Coal is the most carbon-intense of fossil fuels. Reducing use of coal through energy efficiency and renewable energy technologies will be the cornerstones of the solution to global warming, but the plain truth is that hundreds of new coal-fired power plants will probably be constructed around the world in coming years. Coal generates more than half of the electricity we use today, and it is in plentiful supply in such countries as China, India and the United States.

A critical choice remains. Power plants have a long lifespan -- build the new coal plants with dirty, 19th-century technologies and we lock ourselves into high levels of global warming pollution for decades. We can instead choose a 21st-century alternative: Using existing technologies -- each in commercial operation today -- we can convert coal into a clean-burning gas and capture and dispose of the carbon dioxide deep underground, dramatically reducing air pollution from this dirtiest of fuels. If the United States doesn't invest in this technology, neither will China, India and other countries with large coal supplies.

Conclusion: The Tipping Point