Despite what some news reports suggest, here in the United States an energy shortage isn't readily apparent. The flick of a switch brings a seemingly never-ending flow of electricity, gas stations provide what seems like an endless supply of gasoline to our automobiles, and industries churn out synthetic materials and technological gadgets at a staggering rate.

Yet, the energy shortage is real, and the world is quickly running out of the energy resources we rely on most. Eighty-five percent of the energy we use 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. Because these fuels are created geologically over millions of years, they are essentially non-renewable. And, they're running out.

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

Because fossil fuel reserves are shrinking, and because their use is tied to global environmental change, most experts insist on the need for alternative renewable energy sources, including some of those depicted in this video segment. Unfortunately, no perfect alternative has been identified. Each energy source comes with its own set of problems.

For example, although the Sun offers an almost limitless supply of energy, solar panels are expensive, take up a lot of space, and convert only 15 percent of the energy they capture into usable electricity. They also produce power only during cloudless days. Windmills capture another seemingly infinite energy source, but they also produce electricity inconsistently. Other energy alternatives, including nuclear fusion and hydrogen power, hold promise for the future. At present, however, the input of energy to these types of 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.