Remember how in 2009 there was a marked drop in US greenhouse gas emissions, 6.58% below 2008 levels? While the start of the Great Recession had something to do with it, new analysis from the Harvard School of Engineering and Applied Sciences shows that, when it comes to reductions in emissions from electricity production, which dropped 8.76% from 2008, cheaper natural gas prices were behind the decline, with natural gas displacing coal.
Report lead author Xi Liu:
When the natural gas prices are high, as they were 4 years ago, if the gas prices come down a little bit, it doesn't make any difference. But there's a critical price level where the gas systems become more cost-effective that the oldest coal-fired systems. If the gas price continues to drop, you'll continue to go down this curve so that you'll knock out not just the really ancient coal-fired power plants, but maybe some of the more recent coal-fired plants. (Science Daily)
It is true that natural gas produces fewer greenhouse gas emissions than coal. Even taking into account methane leakage natural gas comes in at 47% the climate pollution of coal. But here's where the expansion of fracking and shale gas comes into the picture.
A number of studies have shown that greenhouse gas footprint of shale gas is higher than conventional sources of natural gas. The exact amount varies widely with the studies, but suffice it to say that as shale gas becomes a greater and greater part of the US energy supply, the greenhouse gas reductions of switching from coal to gas will decline.
Which is all to say, past performance is no guarantee of future success here.