Doubling time is the time required for a population to double in number if the current rate of growth continues. If the rate is 1.4, then the doubling time is about 49 years. The slow growth of early years was because the mortality was very high. The result was a very slow growth of population. In more recent years, world population grows at a much faster rate due the declines in mortality.
Fertility rates are lower today, but mortality rates are even lower.
The result is a fast growing world population. You actually only need the total population in order to get a rough idea of the growth rate. However, ideally you need to look at the death rates, migration, and more importantly the age-structure of women of reproductive age. 5 Describe six consequences of population growth and how they might affect you personally. 6 Describe the four major sources of demographic information.