Did anyone tell you the good news? The imminent disaster from a population explosion turns out to be a transient phenomenon. Since the early 1980s, population growth has shown clear signs of slowing down, and the slowdown became truly significant in the mid-1990s (see table at end).

The overall population trend, it turns out, is not a single geometric trend; instead, it is four subsidiary trends that have ebbed and flowed over millennia and have differing impacts on population. The four component trends are food supply, health conditions, wealth, and birth-control technology. The Encyclopedia Britannica is a good reference source for all of these trends.

Before A.D. 1,000, the global population had one limiting factor&emdash;food supply&emdash; and in this matter the world could be divided into two distinct parts: (1) China and the surrounding rice countries; and (2) the rest of the world. China could grow rice on the same plot of land, nearly every year, as long as it had irrigation (making it largely independent of weather conditions) and fertilizer. The fertilizer was mostly human excrement which was made sterile by mixing it with lye and ash and by drying it before use.

China and the surrounding rice nations were very densely populated by A.D. 700. One province in China had more residents than all of Europe. The rest of the world subsisted on hunting-gathering and fishing in most regions, and in the settled agricultural areas (much of Europe and its colonies) on agriculture without the benefit of irrigation or fertilizer. In Europe (except for some Basque regions of Spain, where irrigation was first used in Europe), the main crops of wheat, rye, and barley could only be grown once every five years on the same plot of land. The idea of fertilization was unknown.

The world population slowly doubled between A.D. 700 and 1400, from 250 million to 450 million. With the adoption of Chinese agricultural methods in Europe, specifically new crops, irrigation and fertilizer (partly due to Marco Polo), there was a slow but noticeable improvement in agriculture. The expansion of agriculture to new continents, along with the rapid growth of trade in the colonies, increased the food supply and the population to 800 million by the time Malthus wrote about the subject three centuries later.

The nineteeth century saw a radical improvement in European agriculture, together with the industrial expansion of trade. The rapid growth of the food supply resulted in a rapid growth in population, which has abated in the past several decades.

Health conditions began to change radically between 1850 and 1900, due almost entirely to an emphasis on cleanliness, specifically sterilization in surgical procedures (especially childbirth), and in developing efficient sewage systems. The average American lifespan in 1850 was 47; today it is 76. In 1850, the average life span of a male in the United States who lived to age 20 was age 60. Today, it is age 74. High death rates existed for infants and for women in childbirth in the 19th century.

The life span of an American is a good standard for most of the industrial world. By 1920, this result of greater longevity due to improved sanitation is very visible in lifespan data, and 20-year-old women began to have longer estimated future life spans than men. By 1920, an American male or female who made it to age 20 would live to be 65.

From 1920 on, the increased life span begins to reflect the higher levels of medical care and the greater degree of training required for medical doctors. My grandfather, for example, was a dentist who graduated from U.C. Berkeley after three years of college, while my grandmother was a doctor who had to take only two years at U.C. Berkeley to get her medical degree. By 1920, the length of time needed to achieve a medical degree had increased to five years.

Today, a 20-year-old American male will live on average to the age of 74, while a 20-year-old woman can be predicted to live to the age of 80, largely due to improved medical practices over the intervening fifty years.

At the same time, the world's population has experienced stunning increases, going from about 900 million in 1800 to 1.8 billion in 1940 and 3.7 billion in 1970. This astounding change is a result of both an increased food supply and a greater degree of medical care, which together reduced infant mortality and lengthened life spans.

Wealth and trade made a modest contribution to this growth, because they allowed for the global specialization in food supplies, such as sugar and oils from the tropics, and grains from the central United States and Canada. But wealth eventurally has a negative effect on population growth. As the wealth of a country increases, its population growth slows, so that the wealthiest countries have negative population growth. Increased wealth, along with an increased food supply and improved medical care, is one of the major countervailing forces to population growth.

Today, we can see some changes in the effects of food supply and medical care on population growth. The improvements in life span and infant mortality since 1960 have been very modest and will contribute very little to future population growth. The increase in the food supply continues unabated, but it no longer drives population growth to the same degree that it did initially. The impact of improvements in health care seems to have peaked now that most of the world has greater access to it. At the same time, birth control technology continues to exert influence in stabilizing population growth. Finally, increased wealth is acting to lower the

Deconstructing the single geometric population trend into its four components turns out to provide very helpful explanation of why the population bomb has been defused. The following table makes clear the extent to which this has happened.

Table: Annual Percentages of Global Population Growth in 1990

* Source: 1996 U.S. Statistical Abstract

The actual condition of the world is even better than this data indicates. Reproduction rates, which lead population growth rates by several decades, are so low that most of Europe, Japan, Mexico, and Russia will start shrinking in the next twenty-five years. The educated southern half of India has a negative reproduction rate, which is indicative of the potential for the north and for neighboring Pakistan and Bangladesh.