Today, renewable energy sources generate 12 percent of electricity in the U.S. But wind, wave, sunshine and others represent more than 93 percent of the energy the country could be producing, according to the Energy Information Administration of the U.S. Department of Energy.

If renewable energy is going to be a bigger player and have a significant impact in cutting the greenhouse gas emissions from power plants that are driving climate change, it's going to have to grow quickly. According to Princeton University scientists Stephen Pacala and Robert Socolow's "wedge" strategy of climate change mitigation—which quantifies as a wedge on a time series graph various sets of efforts to maintain flat global carbon emissions between now and 2055—at least two million megawatts of new renewable energy will have to be built in the next 40 years, effectively replacing completely all existing coal-fired power plants as well as accounting for increases in energy use between now and mid-century.

"It's a goal that's beyond anything probably the world's ever undertaken," says Keely Wachs, senior director of corporate communications at BrightSource Energy, a company that hopes to build 2,600 megawatts-worth of power plants that use the sun's heat to generate electricity.

Here are 10 massive projects already producing energy.

Slide Show: The World's 10 Largest Renewable Energy Projects