Nuclear power plants don't emit any greenhouse gases. The technology is already in widespread use, providing about 19 percent of the nation's total energy supply; countries such as France get a much higher share of their power from nuclear. Read more »

Safety is a major concern. Since 2001 federal inspectors have caught nuclear plants violating safety regulations on more than 4,000 occasions, according to the U.S. Government Accountability Office. Creating a secure and permanent system for disposing of spent nuclear fuel remains a challenge for the industry.
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Clean coal power plants -- called integrated gasification combined cycle facilities (IGCC) -- spew very little acid-rain-causing sulfur dioxide and smog-inducing nitrogen oxides into the atmosphere. And scientists are working on technologies to "sequester" carbon dioxide from clean coals plants by piping it underground or into the ocean.
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The problem is nobody's come up with a safe and cost-effective sequestering method, and it may be years or even decades before engineers and scientists figure it out. Plus, pumping the gas through pipelines is very energy-intensive.
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According to the U.S. EPA, diesel vehicles tend to be 30 to 35 more fuel-efficient than comparable standard gas vehicles. That's because diesel engines are tremendously efficient, and the fuel itself simply packs more energy per gallon. Government scientists at the Argonne National Laboratory say the typical diesel car emits 28 percent fewer greenhouse gases than a standard gas-fueled car.
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A key problem with diesel is the soot and sulfur particles spewed by diesel vehicles. Clean diesel -- also called ultra-low-sulfur diesel -- contains, as you would expect, far less sulfur. But clean diesel vehicles still release climate-changing CO2, and petroleum is a dwindling resource. Read more »

Derived from vegetables or animal fat, biodiesel can be used to power diesel engines, while ethanol, a veggie-based fuel, can be burned in gasoline engines. Both biodiesel and ethanol are blended with conventional petroleum fuels, saving oil and cutting CO2 emissions. Scientists are still debating the greenhouse gas-curbing benefits of these alt-fuels, but one U.S. Department of Energy study found biodiesel generated 78 percent less carbon dioxide than traditional diesel. Read more »

It takes huge amounts of farmland to generate biodiesel and ethanol -- an acre of soybeans produces only 50 gallons of biodiesel, according to some reports -- which could pose problems if these farm fuels catch on. Scientists are currently studying whether biodiesel and ethanol are net energy losers -- meaning, on a per gallon basis, does it take more energy to grow, process, and refine biodiesel and ethanol than the fuels actually provides? Also, experts say biodiesel may actually emit more nitrogen oxides, a key ingredient in smog, than traditional diesel.
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Capturing sun rays -- an inexhaustible energy source -- and transforming them into electricity creates no greenhouse gases or other air pollution. In recent years, the technology has been becoming more cost-effective and efficient. Read more »

Obviously, solar power is less effective in cloudy regions. Plus, homeowners and businesses who want to go solar must now pony up thousands -- or tens of thousands -- for photovoltaic arrays and storage equipment. While the technology is getting cheaper, it still may be out of reach for the average consumer. Read more »

Like solar, wind power farms generate no greenhouse gases and are relatively cheap to construct. Off-shore wind projects, which harness the almost-constant wind on the ocean, have steadily gained support in recent years.
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Wind turbines' whirring blades can kill birds and bats, which has prompted some environmentalists to oppose them. Residents near existing and proposed wind farms have also complained that the tall metal structures are unsightly.
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Hydrogen fuel cells are essentially batteries built to turn the chemical reaction of oxygen and hydrogen into electricity. The fuel cells, which emit only water, can be used to power cars and other vehicles or to supply electricity to buildings. Hydrogen isn't a greenhouse gas.
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Researchers are still searching for cheap, environmentally sound methods for manufacturing large amounts of hydrogen. Hydrogen gas is very combustible, bulky and can weaken metal; it must be cooled to -423 degrees Fahrenheit to be liquified. Auto designers, who've poured billions into hydrogen projects, are still struggling to build an affordable, reliable hydrogen vehicle.
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