Officially, climate policy targets are aiming to cap emissions at around 450 parts per million (ppm), which would theoretically prevent global average temperatures rising beyond a 'safe' 2 degrees Celsius. The first problem is that we are long passed the danger point. In mid-2005, the Intergovernmental Panel on Climate Change (IPCC) confirmed that the total atmospheric concentration of greenhouse gases (accounting for nitrous oxide, methane and so on) was already 455 ppm. This implies that we are already well on course to surpass 2 degrees.

The second problem is that as a growing number of leading climate scientists are now telling us, climate policy targets lag far behind the peer-reviewed science. The IPCC's and most other conventional climate models used to inform policy, do not sufficiently account for the complex role of uncertainties linked to positive-feedbacks - that is, the capacity of global warming to trigger changes which, in turn, trigger further changes, leading to a self-reinforcing feedback-cycle.

A disturbing body of data indicates that if we stray for too long above 350 ppm, as we are already, we are in grave danger of raising global temperatures by 1C (we are now at 0.8C), triggering exactly such positive-feedbacks with the potential to lead to dramatic, irreversible changes that could possibly culminate in runaway global warming. We don't even need to get to 2C.

James Hansen, who heads the NASA Goddard Institute for Space Studies, warns that if our "present overshoot" of the 350 ppm upper limit "is not brief, there is a possibility of seeding irreversible catastrophic effects." According to a 2009 paper in Nature co-authored by 28 international climate scientists, these effects would include "the risk of irreversible climate change, such as the loss of major ice sheets, accelerated sea-level rise and abrupt shifts in forest and agricultural systems."

It is likely that some of these feedbacks are already underway. The IPCC had originally projected the disappearance of the Arctic's late summer sea ice by the end of the century. But this year, Mark Serreze, head of the US National Snow and Ice Data Center, reported: "The Arctic sea ice has reached its four lowest summer extents (area covered) in the last four years. I stand by my previous statements that the Arctic summer sea ice cover is in a death spiral. It's not going to recover." Scientists fear the summer sea ice could disappear within three years.

The implications could be catastrophic. The accelerating sea ice melt is linked to the thawing of the Arctic permafrost, beneath which is trapped in the form of methane double the amount of carbon in the atmosphere. Current emissions levels, if unchanged, would lead local Arctic temperatures to rise up to around 8C. World permafrost expert Vladimir Romanovsky of the University of Alaska notes that this would be enough for half the world's permafrost to thaw to a depth of several metres, releasing the vast stores of methane into the atmosphere. Another permafrost expert, Ted Schuur of the University of Florida, observes that the process of thawing and methane release could rapidly accelerate over decades, most likely in the form of a 50-year meltdown intensifying due to rapid feedbacks. The result would be a process of irreversible, runaway warming that would make life on earth largely uninhabitable - "Venus syndrome", in Hansen's words.

Unfortunately, even peak oil will not save us. While the International Energy Agency recently confirmed that world crude oil production most likely peaked in 2006 - leading the watchdog's chief economist Fatih Birol to observe that "the age of cheap oil is over" - there is still enough oil shale, tar sands, coal and natural gas to burn through the first quarter of this century. To be sure, that is not long - but it is long enough to potentially push us off a climate cliff.

Rapid decarbonisation of the economy is therefore not an option. It is a last ditch emergency response necessary for our survival in the emerging post-carbon age. But we cannot achieve this as long as we cling to the mantra of unlimited economic growth on a finite planet. As University of Surrey economist Tim Jackson proves in his Prosperity Without Growth, efforts to 'decouple' growth from availability of cheap fossil fuels have not only failed, they have actually gone in reverse.

This means we need to fundamentally re-think the very definition of prosperity if we are to ensure that our children inherit viable societies on a liveable planet. The economics of the fossil fuel age is now obsolete. It needs to be written for the post-carbon age.