THE LIVING WORLD

Unit Eight. The Living Environment

38. Human Influences on the Living World

38.8. Curbing Population Growth

If we were to solve all the problems mentioned in this chapter, we would merely buy time to address the fundamental problem: There are getting to be too many of us.

Humans first reached North America at least 12,000 to 13,000 years ago, crossing the narrow straits between Siberia and Alaska and moving swiftly to the southern tip of South America. By 10,000 years ago, when the continental ice sheets withdrew and agriculture first developed, about 5 million people lived on earth, distributed over all the continents except Antarctica. With the new and much more dependable sources of food that became available through agriculture, the human population began to grow more rapidly. By the time of Christ, 2,000 years ago, an estimated 130 million people lived on earth. By the year 1650, the world’s population had doubled, and doubled again, reaching 500 million. Starting in the early 1700s, changes in technology have given humans more control over their food supply, have led to the development of cures for many diseases, and have produced improvements in shelter and storage capabilities that make humans less vulnerable to climatic uncertainties. Recall from chapter 35 that populations grow exponentially until they reach the limits of their environment, called the carrying capacity. These changes allowed humans to expand the carrying capacity of the habitats in which they lived and thus to escape the confines of logistic growth and reenter the exponential phase of the sigmoidal growth curve, shown by the explosive growth in figure 38.11.

Figure 38.11. Growth of the human population.

Over the past 300 years, the world population has been growing steadily. Currently, there are over 6 billion people on the earth. Mexico City (inset photo), one of the world's largest cities, has about 26 million inhabitants.

Although the human population has grown explosively for the last 300 years, the average human birthrate has stabilized at about 21 births per year per 1,000 people worldwide. However, with the spread of better sanitation and improved medical techniques, the death rate has fallen steadily, to its present level of 9 per 1,000 per year. The difference between birth and death rates amounts to a population growth rate of 1.2% per year, which seems like a small number, but it is not, given the large population size.

The world population reached 7 billion people in 2011, and the annual increase now amounts to about 78 million people, which leads to a doubling of the world population in about 58 years. Put another way, more than 214,000 people are added to the world population each day, or almost 150 every minute. At this rate, the world’s population will continue to grow and perhaps stabilize at a figure around 10 billion. Such growth cannot continue, because our world cannot support it. Just as a cancer cannot grow unabated in your body without eventually killing you, so humanity cannot continue to grow unchecked in the biosphere without killing it.

Most countries are devoting considerable attention to slowing the growth rate of their populations and there are genuine signs of progress, but the world population may still gain another 1 to 4 billion people before it stabilizes. No one knows whether the world can support so many people.

Population Growth Rate Declining

The world population growth rate has been declining, from a high of 2.0% in the period 1965-70 to 1.2% in 2004. Nonetheless, because of the larger population, this amounts to an increase of 78 million people per year to the world population, compared to 53 million per year in the 1960s.

The United Nations attributes the decline to increased family planning efforts and the increased economic power and social status of women. As family size decreases in developing countries, education programs improve, leading to increased education levels for women, which in turn tends to result in further decreases in family size.

No one knows whether the world can sustain today’s population of 7 billion people, much less the far greater numbers expected in the future. We cannot reasonably expect to expand the world’s carrying capacity indefinitely. The population will begin scaling back in size, as predicted by logistic growth models; indeed it is already happening. In the subSaharan area of Africa, population projections for the year 2025 have been scaled back from 1.33 billion to 1.05 billion because of the impact of AIDS. If we are to avoid catastrophic increases in death rates, such as the tragedy we are seeing in sub-Saharan Africa, the birthrates must continue to fall dramatically.

Population Pyramids

While the human population as a whole continues to grow rapidly, this growth is not occurring uniformly over the planet. Some countries, like Mexico, are currently growing rapidly. Figure 38.12 shows how Mexico’s birthrate, while declining (the blue line), still greatly exceeds its death rate (the red line), which is stabilizing. There is often a correlation in how developed a country is and how rapidly its population grows. Table 38.1, on the next page, compares three countries that differ in how well developed they are.

Figure 38.12. Why Mexico's population is growing.

The death rate (red line) in Mexico has been falling, while the birthrate (blue line) remained fairly steady until 1970. The difference between birth and death rates has fueled a high growth rate. Efforts begun in 1970 to reduce the birthrate have been quite successful. Although the growth rate remains rapid, it is expected to begin leveling off in the near future as the birthrate continues to drop.

