What if Natural Gas Produced All Our Energy?

I was thinking about something Roger Pielke Jr wrote on his blog:

The fact of the matter is that the world will accelerate the decarbonization of its economy when clean energy is cheaper than dirty energy. The remarkable transition to natural gas shows that.

That's probably almost certainly true, at least if you ignore negative external costs (which are very difficult to get people to pay for anyway -- or, at least, to pay for up front). Anyway I was thinking, what if we got all our energy from natural gas? That is, how much carbon would we save if all the energy we now obtain from oil and coal were instead obtained from natural gas?

The carbon savings aren't that large: about 700 Mt CO2/yr less than the 5,471 Mt we emitted last year, or 13%.

The numbers: 
The CO2 numbers for energy consumption are here; in 2011 the US emitted

coal: 1,867 Mt CO2

natural gas: 1,294 Mt CO2

oil: 2,299 Mt CO2

for a total of 5,471 Mt CO2.

These can be converted into energies via conversion factors; I'll use

coal: 94 g CO2e/MJ

natural gas: 68 g CO2e/MJ

oil: 73 g CO2e/MJ

(MJ = Megajoule), which works out to 17.5 metric tons per person per year. (Incidentally, that's down 21% from 1973's 22.3 tonnes, and down significantly even from 2004's 20.4 t.) Admittedly this ignores subtleties about different types of coal, the energy required to produce energy, that we can't currently fly planes on natural gas, etc. This is good enough for blog work -- or, if you will, a gebloggen experiment.

Converting, I estimate the US used 70 trillion MJ of energy in 2011, or a power consumption of 2.2 terawatts -- 7,100 Watts per person!

Converting back, if all this energy were produced by natural gas, we'd have emitted 4,800 Mt CO2, which is still a per capita emission of 15.3 t CO2, or 13% less than what we emit today.

So we would save some significant carbon emissions. But it would not nearly be enough to stabilize climate, which requires emissions cutbacks of roughly 80% by the US.

Transitioning to natural gas is a good thing (as long as your drinking water isn't getting fracked up). But really solving the carbon problem requires a game changing technology.