If you’ve been following along, you know that over the last several years advances in hydraulic shale fracturing (“fracking”) technology have opened up large new sources of natural gas to the energy production market. The Law of Supply being what it is, this has driven down the price of natural gas, and consequently electricity generation companies have switched from preferentially burning coal to preferentially burning natural gas, as that’s now the cheaper way to get a therm. The unexpected result of this is that the United States’s CO₂ emissions from electricity generation have significantly dropped.

This is surprising at first. Why does switching from coal to natural gas reduce emissions so much? After all, they’re both fossil fuels. Well, coal is polycrystalline graphite with impurities, i.e., energetically essentially all carbon. To burn graphite, you put in energy to break C-C bonds then get energy when you form C-O bonds, and your only byproduct is CO₂. Natural gas is primarialy methane, with other hydrocarbons and impurities. To burn methane you break C-H bonds, which are weaker than C-C bonds, and get energy from not only C-O bonds but also the stronger O-H bonds, so methane is a more efficient fuel. On top of that, much of your byproduct is water vapor, which doesn’t contribute to global warming.

Some numbers: Burning 1 mol of either graphite or methane produces 1 mol of CO₂. Graphite’s molar heat of combustion is 393.5 kJ, while methane’s is 802.3 kJ. That means you get slightly over twice as many therms from burning methane as you do graphite for a given quantity of CO₂ produced. So it’s no wonder that the switchover to natural gas has reduced CO₂ emission so drastically!

An unregulated market is a double edged sword for your greenhouse gas policy, however. It’s just blind luck that a lower-carbon-footprint energy source became economical. The market can just as easily make natural gas less economical, or make a higher-carbon-footprint source (e.g., tar sands) economical.