What is meant by population momentum?

Population momentum refers to the phenomenon where a population continues to grow even after birth rates have declined to replacement level. This occurs because a significant proportion of the population is composed of young individuals who are approaching reproductive age. As these young people enter their reproductive years, they contribute to an increase in the overall population, despite lower fertility rates. This growth isn't immediately noticeable in the short term but can lead to a significant increase in population over time.

Understanding population momentum is crucial in demographics and environmental studies because it highlights the challenges related to resource management and social services that a country might face even when the birth rate declines. The presence of a large youth demographic can lead to sustained population growth that may strain resources, infrastructure, and environmental capacity, affecting sustainability efforts.

The other choices do not encapsulate the true essence of population momentum. For instance, the notion of decreasing populations and urbanization does not accurately portray the implications of a young and growing population, while the effect of fertility on aging refers to different demographic transitions rather than the ongoing growth trends influenced by a youthful age structure.