Sad to have to say it but some of the ideas that come from our current political class are just so stupid that they should have remained unuttered. Like this one that's just come from the green wing of the Lib Dems:

Liberal Democrats want to ban all conventional cars by 2040 and allow only electric and ultra-low emission vehicles on the roads. Every diesel and petrol car would be scrapped by then or earlier if there are enough technological advances. The extraordinary policy, contained in the Lib Dems’ ‘green manifesto’ launched this week, has the backing of senior figures including party president Tim Farron.

Let us start from the idea that the IPCC is correct, climate change is a problem that we need to do something about. Let us also, arguendo, agree that Lord Stern was correct as to what we should now do about matters. Excellent, what we should do is have a carbon tax equal to the social cost of the damage that emissions will do. And, here in the UK, we do indeed have a tax that performs that function for us. The fuel duty escalator was specifically brought in to "meet our Rio commitments" and it is now higher than that $80 a tonne CO2-e that Stern insisted that the tax should be. So, by the very arguments that have been used to persuade us that something must be done we are already doing that something.

Further, Stern (and all other economists) insist that this sole interference into the price system is all that we should do: and they also all make it very clear indeed that near random bureaucratic interference into that  price system fixing is going to be welfare destroying.

For here's what we're trying to do. We want to prevent people doing what causes more damage in the future than that thing produces benefits in the present. And we also want people to carry on doing things that produce greater benefits now than damage in that future. Because this is the method by which we maximise utility over time. Which is what the carbon tax achieves. People are priced out of doing things with greater costs than benefits while still enjoying those actions which produce greater benefit than cost.

Then along comes some nitwit who says that even though the use of petrol engines will produce greater benefits than costs we must still ban them. This must be the action that is being proposed here: if technology has advanced enough in 25 years time that non-petrol engines will provide greater utility than petrol ones then we will all naturally switch. If, on the other hand technology has not so advanced then the banning will be an obvious reduction in utility.

And that is why this is such a stupid proposal. If technology advances sufficiently then the ban is unnecessary. If it does not then the ban is simply making people poorer, that being the same statement as reducing utility.

What makes the proposal monstrously stupid is that we've already done everything we were supposed to do to beat climate change from fossil fuel powered vehicles. We've already got that carbon tax at over and above the social cost of emissions. Anyone who doesn't realise that just hasn't been paying attention to all that we're being told about climate change. And people who haven't been paying attention really shouldn't be proposing political policy, should they?