“The message [that the synthesis report] contains, could not be simpler,” said Ban Ki-Moon, United Nations Secretary General. “The threat of climate change is real and there are concrete and affordable ways to deal with it. Scientists have now done their work and I call on political leaders to do theirs and agree not only to launch negotiations, but also to conclude them by 2009.”

The IPCC's Fourth Assessment Report will feed into the UN climate-change negotiations that are scheduled to take place in Bali in early December.

“The breakthrough needed in Bali is an agreement to launch negotiations for a comprehensive climate-change deal that all nations can embrace – developed and developing alike,” said Ki-Moon.

The IPCC met last week in Valencia, Spain, to agree the final wording of the synthesis report. According to the document, unmitigated climate change would in the long term be likely to exceed the capacity of natural, managed and human systems to adapt. What’s more, there is "much evidence" that mitigation actions can bring near-term co-benefits – such as improved health resulting from reduced air pollution – that could offset a substantial fraction of the costs. Modelling studies show that carbon prices of $20 to $80 per tonne of carbon dioxide equivalent by 2030 would lead to stabilization of atmospheric carbon dioxide equivalent levels, at around 550 ppm by the end of the century.

“A wide variety of stabilization levels can be achieved by deployment of a portfolio of technologies that are either currently available or expected to become commercialized within the coming decades,” said Raj Pachauri, chair of the IPCC. “If we want action, then clearly there has to be a price atttached to carbon.”

The macroeconomic costs of mitigation generally rise as the stabilization target becomes more stringent. The IPCC predicts that, in 2050, global average macro-economic costs for mitigation towards stabilization between 710 and 445 ppm carbon dioxide equivalent are between a 1% gain and 5.5% decrease of global gross domestic product (GDP). This corresponds to slowing average annual global GDP growth by less than 0.12%.

“Delayed emission reductions significantly constrain the opportunities to achieve lower stabilization levels and increase the risk of more severe climate change impacts,” says the report.

But what of the irreversible impacts? The scientists found that around 20–30% of species are likely to be at increased risk of extinction if global average warming exceeds 1.5–2.5°C relative to 1980–1999. And it’s likely that the meridional overturning circulation (MOC) in the Atlantic Ocean will slow during the 21st century. The report says that the MOC is unlikely to undergo a large abrupt transition during that period, but that longer-term changes cannot be assessed with confidence. Large-scale changes in the phenomenon would affect marine ecosystem productivity, fisheries, ocean carbon dioxide uptake, oceanic oxygen concentrations and terrestrial vegetation.

What’s more, partial loss of ice sheets on polar land could lead to metres of sea level rise, major changes in coastlines and inundation of low-lying areas such as river deltas and islands. Although such changes are projected to take place over millennia, scientists cannot rule out more rapid sea level rise on century time scales.

Speaking of the synthesis report as a whole, Bob Spicer, of the UK’s Open University, said: "This is a tremendous achievement, but it has to be remembered that because of the political element in the process, and the fact that the geological record of how the real Earth system has actually behaved in the past has been largely ignored, the predictions for any given emissions scenario are likely to be underestimates – the most benign view – of future change. Those areas undergoing most rapid change now – the Arctic and continental interiors – are likely to be where the biggest errors in the predictions lie.”

Spicer believes that this “has worrying implications for poorly understood processes, which are played down in the IPCC because they cannot be quantified sufficiently to pass political vetting”.

The last word goes to IPCC chair Raj Pachauri: “Gandhi said ‘be the change you want to see’ and I think therefore what we really need is a new ethic by which every human being realizes the importance of the challenge we’re facing and starts to take action, through changes in lifestyle, in attitude and behaviour.”