Three nonrenewable fossil fuels—petroleum, natural gas, and coal—have supported more than 80 percent of the U.S. energy consumption in the past 100 years (EIA 2015a). Around 1950, petroleum emerged as the leading U.S. fuel source and continues to grow (EIA 2015a), despite critically depleting reserves. It is now a crucial component of the transportation and chemical industry sectors, supporting ~40 percent of U.S. energy consumption (Pimentel et al. 2002). In 2015, the United States imported 9.4 million barrels of petroleum every day, 78 percent of which contained crude oil (EIA 2015b). Harmful greenhouse gases such as carbon dioxide are released by the burning of fossil fuels. Although the United States makes up just 4 percent of the global population, it emits ~22 percent of the world’s carbon dioxide (Pimentel 2002). Greenhouse gases like carbon dioxide trap solar thermal energy within the atmosphere, exacerbating global warming. Thirteen of the 15 warmest years in instrumental history have occurred since 2000 (Cate and Ball 2016).

In the mid-1980s, new forms of renewable energy (i.e., those that do not come from the decomposition of ancient living organisms) began to slowly emerge (EIA 2015a). By 2014, approximately 10 percent of U.S. energy came from renewable sources, the most we’ve seen since the 1930s when wood was commonly used (EIA 2015a). Wind and solar energy represent the most successful sectors of renewable energy, with geothermal and biomass sources trailing behind (Nigam and Shukla 2015).

There are two main categories of biotechnology-based renewable biofuels: bioethanol to replace gasoline and biodiesel to replace diesel. Collectively these are called biofuels. Biofuels “burn clean”—meaning they produce less carbon dioxide and sulfur dioxide than their conventional petroleum-based counterparts (Pimentel et al. 2002).

Box 5.3. Renewable versus sustainable

When discussing fuel alternatives, it is important to make a distinction between the terms “renewable” and “sustainable.” Definitions vary greatly in the literature. Renewable fuels can be replenished in contemporary times, with or without the activities of humans. Sustainable fuels are consumed at a rate that is less than or equal to the rate

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at which resources are used to produce them. Thus, the preservation of future resources is paramount. These definitions are based upon those found in The Oxford English Dictionary for “renewable” and “sustainable.”

Fossil fuels are not sustainable because (1) the rate of natural decomposition of organic matter is much slower than the global rate of fossil fuel consumption and (2) the combustion of fossil fuels dramatically compromises essential global resources such as clean water, air, and land to support life. Although renewable, corn-based bioethanol is also not sustainable because the food, fossil fuels, arable landmass, and water consumed in production outweighs the energy yield.

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