EcoWatch: For decades, coal-fired power dominated the U.S. energy supplyand was a leading source of greenhouse gas emissions. But a combination of factors including cheap natural gas, stronger pollution controls, increasing investment in renewable power and improved energy efficiency have pushed coal's share of the energy market down to 33 percent and with it the carbon-intensive fuel's share of the nation's greenhouse pollution. As a result, this year will be the first when CO2 emissions from coal-fired...