Although carbon dioxide (CO₂) is a naturally occurring and non-toxic gas, human activities have increased its concentration in the atmosphere far beyond natural levels. CO₂ is a greenhouse gas. It allows solar radiation to pass through the atmosphere and be absorbed by Earth's surface but prevents the energy that is reradiated from escaping back into space. The burning of fossil fuels, which releases the energy we use to power our cars — and many other things — is widely believed to be the primary culprit behind the recent CO₂ buildup.

Hydrogen is a potential alternative source of energy for vehicles, portable electronics, and even home power generators. Hydrogen can be used as an energy source for fuel cells, which, like gasoline engines and household batteries, are a type of energy conversion device. However, unlike engines and batteries, fuel cells don't produce harmful emissions, and they don't "go dead." With a fuel cell, as long as there is a flow of the chemical elements hydrogen and oxygen into the cell, electricity will flow out of the cell.

Fuel cells designed to run on hydrogen combine hydrogen with free oxygen in the air and produce water. Electricity is released during the reaction. Because hydrogen fuel cells produce energy with no harm to the environment — water and heat are the only by-products — it has been touted as a replacement for the internal combustion engine.

Although hydrogen is abundant on Earth, it is combined with other molecules in some other form — such as with oxygen as water. Because a fuel cell requires pure hydrogen, the great technological challenge is to separate hydrogen from other molecules for use. Even though scientists and engineers have proven they can remove hydrogen from water, the separation process requires a great deal of energy that must be provided by another fuel source. Using natural gas or coal for this purpose would, of course, produce more CO₂ emissions — precisely what fuel cells are designed to lessen.

Non-polluting, renewable sources of energy, including solar cells, wind turbines, or hydroelectric dams, might one day fuel the hydrogen extraction process. But such a solution has not yet proven to be efficient — more energy is spent than is generated. In 2004, Iceland opened its first commercial hydrogen filling station. Iceland already derives 90 percent of its electricity from turbines driven by steam from renewable hot water springs that originate deep underground. Iceland's government has committed to powering its cars and fishing fleet with cleanly produced hydrogen within the next 25 years.

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