By Richard Black
Environment correspondent, BBC News website, Valencia
Environmentalists urge a decrease in fossil fuel burning
So here, as Australians say, is "the big ask".
You have a global economy that depends on fossil fuel use.
Our economies grow primarily by increasing fossil fuel use, particularly coal, the most polluting form.
And you have a decade to turn it around without letting economic growth slide away.
This, in a nutshell, is the challenge set by the Intergovernmental Panel on Climate Change's (IPCC) synthesis of its 2007 global assessment.
"There is real urgency," said Bert Metz from the Netherlands Environmental Assessment Agency, who co-chaired the IPCC working group on options for mitigating climate change.
"We need to peak emissions within 10 years if we are to keep the global temperature rise to 2C. If we leave it for 25 years, we're already committed to 3C."
Handily, the IPCC summary published here also tells you what those temperature rises translate to in terms of impacts.
Grounds for optimism?
Two Celsius means about one third of species at risk of extinction, decreased cereal production in the tropics, most coral reefs bleached.
Three Celsius puts millions more people at risk of coastal flooding, decreased cereal production at all latitudes and widespread death of coral reefs.
The IPCC says more heat waves are very likely in the future
Those are just a selection; read the document itself for a more graphic picture, and read the entire Working Group 2 report for a still fuller exposition. It is not pretty.
You can see what IPCC chief Rajendra Pachauri meant when he described the consequences of not reversing the fossil fuel juggernaut as "disastrous".
Unfortunately, that juggernaut is exactly what has delivered the high living standards that Western nations have come to enjoy, and to which developing countries aspire.
The UN message is "it can be done".
"We can achieve significant greenhouse gas reductions simply by looking at energy efficiency," said UN Environment Programme executive director Achim Steiner.
"The simple light bulb has become a symbol - with a change of light bulb you can reduce your energy consumption [on lighting] by four-fifths.
"And if we look at reducing emissions by reducing deforestation, it's a win-win situation."
United Nations figures have to remain "on message" with this.
The IPCC is a UN agency, the Convention on Climate Change and Kyoto Protocol are UN treaties, so it would be a bit odd if a senior UN official came out and said "it's a waste of time, it's hopeless, we're never going to do it".
Looking at the real world, it is hard to see any real grounds for their optimism, however.
The IPCC analysis implies that we need to curb fossil fuel use within a decade.
The International Energy Agency predicts energy use will increase by 50% by 2025, with fossil fuels still the dominant component.
The IPCC says vehicles should become more fuel efficient. The Asian Development Bank projects that the number of vehicles in some Asian countries including China could rise by a factor of 15 within 30 years, dwarfing any efficiency gains.
The IPCC says protecting and replanting forests is the most efficient way to curb emissions.
The Food and Agriculture Organization finds that forests are being lost at "an alarming rate" - lately, of course, helped by the craze for biofuels.
There is the myth of hydrogen, the limits of wind, the cost of solar, the ecological impacts of biofuels and so on, and so on.
Yet the IPCC says fossil fuel use can be curbed; so how, precisely?
"You have to establish a global price of carbon, so that becomes a factor in the decisions that companies make," said Dr Metz.
"One way that is happening with the Kyoto treaty is through quotas for emissions and establishing a market. But you can also do it through taxation, or through subsidies to make the things we need to come in more attractive - anything that creates an incentive."
A global carbon market was one of the Stern Review's principal recommendations too.
But here is the rub; in order to have a meaningful global carbon price, you need to have a meaningful global system of mandatory emissions targets.
"You are not going to do this with regional pacts of voluntary measures," observed Dr Metz, in what could be interpreted as a comment on US-led initiatives such as the deal clinched at September's Asia-Pacific Economic Co-operation (Apec) forum meeting in Sydney.
And what would a global framework of mandatory emissions targets look like?
Well, probably something like the Kyoto Protocol, many of whose members are struggling to meet even the modest reduction targets that they signed up to a decade ago, and which many countries including the US abhor as a model for any future global agreement.
The environmental movement, which now finds some of its long-standing fears endorsed by the IPCC, is adamant on what the next steps must be.
"You couldn't get a clearer picture of where the world is heading than this report," said Stephanie Tunmore of Greenpeace International.
"It sets out a compelling case for early action, and it must take centre stage at the next round of UN talks in December in Bali."
UN Secretary General Ban Ki-moon is also hoping for a sunny outcome in Bali in which every country is prepared to say something about controlling its emissions - the rich reducing, the poor swelling more slowly.
"Industrialised countries need to continue to take the lead in climate change abatement," he said.
"But at the same time, we cannot ignore the reality that if developing countries fail to join the effort, there can be no viable solution.
"In Bali, let us not point fingers or apportion blame; rather, let us find common ground."
On the face of it, the minimum outcome from Bali - an agreement about when to start and when to finish talks on another round of emission targets to follow the existing Kyoto Protocol set - is not terribly demanding.
The last talks, a year ago in Nairobi, were conducted in an incredibly polite manner.
So polite, in fact, that not a single government elbowed its way through the door marked "get your emissions reductions here" - the hubbub of "after you" - "no, after you" could have been truly life-affirming in a different context.
The IPCC has told us why things need to be different now, and given us some ideas of how to point our eco-friendly hybrid towards the promised low-emission land.
One suspects that in the real world it is still liable to get side-swiped by the seductive juggernaut of business-as-usual.
In that case, we will have a trackside view if the impacts projected in this seminal IPCC climate treatise come to pass.