UN:Behind the picture of a serious climate change threat, painted in a United Nations report to be published tomorrow, just how fast mankind can act remains unsure.
"There are so many reports," said Fatih Birol, chief economist at the International Energy Agency and author of one of the biggest, the IEA's 596-page World Energy Outlook 2006. "Some are much more in-depth, some have a different focus. The thing is how much they link with reality."
Inconsistencies abound - a European Commission study, for instance, contradicts the European Union executive's stated policy.
Tomorrow's report by the UN's Intergovernmental Panel on Climate Change (IPCC)follows on the heels of a clutch of papers in the past four months, all predicting different shades of gloom but no consensus on cost and urgency.
The UN report carries extra weight because it pulls together all available research, and all governments have to sign off on it. The final draft is clear - humans are almost certainly causing climate change and will have to cope with an average warming of around 3 degrees centigrade by the end of this century even if the issue is tackled promptly.
The IEA says that unless policymakers make an unprecedented effort, the best the world can expect is to keep carbon emissions around one-third above present levels by 2030. "We took policies and measures already being considered by governments, and likely to be implemented," said Birol, emphasising that the IEA's focus is on being realistic.
Such a scenario would consign humanity to a roughly 3 degrees Celsius temperature rise by 2100, according to Malte Meinshausen, climate scientist at the Potsdam Institute for Climate Impact Research.
A rise of that order could cause a series of dangers, according to a report commissioned by the British government, the Stern review, published last October.
These included: 1-4 billion more people suffer water shortages; 150-550 additional million at risk of hunger; up to 170 million more people affected by coastal flooding; and up to 50 per cent of species faced with extinction. For this reason the European Union wants global average temperatures not to exceed 2 degrees above pre-industrial levels, something one commission report says is too costly.
Published this month two days before the European Commission affirmed its 2 degrees target, the commission's own research report, the World Energy Technology Outlook, rejected this 2 degree limit as too expensive to implement.
In discussions before writing the report, the authors estimated this would require a carbon price of €400 per tonne by 2050 - compared with €15 at present. That would add €1 to the price of a litre of petrol, said Domenico Rossetti di Valdalbero, the commission scientist who supervised the work.
The report was prepared by a consortium of six European research organisations, and concluded that a 75 per cent cut in carbon emissions by developed countries by 2050 was consistent with 2 degrees, but did not analyse it on the basis of its cost.
"I feel it is feasible but relatively expensive, which I feel most scientists involved in this report would agree with."
Even a softened target, which would see developed countries halve their carbon emissions by 2050, would still require a carbon price of €200 per tonne by 2050 to encourage the adoption of low carbon-emitting energy technology.
The Stern review, commissioned by the British government and published last year, recommends a 60-80 per cent cut in greenhouse gas emissions by rich nations by 2050, versus 1990 levels.
The IPCC report released this week focuses on the scientific evidence for climate change, and will be followed in April by a report on the likely impacts, and in May by a third report on how to fight it.
The Eiffel Tower is to turn its 336 night-time lights off for five minutes tonight to draw attention to energy consumption and the environment on the eve of the publication of the UN report.
The tower's lights account for 9 per cent of the seven megawatts consumed hourly by the structure.
Earlier this week environmental campaigners from Greenpeace hung a banner from the tower showing a giant thermometer to publicise the global warming issue.