it won't save our bacon. the entire human race only emitts 3% of the planets CO2 output. we have next to no effect on atmospheric CO2 whatsoever. it's all scare-mongering boll0cks that do-gooders use to winge and politicians use to tax the sh!t out of us.
in summary, if the human race HALVE our CO2 output, we will still only reduce it by an overall 1.5%...
*just thought* after re-reading your post si, did you mean if we run out of petrol, or somehow, the means to manufacture petrol?