1. ¹The growth of human populations, like that of other species, depends on birth rates and death rates. ²About 10,000 years ago, agricultural societies replaced a lifestyle of hunting, gathering, and scavenging. ³With this change, human birth rates increased and death rates decreased, leading to a steady, but relatively slow, rate of population growth. ⁴Since the mid-1700s, when the Industrial Revolution began in England, the global human population has skyrocketed. ⁵This unprecedented growth has resulted mainly from falling death rates, especially infant mortality, throughout most of the world. ⁶Improved nutrition, better medical care, and sanitation—all technological, rather than biological attributes of our species—have contributed to an increased percentage of newborns that survive long enough to leave offspring of their own. ⁷The effect of decreasing mortality on population growth is compounded in most developing countries, which tend to have relatively high birth rates. ⁸For example, the birth rate in Sri Lanka has decreased fairly steadily for nearly 50 years, but it has never declined enough to offset the drop in the death rate. ⁹Thus, the overall effect is a growing population.
2. ¹For hundreds of years, agricultural and industrial technology have enabled us to exploit environmental resources more and more efficiently and to inhibit the mechanisms that would normally control our population growth. Put another way, technology has increased Earth's carrying capacity for people, allowing our long-term explosive growth.
3. ¹What is the future of the human population? It is difficult to predict the future size of any population accurately, and the human population is no exception. What we do know is that no population can continue to grow indefinitely. Exactly what the world's carrying capacity for humans is and under what circumstances we will approach it are topics of great concern and debate. Is our population already greater than that which the Earth can sustain, or is there room for growth? Ideally, human populations would reach carrying capacity smoothly and then level off. But if the population has already exceeded carrying capacity, a sharp decline might occur in the future. If the population fluctuates around carrying capacity, it might mean periods of increase would be followed by mass death, as has occurred during plagues and famines.
4. ¹Barring some worldwide calamity, it is likely that the human population will continue to grow well into the next century. Whatever happens, we know that the human population must eventually stop growing. Unlike other organisms, we have the ability to decide whether this occurs mainly as a result of a decrease in the birth rate or an increase in the death rate.

Source: Campbell, N.A., Mitchell, L.G., & Reece, J.B. (2000). Biology: Concepts & Connections, 3/e. New York: Addison Wesley Longman, pp. 708-709.