More variable tropical climates have a slower demographic growth

Résumé En

A classical approach to assess the amplitude of rainfall variations is based on the coefficient of variation. Using worldwide station data for the twentieth century, an alternative method, involving the development of a polynomial fit, was shown to be more relevant to semi-arid climates. The results singularised the tropical and subtropical regions, whose amplitude of interannual rainfall variability was larger than that of the extra-tropical regions, for a given mean rainfall value. However, the tropical belt also showed large contrasts between highly variable climates—corresponding mainly to regions where the sea-surface temperature forcing is strongest—and more steady climates. A separate analysis documented the relationship between the amplitude of rainfall variations and human demography. Population densities did not show any systematic decline with increasing variability. However, in the tropics, there was often a coincidence between reduced demographic growth and high rainfall variability. This smaller demographic growth may result from both reduced natural growth (especially enhanced mortality) and out-migration from regions affected by strong rainfall variations, as evidenced from a number of African cases studies. In contrast, tropical regions with a fast-growing population had on average a lower rainfall variability.