 In the 1950s Canada's population growth rate was over 3% per year, so extrapolating the current growth rate of 0.9% for many decades (implied by the doubling time) is unjustified unless we have examined the underlying causes of the growth and determined they will not be changing significantly over that period.The equivalent concept to "doubling time" for a material undergoing a constant negative relative growth rate or exponential decay is the half-life.The equivalent concept in base-"e" is "e"-folding.