Carbon dioxide is a major contributor to greenhouse gases, which are largely responsible for global warming. Last year, nearly 40 percent of U.S. greenhouse gas pollution came from fossil fuels.

The good news is that nationwide, from 2005 to 2012, carbon dioxide emissions from coal-fired power plants decreased by 13 percent. Wind energy, natural gas, lower demand and the closure of aging coal-fired power plants account for the decline.

The bad news is CO2 emissions from coal are up 7.1 percent over this time last year, likely because of higher natural gas prices, according to a report by the Environmental Integrity Project. Meanwhile, the price of coal has flatlined, making that fuel source attractive to utility companies.

Conservation and increased use of renewables, such as solar and wind, could help offset the troubling uptick in CO2 emissions.