Ethiopia, a developing country, has a higher fertility rate, which results in a higher birthrate than either Brazil or the United States. But Ethiopia also has a much higher infant mortality rate and a lower life expectancy. Overall, the population in Ethiopia will double much more quickly than the population of Brazil or the United States. The rate at which a population can be expected to grow in the future can be assessed graphically by means of a population pyramid— a bar graph displaying the numbers of people in each age category (some examples are shown in figure 38.13). Males are conventionally shown to the left of the vertical age axis (colored blue here) and females to the right (colored red). In most human population pyramids, the number of older females is disproportionately large compared with the number of older males, because females in most regions have a longer life expectancy than males. This is apparent in the upper portion of the 2005 U.S. pyramid.

Figure 38.13. Population pyramids.

Population pyramids are graphed according to a population's age distribution. Kenya's pyramid has a broad base because of the great number of individuals below child-bearing age. When all of the young people begin to bear children, the population will experience rapid growth. The 2005 U.S. pyramid demonstrates a larger number of individuals in the "baby boom" cohort—the pyramid bulges because of an increase in births between 1945 and 1964, as shown at the base of the 1964 pyramid. The 25 to 34 cohort in the 1964 pyramid represents people born during the Depression and is smaller in size than the cohorts in the preceding and following years.

TABLE 38.1. A COMPARISON OF 2006 POPULATION DATA IN DEVELOPED AND DEVELOPING COUNTRIES

Viewing such a pyramid, one can predict demographic trends in births and deaths. In general, rectangular pyramids are characteristic of countries whose populations are stable; their numbers are neither growing nor shrinking. A triangular pyramid, like the 2005 Kenya pyramid, is characteristic of a country that will exhibit rapid future growth, as most of its population has not yet entered the child-bearing years. Inverted triangles are characteristic of populations that are shrinking.

Compare the differences in the population pyramids for the United States and Kenya in figure 38.13. In the somewhat more rectangular population pyramid for the United States in 2005, the cohort (group of individuals) 40 to 59 years old represents the “baby boom,” the large number of babies born following World War II. When the media refers to the “graying of America,” they are referring to the aging of this disproportionately large cohort that will impact the health-care system and other age-related systems in the future. The very triangular pyramid of Kenya, by contrast, predicts explosive future growth. The population of Kenya is predicted to double in less than 20 years. However, it is important to note that these estimates do not take into account the huge impact that natural disasters and epidemics such as AIDS will have on population sizes. In sub-Saharan Africa, the AIDS epidemic has reduced the life expectancy at birth by 20 years. Figure 38.14 shows two population pyramid projections for Botswana, Africa, where over 36% of the population is living with HIV or AIDS. The uncolored portions of the bars indicate the population projections in 2025 without the effect of the AIDS epidemic, and the colored bars reflect actual projections with AIDS.

Figure 38.14. Projected AIDS effect on Botswana population (year 2025).

The Level of Consumption in the Developed World Is Also a Problem

The world population is expected to stabilize sometime in this century at about 10 billion. We in the developed countries of the world need to pay more attention to lessening the impact of our resource consumption. Indeed, the wealthiest 20% of the world’s population accounts for 86% of the world’s consumption of resources and produces 53% of the world’s carbon dioxide emissions, whereas the poorest 20% of the world is responsible for only 1.3% of consumption and 3% of CO₂ emissions.

One way of quantifying this disparity is by calculating what has been termed the ecological footprint, which is the amount of productive land required to support an individual at the standard of living of a particular population through the course of his or her life. As figure 38.15 illustrates, the ecological footprint of an individual in the United States is more than 10 times greater than that of someone in India. Based on these measurements, researchers have calculated that resource use by humans is now one-third greater than the amount that nature can sustainably replace; if all humans lived at the standard of living in the developed world, two additional planet earths would be needed.

Figure 38.15. Ecological footprint of individuals in different countries in 2003.

The ecological footprint calculates how much land is required to support a person through his or her life, including the acreage used for production of food, forest products, and housing, in addition to the forest required to absorb the carbon dioxide produced by the combustion of fossil fuels.

Key Learning Outcome 38.8. The problem at the core of all other environmental concerns is the rapid growth of the world's human population. Serious efforts are being made to slow its growth.