It is silly. No-one wants to burn fuel needlessly. But having vision is essential.
Let's say you started the engine and drove straight off - unable to see through an iced up and misted screen - killing four pedestrians as a direct result of having no vision.
I'm quite sure the relatives of those four fatalities would not take much solace in the fact that you saved the environment from a couple of micrograms of needless CO2.
I doubt that you'll feel too good about yourself. I very much doubt that the Her Majesty's Courts Service would grant you much leniency when it comes to sentencing for causing death by dangerous driving.
What an insane (thanks, TN, for that word) situation.
What a stupid country it is in which we live. Governed by stupid mindless F.W.s like GORDON BROWN!!!!!
There is, of course, one way to avoid idling - place the footwell mat on the pedal and let it bounce off the rev limiter at 7000+ rpm
Furthermore, who cares anyway? If it's cold enough that your car needs 4-5 minutes to warm up then global warming CANNOT be a particularly big deal, can it?