Picking on SUVs ignores the price of the low-cost elephant in the sky

London Briefing: The energy minister, Malcolm Wicks, is taking aim at drivers, particularly those who roam around UK cities in vehicles designed for the country. In the US they call them SUVs, in Britain, they are better known as 4x4s.

Either way, cars that UK ministers can't afford appear to be going the same way as fox hunting, at least if Wicks gets his way.

The message from such policy pronouncements is rather profound. The government is ratcheting up its efforts to cut CO2 emissions and is clearly going to focus on cars, particularly of the larger engine variety.

The connection between global warming and carbon dioxide is, these days, taken as given. Sceptics are regarded as dangerous cranks and are held in as much contempt as those who supported the Iraq war. Indeed, most environmentalists assume their critics are died in the wool neo-conservative supporters of George Bush. All of which is a shame, since it obscures the underlying debate.

As far as this non-scientist can see, the evidence about global warming is pretty clearcut. It is happening. While there may be one or two doubts about the statistical evidence (how do they know, with any accuracy, the average temperature of northern Europe 200 years ago?), it is clear that we can draw lots of graphs that show average temperatures rising and the rise looks to have accelerated over the recent past. Climate change is real.

It is also statistically true that emissions of CO2 have been rising, particularly since the industrial revolution. So, we have two highly correlated events: global warming and higher levels of CO2. Now, as any junior statistician will tell you, correlation is not causation. Something else is needed to prove one thing causes another.

Many years ago, I attended the inaugural professorial lecture of one of Britain's leading econometricians (economists who deal with statistics), David Hendry. He spent his hour showing, with impeccable precision, that changes in the amount of rainfall in the Scottish highlands cause the rate of inflation in the UK to go up and down. The logic was impeccable and the correlation, of course, was totally spurious. Spurious correlation is taken so seriously by economists and statisticians that they talk about it in Latin. Post hoc ergo propter hoc (after this, therefore because of this) is the way academic economists insult each other (nobody ever said that these people would make good reality TV).

We know that global warming and CO2 emissions are correlated. Scientists have theories about why they might be linked. A small minority argue they are not linked.

One thing that nobody says is that we don't know. That reminds me about the long-running debate about monetarism: since time immemorial, economists have argued about whether changes in the money supply cause changes in inflation or vice versa. We know that the two variables are correlated (sort of) and have all sorts of competing theories about why one might cause the other. But nobody really knows. And nobody admits this.

Scientists are not bad people and are not stupid: most rational human beings when confronted by the facts of global warming might conclude that it would be wise to cut back on CO2 emissions just in case the connection is real. That, I suspect, is the substance of the debate.

But it might be equally valid to conclude that efforts - especially cash - would be better directed at coping with the effects of global warming rather than on the futile effort to stop inevitable climate change.

But why attack the poor old SUV? The minister would be much better advised to have a go at aeroplanes. We could drive a fleet of 4x4s around the world several times and still emit less CO2 than a plane flying one journey from London to Malaga.

It strikes me that we should not take the pronouncements of Wicks seriously until he works out where the real problems lie. Eventually, he will get it and taxes on airline travel will rise.

One inevitable consequence of the correlation between global warming and CO2 is that he era of low-cost air travel is now drawing to a close.

•Chris Johns is an investment strategist with Collins Stewart. All opinions are personal.