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Two mid-range scenarios were developed. They differ only 

 in their estimates of snow and ice contributions to sea level 

 rise; the estimates of the mass transfer of ice and snow from 

 land to sea are much less reliable than estimates of thermal 

 expansion. The mid-range scenarios used assumptions that fell 

 between those used in the low and high scenarios (except in 

 the case of fraction airborne, where we used the ORNL model). 

 Although it is currently impossible to estimate probabilities, 

 we believe that the actual sea level rise will probably fall 

 between the two mid-range scenarios. 



The remainder of this chapter presents the assumptions 

 and models used to develop the scenarios. Technical details 

 about the assumptions and models are provided in Appendix B. 



YEARLY RISES IN SEA LEVEL WERE SIMULATED 



The yearly rise in sea level can be estimated by project- 

 ing yearly changes in the natural systems that determine it. 

 Four systems must be considered: atmospheric composition, 

 the climate, the oceans, and the cryosphere (the part of the 

 world that is ice or snow). Changes in the atmosphere will 

 produce changes in temperature, which in turn produce thermal 

 expansion and the transfer of snow and ice from land to sea 

 (Figure 3-2). Below, we also discuss possible changes in 

 other factors that determine global temperature, such as changes 

 in solar irradiation or volcanic aerosols. (A permament 

 increase in the average level of aerosols would block sunlight 

 and cool the earth.) 



