According to the Energy Information Administration (EIA) since 2005 renewables have risen from 8% of total electricity generation in the U.S. to 13% in 2015. In 2015 non-emitting sources, renewables and nuclear, provided a record 33% of overall U.S. electricity production. Roughly another third was generated by natural gas and a third by coal.
In March 2015, the United States submitted its Intended Nationally Determined Contribution (INDC) for GHG emissions reduction to the United Nations Framework Convention on Climate Change targetting emissions reduction of 26% to 28% below 2005 levels by 2025. Electricity generation is the largest source of emissions in the U.S. In 2015 according to the EIA carbon dioxide (CO2) emissions from electricity generation totaled 1,925 million tonnes, 21% below their 2005 level so the INDC goals appear achievable, at least for electric power.
There are two reasons why energy produced with natural gas is less carbon intensive than coal. Coal plants consume more energy than a combined-cycle natural gas plant to produce the same amount of electricity. Secondly, coal's carbon content per unit of energy is nearly twice that of natural gas. The bottom line is that to generate the same amount of electricity, natural gas emits 40% of the carbon dioxide that would be emitted from a coal-fired unit.