By Roger Harrabin
BBC environment analyst, Bali
In public, climate scientists and European politicians are generally optimistic that rising carbon dioxide levels and temperatures can be curbed.
If global temperatures rise, billions will face water shortages
In private, some are less sanguine; but there has been a widespread unwritten code of optimism to avoid being accused of scaremongering or creating despair.
Now, science advisors to two governments with claims to leadership in global climate politics, Germany and the UK, have told BBC News it is unlikely that levels of greenhouse gases can be kept low enough to avoid a projected temperature rise of 2C (3.6F).
Professors Sir David King and John Schellnhuber say the world is more than 50% likely to experience dangerous levels of climate change.
They believe politicians have been too slow to cut emissions.
Current science suggests that above 2C, billions of people will face water shortages, the world's food supplies could be threatened and widespread extinction could be triggered.
Neither scientist believes that the world would achieve the goal of the Intergovernmental Panel on Climate Change (IPCC) of stabilising emissions by around 2015.
Lack of optimism
Prof King said he believed there was a 20% chance of temperature rise exceeding 3.7C - an increase that could seriously damage the global economy.
"Ask yourself the question," he said, "if you got in an aeroplane and the pilot said you've got an 80% chance of landing this plane safely, I doubt if you'd get in the plane."
Prof James Marburger, the US chief scientist, previously told the BBC that carbon emissions should be cut immediately - but that it was impossible to be sure what a dangerous level of climate change might be.
The scientists' warning comes as politicians begin to arrive in Indonesia for the latest climate talks - and as a Mori poll suggests that two-thirds of people in the UK do not trust world leaders will solve climate change.
The history of climate negotiations do not inspire optimism.
World leaders first pledged to avoid dangerous climate change at the Rio Earth Summit in 1992 when they signed the non-binding UN Framework Convention on Climate Change. Emissions continued to rise.
Then came the legally-binding Kyoto protocol. But the USA and Australia pulled out, which undermined the effort to reduce emissions, and corroded the will of other governments.
Japan - a signatory to Kyoto - should have cut by 6% but it has increased emissions by 7%.
Can greenhouse gas emissions be kept to stable levels?
Italy (+7.4%) and Spain (+59.8%) are missing their targets by a mile.
In the UK, carbon emissions have recently been going up despite all the government's green rhetoric.
And meanwhile the big developing nations which signed the Kyoto Treaty but were not obliged to cut emissions under it have been doing their catching up.
Still a long way behind rich nations in terms of pollution per person but posing now a mighty threat.
It all means that since the world committed to avoid dangerous climate change, emissions globally are up around 22%, the highest levels of CO2 since dinosaurs roamed a sweltering earth.
In his interview with the BBC, Prof King warns that we will have to spend more on adaptation as well as on cutting emissions.
He says it will not be cheap - and that is not a message you hear often from his political masters.
He also said it took until 2005 before the UK cabinet really understood the implications of climate change for all departments (an implicit criticism of Gordon brown and the Treasury).
Prof King said he believed the UK now had the most comprehensive plan for tackling climate change of any major economy.
Britain is putting money into a "global audit" of climate adaptation
He also said he was optimistic that politicians globally would now take much more urgent action to tackle emissions.
Prof Schellnhuber agreed - and said Germany would unveil a plan to cut emissions 40% by 2020, a more ambitious target than the UK.
Prof King said there was much more chance of action on climate as President Bush was approaching the end of his term of office.
He said the US government climate strategist James Connaughton had positively obstructed progress on tackling climate change.
The two men have an adversarial history - Prof King was described by Republican politicians as a scare-monger, and he believes it was Mr Connaughton who banned him from private talks at Camp David between Mr Bush and Tony Blair on climate.
But as the world's politicians begin to face up to the need to cut emissions, they may face unpleasant surprises.
Buried in the latest IPCC document is a little-noticed sentence admitting that our projections for emission reductions might be underestimated due to missing carbon cycle feedbacks.
That means the earth may already be turning against us - as our emissions heat the world, the Arctic sea ice melts, the dark water absorbs more heat and causes further melting. And so on in many different ways.
That means we may within 50 years need to take all, or almost all, the carbon out of the way we live. That would need an extraordinary technological and social revolution.
Of course the mainstream science may be wrong. There is still huge uncertainty in climate modelling.
In a recent survey of climate scientists conducted by a leading sceptical scientist, Dr Roger Pielke Sen, 18% of those who responded said the IPCC had exaggerated.
But 65% said the IPCC had got it right. And 17% said the prognosis was even worse.
Meanwhile, the UK still plans a huge airport expansion, there is not the slightest hint of a deal that would see rich nations pay poor nations to capture their emissions from coal and even Democrats in the US Congress want to postpone any tough action on emissions until after 2020.
That may be why the scientists' mask of optimism is beginning to slip.