Population growth affects global environmental change. Other things being equal, an increase in the world’s population will lead to, among others, more carbon emissions, more deforestation and less biodiversity, although the effects can go in both directions – environmental change can influence population growth. What is important to remember though, is that it is not just adding people that matters, but also how those people produce and consume. To understand this, imagine adding 10 people of a hunter-gatherer nature to the world’s population vis-à-vis adding 10 who have chainsaws, fly around the world for their holiday(s) and live in 2,349 sq ft (218 m2) houses. Finally, it also matters whether people are organized into a small number of larger households or a large number of smaller households. Understanding basic population trends such as growth, fertility and mortality, can further help in defining the population-environment relationship.

Concerning population growth, we are approaching the end of the demographic transition, that is, the transition from high to low levels of fertility and mortality. Because mortality declined first, the world’s population experienced significant growth during the transition, increasing from 2.5 billion in 1950 to 6.3 billion today. However, the rate of growth has been declining since the late 1960s. One legacy of our past rapid growth is a young age structure; 30% of the world’s population is below age 15. As these young people enter childbearing age, their reproduction will increase the world’s population even if they only average two children per couple. This is “population momentum.”

The increase in global life expectancy from 47 in 1955 to 65-67 today, results in children, who might have died in the 50’s before having children of their own, now living long enough to produce offspring of their own. Thus, modern mortality rates affect ages past the reproductive years, subsequently raising fertility rates. However, even though mortality rates are expected to decline, continued longevity growth is not guaranteed, as mortality can be responsive to, among others, economic downturns, regime change, and infectious diseases.

Since the late 1960’and early 1970’s, a dramatic transition in fertility rates has occurred in that today, only 16 of 187 countries do not show any evidence of a transition from high to low fertility. Further, the United Nations projects a global total fertility rate of 2.02 in 2050, which is just below replacement level and we are now witnessing a shift to concern about low fertility.

As we look to the future, the world’s population will continue to grow for several decades, but the rate of growth will diminish. As population’s age, the effect of population momentum will diminish, as will growth rates. Looking further into the future, say 2050 or so, there is a realistic possibility that, for the first time since reliable global population counts have been available, the size of earth’s population will decline. This will most likely be the result of fertility reductions rather than increased mortality. If global population decline occurs it will occur within a context of expressed concern about this decline – just as we are now seeing in Japan and several European countries.