The belief that we can stabilise the climate at a specified concentration of greenhouse gases in the atmosphere, with an associated increase in average global temperature, has underpinned all international negotiations over global warming. The idea that greenhouse gas emissions must be limited to prevent 'dangerous' warming is embodied in the 1992 Framework Convention on Climate Change. The official European and Group of Eight goal of aiming to keep warming below 2[degrees]C is based on this idea, as are greenhouse gas concentration targets such as 450 parts per million (ppm) or 550 ppm advocated in the Stern report and Australia's Garnaut report.
But the belief that humans can adopt policies that stabilise the climate rests on assumptions that aren't well founded in the science. Stabilisation requires that annual emissions are eventually reduced to 'the level that balances the Earth's natural capacity to remove green house gases from the atmosphere'. The problem is that global warming is likely to trigger its own 'natural' sources of new emissions and interfere with the Earth's capacity to remove carbon from the atmosphere.
The Earth's climate is not like a machine whose temperature can be regulated by turning some policy knobs; it's a highly complex system with its own regulatory mechanisms. Humans cannot regulate the climate; the climate regulates us. For several years, climate scientists have understood that some of the relationships among variables are non-linear, so that a slight increase in warming can cause a large shift in other aspects of the climate. Paleoclimatologists have known this for a long time, but it's only recently that the idea has been linked explicitly to today's global warming.
If we look at a chart showing the Earth's climate history stretching back over many millennia, we don't see smooth transitions from ice ages to 'interglacial' or warm periods (such as the one we are now in). The transitions are sometimes dramatic, with sharp changes in climate occurring over mere decades, probably due to amplifying feedback effects. So climate states can end abruptly once certain thresholds are crossed, setting off accelerated warming that is stopped only when a natural limit is reached, such as the disappearance of ice from the Earth.
There are numerous tipping points that could induce positive-feedback effects that amplify warming and its effects, including the disappearance of summer sea-ice in the Arctic, the melting of the Greenland ice sheet, the melting of the West Antarctic ice sheet, the release of carbon from melting permafrost, and large-scale dieback of the Amazon rainforest. As they occur, these changes will be effectively irreversible, at least for thousands of years.
A recent paper has destroyed any idea we might have that we can take radical corrective action once things become intolerable. It reaffirms that a large proportion of the C[O.sub.2] we are putting into the atmosphere will still be there in 1,000 years, so the level at which emissions peak makes a huge difference. Both the warming and the sea-level rise associated with that peak will not decline, even if emissions fell to zero, but will stay virtually constant for more than a millennium. The authors conclude:
'It is sometimes imagined that slow processes such as climate change pose small risks, on the basis of the assumption that a choice can always be made to quickly reduce emissions and thereby reverse any harm within a few years or decades. We haw" shown that this assumption is incorrect ...'
The lag between emissions and their effects on climate and the irreversibility of those effects make global warming a uniquely dangerous and intractable problem for humanity. Among other things, these features of climate change render standard economic analysis of the problem hopelessly inappropriate. Indeed, it is positively dangerous. …