The world's human population is rapidly approaching seven billion. Human beings have increased the concentration of GHGs in Earth's atmosphere via many processes, including agricultural practices and the burning of fossil fuels. The sheer size and growth rate of the human population is a fundamental driver of global change, including global warming.
The human population is a fundamental driving force of climate change. Human activities ranging from agricultural practices to the burning of fossil fuels contribute greenhouse gases (GHGs) to the atmosphere. Both the number and the behavior of human beings are basic drivers of climate change. In terms of numbers only, in the early twenty-first century, the global human population increased by about 200,000 people per day.
Hominids that painted cave walls (Homo erectus) appeared almost one million years ago. By 8000 BC modern humans (Homo sapiens) numbered around 8 million. The first 990,000 years of human existence were characterized by a very low population growth rate (15 persons per million per year). The pro-natal fertility beliefs of early humans (whether conscious, unconscious, or instinctive) were undoubtedly necessary to maintain the tenuous presence of humanity on the face of the Earth. Pro-natal fertility beliefs served humanity well for 990,000 years, which perhaps explains why they continue.
Historical estimates of the Earth's total population are problematic. Nonetheless, there is little argument that human numbers have increased dramatically in the past three hundred years. The conventional wisdom regarding the dramatic changes in the growth rate of the human population typically attributes them to three significant epochs of human cultural evolution: the agricultural, industrial, and green revolutions.
Prior to the agricultural revolution, the human population was probably less than ten million individuals, who survived primarily by hunting and gathering. With the domestication of plant and animal species about ten thousand years ago, the human population experienced an increase in its growth rate. Jared Diamond's book Guns, Germs, and Steel (1997) suggests that the geographic endowments of domesticable plants and animals, climate, and other environmental variables have had a profound influence on the fate of human societies, including their ability to engage in agricultural innovation. By about 5000 BC food production gains caused by the agricultural revolution enabled the planet to support about 50 million individuals.
For the next several thousand years, population continued to grow at a rate of about 0.03 percent per year. By the year 0, the population numbered about 300 million. Through the Roman Empire and the Middle Ages (0-1300 AD), the rate of population growth began to increase slightly because of colonization and agricultural expansion. However, there is evidence from this period to suggest that the size of the population was actively controlled by factors such as disease, famine, and war for short periods of time (Harrison, 2001).
At the end of the Middle Ages, the human population numbered about 400 million. As people became more concentrated in denser urban environments, the effects of disease increased. Starting in 1348 and continuing to 1650, the human population was subjected to massive declines caused by the effects of the bubonic plague. At its peak in about 1400, the Black Death may have killed 25 percent of Europe's population in just over fifty years. However, by the end of the last great plague in 1650, the human population numbered 600 million, and this expansion may have been the outcome of the introduction of new species for agricultural cultivation. Agricultural cultivation expands with human population and produces climate change both by increasing GHG emissions and by deforesting and desertifying the surface of the Earth.
The Industrial Revolution began sometime between 1650 and 1750. In the period following 1700, the human population expanded at an accelerating rate (Diamond, 1997). In just under three hundred years, the population of the Earth went from 500 million to well over 5 billion individuals, and the annual rate of increase went from 0.1 percent to 1.8 percent. It is believed the global population growth rate peaked at around 2.1 percent in the early 1970's.
A third revolution occurred sometime in the 1960's. The development of various vaccines and antibiotics in the twentieth century and the spread of their use to most of the world after World War II caused dramatic reductions in the crude death rate. This resulted in increased population growth rates. The third revolution is often called the Green Revolution because of the technology used to increase crop yields. The Green Revolution was really a combination of improvements in health care, medicine, and sanitation, in addition to an increase in the production of food.
The last few hundred years of human history was also a time of change in the human demographics. During this period, the concentration of industry in urban areas and the efficiency gains of modern agricultural machinery caused large numbers of individuals to move from rural areas to cities to find jobs. From 1900 to the present, the percentage of people living in cities went from 14 percent to about 50 percent. Demographers estimate that by the year 2025 more than 60 percent of the Earth's human population will be living in cities. Scientists estimate that the human population will continue to increase until the year 2050, at which time it will level out at between 8 and 15 billion. In this projection, it is assumed that 90 percent of this growth will take place in the developing world.
Every human being contributes GHGs to the atmosphere because of their food, shelter, and transportation needs. Increasing human numbers means increasing GHGs in the atmosphere. As the world's inhabitants have increased in wealth, GHG emissions have also increased. Increasing wealth historically has been related with increasing energy consumption per capita for housing, transportation, and agriculture. The significance of myriad environmental impacts associated with the size, growth, and spatial distribution of the human population should not be underestimated.
1. Diamond, Jared. Guns, Germs, and Steel: The Fate of Human Societies. New York: W. W. Norton, 1997.
2. Harrison, Paul, and Fred Pearce. AAAS Atlas of Population and Environment. Berkeley: University of California Press, 2001.
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