The broadest definition of energy is the ability to do work. Human societies tap into various forms of energy, including chemical energy in biomass, natural gas, coal, and petroleum; nuclear energy in uranium; gravitational energy captured in hydroelectric plants; wind energy; and solar energy. Energy is usually measured in British thermal units (BTUs). A BTU is defined as the amount of heat energy that will raise the temperature of one pound of water by one degree Fahrenheit. In 2005 the world economy obtained about 40 percent of its nonsolar energy from petroleum, about 23 percent each from natural gas and coal, 8 percent total from hydroelectric, wind, and thermal sources, and about 6 percent from nuclear. Most of this energy is used in the industrialized world, although the most rapid growth in energy use is occurring in the industrializing world, especially China. The largest use of energy by far is for industrial production and transportation.
Energy has been a crucial factor in human cultural evolution. The evolution of increasingly complex human societies was driven by the capacity to harness energy. Harnessing energy may have also played a key role in our biological evolution. The large human brain, unique even among primates, has enormous energy requirements. The human brain represents about 2.5 percent of body weight and accounts for about 22 percent of resting metabolic needs. This large energy requirement was met by a much higher proportion of protein in the diet of early humans and the use of fire to predigest meat. The use of fire played a role in the anatomical development of our species— larger brains and shorter guts—and paved the way for further advances in technological and cultural evolution.
Beginning about 10,000 years ago, early agricultural technology harnessed flows of solar energy in the forms of animal-muscle power, water, and wind. With the wide spread use of wood for fuel, humans began to tap into stocks of solar energy rather than flows. The use of stocks of energy made it possible to capture ever larger amounts of energy per capita with smaller amounts of effort. Wood, wind, and water power fueled the industrial revolution, which began in the early eighteenth century. In the nineteenth century, ancient solar energy, fossil hydrocarbons in the form of coal, rapidly became the fuel of choice. During the twentieth century, petroleum and natural gas replaced coal as the dominant fuel. Each step in the history of energy use has been characterized by a dominant fuel type that is increasingly flexible and substitutable.
Since our industrial economy depends so heavily on fossil fuels, an obvious question is, "Are we running out of it?" Most economists answer this question with an emphatic "No!" As energy becomes scarce, its price will increase, calling forth substitutes, increasing conservation efforts, and encouraging more exploration for new supplies. Economists point out that past warnings of impending shortages have proved to be greatly exaggerated. Critics of the economic argument counter that the inverse relationship between energy supply and energy demand may be trivially true, but this does not mean that the increasing scarcity of an essential resource like petroleum can be easily accommodated. The economic argument also ignores the geopolitical consequences of the waning of the petroleum age.
A useful supplement to the price-based analysis of economists is the concept of energy return on investment (EROI). This is a measure of how many units of energy can be obtained from a unit of energy invested. If the EROI is less than one, it makes no sense to tap that energy source, no matter how high the price.
Although the world uses many types of energy, none of them have the flexibility and high EROI of petroleum. Of paramount concern is when world petroleum production will peak and start to decline. Most predictions of when worldwide oil production will peak are based on variations of a model developed by the geophysicist M. King Hubbert in the 1950s. He created a mathematical model of the pattern of petroleum exhaustion assuming that the total amount of petroleum extracted over time would follow a bell-shaped pattern called a logistic curve. Past experience for individual oil fields shows that once peak production is reached, production tends to fall quite rapidly. A number of petroleum experts argue that technological advances in the past decade or so have extended the peak of the Hubbert curve for specific oil fields, but this has made exhaustion more rapid after the peak occurs. Since oil is limited, policies promoting technology to make more energy available today mean that less will be there in the future.
Estimates of when world oil production will peak run from 2005 (production has already peaked) to 2030, with most predictions clustering around the years 2010—2012. Predicted consequences of declining oil production range from catastrophic scenarios as agricultural and industrial outputs plummet, to relatively mild scenarios as the world’s economies endure inflation and temporary economic hardships to adjust, to the rosy scenarios of free-market fundamentalists who claim that markets will quickly call forth substitutes and conservation that overcome the scarcity of any particular fuel type.
It is impossible to predict how the world’s economies will adjust to the end of the fossil-fuel age. So far energy policies in the developed and developing worlds have shown little concern for the limited amount of fossil fuels. What happens in the future depends on how much developing economies (especially China) grow and how energy-dependent they become. Also of concern is how the rest of the world will react to the growing concentration of petroleum reserves in politically volatile areas and to the increasingly ominous effects of global climate change.
