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Several research groups have developed GCMs to explore 

 the effects of changes in the atmosphere's composition: 

 NOAA Geophysical Fluid Dynamics Laboratory (GFDL), the National 

 Center for Atmospheric Research (NCAR), and the Goddard Insti- 

 tute for Space Studies (GISS) have undertaken three well-known 

 efforts in the United States. _^£/ Each group has run its GCM 

 assuming the current atmosphere and doubled CO2 concentrations. 

 While some of the model predictions differ about climatic 

 change, they are qualitatively similar. 



The GISS group has used its model to estimate the size 

 and geographical distribution of snow and ice in a warmer 

 world. In fact, the group has done two doubled CO2 experiments, 

 which differed only in their techniques for computing sea ice, 

 a key factor in determining the effects of atmospheric change 

 on Antarctica. One model run started with somewhat more sea 

 ice, and one with somewhat less sea ice than currently exists in 

 Antarctica. This provided a method to check the sensitivity 

 of the models to this initial condition. 



Both doubled CO2 runs showed a substantial net melting 

 of land-based snow and ice. Both "control" runs (i.e., using 

 current atmosphere and climate) showed nearly stable amounts 

 of snow and ice. Thus, the results of the four runs increase 

 our confidence that global warming will significantly decrease 

 snow and ice resting on land. The run deemed most accurate 

 showed a melting equivalent to a sea level rise of about one 



