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characteristics of the ice sheet induced by refreezing will 



lead to faster deglaciation. (A brief discussion appears in 



the next section and in Appendix C.) 



Another important feature of the GISS modeling is that the 



estimated melting of one centimeter per year was computed on 



the basis of a world whose atmospheric levels of CO2 have 



already doubled. Thus, the GCM results do not directly provide 



a realistic time trend of sea level rise as CO2 and other 



greenhouse gases approach and then exceed this level of warming. 



Computation of a time trend would have required estimating 



(1) the percentage of the meltwater that runs off into the 



oceans; (2) the degree to which the meltwater that does not 



run off accelerates glacial movement; and (3) the scale of 



melting that occurs as the earth warms. These steps could not 



be undertaken in this study. Thus, instead of directly using 



the one centimeter per year of melting as a scenario, the GISS 



output was taken as a qualitative confirmation that ice and 



snow can be expected to contribute significantly to sea level 



r ise . 



Deglaciation as a Source of Land-to-Sea Transfer of 

 Snow and Ice 



The possibility of a major deglaciation (the removal of 



ice by a process other than melting in place) cannot be ignored. 



Geological evidence suggests that changes in global temperatures 



can cause the large ice sheets that rest on Antarctica to break 



up and "fall" into the oceans. For example, during the last 



