It is not, admittedly, a trifling sum. The International Energy Agency reckons it will cost US$45 trillion ($60 trillion) to develop and deploy the technologies needed to halve carbon emissions from the energy sector (including transport) by the middle of the century.

That is about what would be required to stabilise the levels of carbon dioxide in the atmosphere at 450 parts per million, which would be half as much again as the air breathed by Captain Cook or anyone before him.

It ought to be enough to keep the rise in average global temperatures below 2.4C and is the target the leaders of the Group of Eight leading industrial powers agreed last year they would seriously consider.

While $45 trillion is a lot of money, it has to be put in perspective.

It would be spread over more than 40 years and across the whole world economy. It would equate to just over 1 per cent of global gross domestic product over that period, the IEA estimates.

And it would be offset by the cost of the fossil-fuel use avoided, which could be of a similar order, the IEA says. As it acknowledges, however, in a world where the oil price can jump $11 in a single day, any estimates of that are "debatable".

The problem for this scenario, it says, is not the cost but the "burden sharing", which is diplomatic language for dealing with free rider problems.

Current trends in areas like fuel switching and improving energy efficiency will not get us there.

"We will need in the coming decade a global revolution in the way we produce and use energy, with a dramatic shift in government policies and unprecedented co-operation among all major economies," the IEA says.

This call for revolutionary change is not coming from a bunch of perfervid, hair-shirted Greens but from a famously conservative body representing the richest energy-consuming countries.

So what are the technologies they think will keep the planet liveable, by eliminating about 50 billion tonnes a year of CO2 emissions?

More than a third of it, the biggest contribution, is expected to come from improvements in the efficiency with which end-users use fuels and electricity. These are also the cheapest gains, often indeed saving the consumer money.

The IEA believes the fuel economy of cars and lighter trucks could be doubled using existing technologies and at reasonable cost.