It is clear that the Earth will warm because of the enhancement of the natural greenhouse effect due to rising atmospheric concentrations of GHG resulting from human activities. But there is still uncertainty as to how much and how fast the atmosphere will warm. In order to estimate possible impacts on the human population more needs to be known about the nature and degree of these uncertainties. World population size is likely to stabilize during the second half of the next century at a level which will be determined largely by the speed of fertility decline. Expert opinion suggests that this level is likely to slightly less than twice present world population size.
Future population aging, like future population growth, is virtually certain to occur.
The current trend of global population aging is an unprecedented event, and its consequences for the economy, for social organization, and for political systems are only now beginning to be studied. There seems little doubt that climate change will make meeting challenges in these areas more difficult than it would have been under stable weather conditions. This leads us to Question 5, regarding how rapid population growth and high fertility might affect coping and adaptation abilities. Perhaps the most robust result to emerge from climate modeling exercises is that impacts on tropical zones are likely to be more severe than impacts on temperate zones, and that impacts on marginal lands within these zones are likely to be worst of all. The most important of these impacts are likely to take the form of increased frequency of extreme events, such as floods or droughts.