You have to put that in perspective bit. Methane is a powerful greenhouse gas but it breaks down in a few years as well. And then there is the notion that there is quite a lot of it leaking in the atmosphere already through natural processes, melting of the permafrost, and indeed as a side effect of oil/gas production. You'd have to produce stupendous amounts of fuel to have leakage to an extent that it actually matters.

So, if we'd actually capture and burned the gas, there would be little to worry about. Especially considering we'd be getting less of it out of the ground in fossil form.