starting with no ice, a descending snow line generates rapid ice mass gains as it meets the mountain regions. Once a full continental ice sheet has formed, the snow line must rise considerably higher to achieve negative net mass balance and initiate broad-scale retreat. This is because the steep outer slopes of the ice sheet and the atmospheric lapse rate require much warmer conditions to produce enough surface melt area around the margins to overcome the net interior snowfall (which at present is balanced by Antarctic iceberg and shelf discharge) [ Oerlemans, 2002 ; Pollard and ]. In the model runs for the EAIS, this hysteresis equates to a difference of 0.5 ? PIC between forward and reverse runs. Although the forward run required atmospheric CO2 to descend past ?2.7 ? PIC for inception to begin, the reverse run required atmospheric CO2 to rise above ?3.2 ? PIC (?900 ppmv) for deglaciation to begin. These runs include orbital variations; without orbital variations, the hysteresis is greater. The model shows formation of ice in a series of steps between multiple quasi-stable states. The snow line has to descend further with additional cooling before there is additional growth. With the continent fully glaciated, further growth is inhibited when the ice sheet reaches the coastline [ Pollard and ].