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An estimate of GNP for each region is used as a proxy for 

 the overall level of economic activity and as an index of income. 

 While the level of GNP is an input to the system, it is derived 

 from assumptions on demographic growth and levels of labor 

 product ivity . 



World population was assumed to stabilize by 2075, based on 

 work done by Keyfitz et al .jV Labor productivity growth for 

 the high scenario was assumed to be 3.2 percent per year in 1980, 

 declining linearly to 2.2 percent per year by 2100. 



In the low scenario, labor productivity growth was assumed to 

 start at 2.2 percent per year in 1980 and to decline linearly to 

 1.7 percent per year by 2100. Both the high and low estimates of 

 productivity growth utilize rates below the average rate of increase 

 around the world in the last thirty years. The mid-range estimate 

 was an interpolation of the two curves. 



The energy technology parameter is a time-dependent index of 

 energy productivity, given constant energy prices and real incomes. 

 That is to say, it reflects improvements in energy efficiency 

 beyond those stimulated by increases in real prices or decreases 

 in real income. In the past, technological progress has had an 

 important influence on energy use in the manufacturing sector of 

 advanced economies. The inclusion of an energy technology para- 

 meter allows scenarios to be developed which incorporate either 

 continued improvements or technological stagnation as an integral 

 part of energy use scenarios. A constant improvement of 25 



