By Richard Black
Environment correspondent, BBC News website
"It's a great report," enthused Bill Hare. "It's very strong, and it shows that it's economically and technically feasible to make deep
emission reductions sufficient to limit warming to 2C.
The IPCC maps a low carbon trip but does not guarantee arrival
"It shows that the costs of doing this are quite modest."
Bill Hare should know. As a researcher and campaigner, he has been on the climate change beat for well over a decade, and has read more reports than the average head teacher - quite possibly, written more too.
Even allowing for the euphoria which comes after a week of long negotiations and little sleep, the enthusiam which he and others have shown for the Intergovernmental Panel on Climate Change's (IPCC) Working Group 3 summary report on mitigation of climate change is, on the surface, well justified.
Here, for the first time, we have the overwhelming mass of the world's governments sitting down with humanity's leading experts and agreeing a price list for tackling climate change.
This is not the Stern Review, commissioned by a single government and visited upon the rest; it is not a dry assessment by scientists which policymakers will keep dry by leaving it on their top shelves forever.
However worthy and weighty such reports are, they are not automatically "bought into" by those holding the reins of political power.
In principle, IPCC reports are. The initial offerings of scientists (and in this case economists) are scanned line by line, clause by clause by representatives of almost all the world's governments.
They have endorsed its findings, and this is what makes the IPCC different; this is what leads to the expectation of action.
IPCC Working Groups 1 and 2, which released their summary reports earlier this year, looked at the science of climate change and the potential impacts on the world.
This third now tells us what are likely to be the most cost-effective ways of reducing emissions, and how much it will cost.
Nuclear may be cost effective, says the IPCC, but concerns remain
The answers to the second question are "it depends", and "not much".
It depends on what you set as a target for greenhouse gas concentrations. Keeping the global rise in average temperature to about 2C, the most radical and expensive scenario addressed here, would shave about 0.12% off the average annual GDP growth rate.
For comparison, the International Monetary Fund (IMF) projects annual global growth of 4.9% this year and next. So taking a small bite out for reducing emissions would leave the expanding cake virtually untouched.
And in its sober language, the report hints that the financial benefits from reducing emissions (in the form of avoiding climate impacts such as floods, droughts and migration) could be on a same scale as the investment.
"Limited and early analytical results from integrated analyses of the costs and benefits of mitigation indicate that these are broadly comparable in magnitude, but do not as yet permit an unambiguous determination of an emissions pathway or stabilisation level where benefits exceed cost," is the judgement.
Furthermore, the IPCC has laid out the areas where reductions could be achieved most cost-effectively, and the technologies for achieving them.
These amount to recipes for keeping the cake cheap and tasty at the same time.
So on transport, for example, the recipe involves a sizeable dollop of energy efficiency, a dash of biofuels, a pinch of increased vehicle occupancy and a switch from roads to rail and waterways.
Follow the recipe, it says, and global transport emissions can by 2030 be kept two billion tonnes of carbon dioxide below the levels they would have achieved under "business as usual" projections.
What is not to like?
There is, however, an elephant in the room, which the IPCC cannot address, because it belongs to national governments.
Investing in emissions control might bring a small reduction in GDP; but it would still be a reduction. The only exceptions are measures such as improving energy efficiency where there could be a net economic gain.
So in order to induce governments, businesses and individuals to make those emissions-curbing adjustments, some incentive is needed.
Although the IPCC acknowledges that social and cultural factors might make us more frugal with energy, it assumes - probably correctly - that money will do most of the talking.
Taxes and subsidies are discussed. But its conclusions really address a situation in which there is global pricing for greenhouse gas emissions; the higher the price, the more cost-effective it would become to invest in low-emissions technologies and lifestyles.
"The report provides a timely and welcome reminder that a long-term carbon price is essential to ensure that present and future energy investment decisions are low carbon ones," observed Michael Grubb, an economics professor at Imperial College London, chief economist of Britain's Carbon Trust and a major contributer to the IPCC report.
But currently the carbon market is not global; it is limited to European countries immersed in the Kyoto Protocol, and to other states, regions and companies of the world which have chosen to get involved.
As Professor Grubb told this website last year, a global carbon market needs to be driven by a global deal limiting emissions.
"In the longer term, spreading [carbon trading] further afield will depend on having further emissions targets spread beyond the countries which already have them."
Now what does that deal sound like? Ah yes - like an extension in time and geographical spread of the Kyoto Protocol targets.
Last time the international community talked about them, at the UN climate summit in Nairobi in November, it could not even agree when to start talks about talks.
Off the road
You might think that by endorsing this IPCC report, the very governments which are involved in the annual UN climate change negotiations have committed themselves to some concrete action.
If this is their intention, we should see signs of it emerging during the G8+5 talks in the middle of the year and at the next UN climate session in December.
Even so, they would have to agree on what scale of climate impacts they want to avoid, what level of greenhouse gas concentrations that equates to, and how soon they want to begin reversing the inexorable rise in emissions.
Perhaps a more accurate view can be deduced from changes to this report's wording made during the week's discussions in Bangkok, where a reference to a "global" carbon market became merely an "international" market, and a reference to the importance of "regulatory and financial incentives and international co-operation" in climate policy was removed altogether, with approval for the effectiveness of "voluntary agreements" inserted instead.
Governments have emerged with a roadmap to a cooler world. Whether they follow it, or get distracted on the way, or argue so much over which route to take that the journey never starts, is a question for another time.