KEY POINTS:
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.
Beyond that, plug-in hybrids and all-electric vehicles have emerged as an important option, it says. But despite rapid progress in battery technology, important technological challenges remain to be overcome before mass production of these vehicles would be an option.
Through efficiency improvement and electrification, both average oil use and CO2 emissions from light vehicles could be cut by 75 per cent or more by 2050, the IEA says.
But as demand for travel is expected to treble by that period, average emissions per kilometre would need to fall by two-thirds just to stay even. And light vehicles account for only about a half of transport sector emissions. Decarbonising trucks, aircraft and ships is a bigger challenge.
The IEA sees just over a fifth of the overall targeted reduction in global emissions coming from renewable energy sources. Globally, as in New Zealand, their share of energy supply has been falling over the past 20 years.
There are signs that is turning around, though.
Last year about 30 per cent of the increase in electricity generation capacity, some 33,000MW, was renewable, mainly wind power.
To get to the target of halving energy emissions, the IEA envisages nearly half of the electricity generated in 2050 will be from renewable sources, compared with 18 per cent now.
It also sees a quadrupling of the use of biomass, to the point where it would provide nearly a quarter of the world's primary energy. That would require 15 billion tonnes, half of it from purpose-grown energycrops, the rest from crop and forest residues.
It is talking about "second generation" biofuels, made from inedible plants or bits of plants, rather than the grains and oilseeds used today - a wicked waste of food and of dubious net benefit to the climate.
Widespread deployment of second generation biofuels technology could begin within five to 10 years, the IEA says.
The other big source of emission reductions, about a fifth of the total, is expected to be carbon capture and storage (CCS).
This involves taking the CO2 produced at large point sources such as thermal power stations or heavy industrial plants and burying it in a suitable geological structure underground where we can be reasonably sure it will stay put.
The sort of timeline the IEA envisages for the power-sector side of this would require 10 demonstration plants to be built over the next seven years, then 20 full-scale demonstration plants by 2030, costing about US$1.5 billion a pop.
By the middle of the century it sees 30 per cent of the world's electricity coming from coal- or gas-fired power stations with CCS.
The remaining reductions needed to halve emissions would come from fuel switching (from coal to gas or from either of them to biomass) and from an expansion of nuclear power, with as many as 30 nuclear power plants being built each year.
All of this raises the question what sort of carbon prices would be necessary to make these technology shifts commercially worthwhile.
The IEA estimates that getting the curve in the growth of carbon emissions to bend and come back to 2005 levels could be done with a carbon price of US$50 a tonne. It would be equivalent to an extra US$20 a barrel on the oil price.
But to go on to halve it from there requires scarier carbon prices - up to US$200 a tonne (or US$80 a barrel), or even higher on more technologically pessimistic assumptions.
If they are even half right the implications are clear. This is turning into a world where you want to be a seller, not a buyer, of carbon.
One man's cost is another man's livelihood (up to a point). The trick is to be the other man.
That US$45 trillion is a big business opportunity. At the very least we need to see wind turbines not as eyesores blighting what is, in any case, a man-made landscape - but the elegant shape of the future.
And plant a lot more trees.
