Defeat is an orphan, but victory has many parents. Uncle Sam's remarkable success in cutting carbon emissions since they peaked in 2007 is an environmental victory, but it has triggered a debate about the contribution of the gas boom to the fact of the big US carbon drop.
The Breakthrough Institute estimates that both increased non-hydro renewables and gas contributed significantly to the substantial reduction of US carbon emissions from 2007 to 2012. It finds that more gas generation, however, cut carbon emissions by 3 to 10 times more than non-hydro renewables. http://thebreakthrough.org/index.php/debunking-rhodium/. Breakthrough's paper links to a blog post that I also published on this topic.
From 2007-2012, coal's generation market share plunged 9 percentage points from about 48% to 37%, as both gas and non-hydro renewables increased their market share but by different amounts. Coal generation's loss was the gain of gas and non-hydro renewables. Non-hydro renewables boosted their generation market share by about 2 percentage points and gas climbed about 8 percentage points.
Cheap gas also led to displacement of some oil in power generation, home heating, and transportation. Biofuels, mainly corn ethanol, too also gained market share from oil. The displacement of oil by gas and biofuels also contributed to the decline in carbon emissions.
The Breakthrough Institute's analysis of the comparative contributions made by both gas and renewables to the big drop in carbon emissions is worth reading. Yet, the most important point is that US carbon emissions declined substantially from 2007 to 2012, thanks to gas and non-hydro renewables gaining market share, and thanks to growing energy efficiency and conservation.