Despite the apparent abundance of electricity and gas we enjoy in the United States, the world is quickly running out of the energy resources we use most. Eighty-five percent of the world's energy comes from fossil fuels, namely coal, oil, and natural gas. Of the fossil fuels used worldwide each year, the United States consumes about 25 percent. And because these fuels are created geologically over millions of years, they are considered non-renewable.

In addition, the burning of fossil fuels creates atmospheric carbon dioxide, a greenhouse gas whose concentration in our atmosphere has risen dramatically in the past century. Today, most climate experts link a global warming trend to this increase in greenhouse gases.

Shrinking fossil fuel reserves and the threat of global warming have led researchers around the world to search for alternative energy sources, including some of those depicted in this video segment. For example, solar panels, composed of photovoltaic cells, convert sunlight into electricity. Water can be harnessed to produce electricity, too. Waterfalls such as Niagara Falls and many dammed rivers drive large turbines that turn electrical generators. Hoover Dam, for example, generates enough electricity to serve 1.3 million people.

In some areas of the world that are consistently windy, wind energy is harnessed to produce electricity. In fact, generators powered by wind are one of the one of the least expensive ways to produce electricity. They currently provide at least 10 percent of the electricity used in Denmark and more than one percent of California's electricity needs. Other alternative energy sources include nuclear, geothermal (which uses Earth's internal heat), and biomass (which uses renewable crops rather than fossil fuels).

Despite the promise that all of these energy sources offer, none is perfect. Each comes with its own set of benefits and limitations. For example, although the Sun offers an almost limitless supply of energy, solar panels currently convert only 15 percent of the energy they capture into usable electricity. Windmills also produce electricity inconsistently. Other energy alternatives, including nuclear fusion and hydrogen power, may hold promise for the future. At present, however, the input of energy needed to run these systems is as great as the output.

To make up for these shortfalls, energy experts continue to call for more research into alternative energy sources. Without it, they say, we are most certainly headed for a serious energy crisis.

To learn more about energy consumption in the United States, check out Snapshot of US Energy Use.

To learn how the world's current use of fossil fuels is contributing to global warming, check out Your Carbon Diet and Climate Change.

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