From "New Scientist" -- usually a reliably "Green" source:
Climate change models, no matter how powerful, can never give a precise prediction of how greenhouse gases will warm the Earth, according to a new study. The result will provide ammunition to those who argue not enough is known about global warming to warrant taking action.
The analysis focuses on the temperature increase that would occur if levels of carbon dioxide in the atmosphere doubled from pre-Industrial Revolution levels. The current best guess for this number - which is a useful way to gauge how sensitive the climate is to rising carbon levels - is that it lies between 2.0 C and 4.5 C. And there is a small chance that the temperature rise could be up to 8C or higher.
To the frustration of policy makers, it is an estimate that has not become much more precise over the last 20 years. During that period, scientists have established that the world is warming and human activity is very likely to blame, but are no closer to putting a figure on exactly much temperatures are likely to rise.
It now appears that the estimates will never get much better. The reason lies with feedbacks in the climate system. For example, as the temperature increases, less snow will be present at the poles. Less snow means less sunlight reflected back into space, which means more warming. These positive feedbacks accelerate global warming and also introduce uncertainty into estimates of climate sensitivity, say Gerard Roe and Marcia Baker of the University of Washington in Seattle. What is more, they found that better computer models or observational data will not do much to reduce that uncertainty. A better estimate of sensitivity is the holy grail of climate research, but it is time to "call off the quest", according to a commentary published alongside the paper.
That is likely to fuel attacks by critics in the oil industry and elsewhere who argue against investing in measures like clean energy until more is known about climate change. Others say that we need to act even if climate sensitivity lies at the low end of the scale, since coastal areas would still be threatened by rising seas, for example.
Ultimately, the papers also illustrate the limits to which models, even those produced by powerful supercomputers, can help politicians make decisions. "This finding reinforces not only that climate policies will necessarily be made in the face of deep, irreducible uncertainties," says Roger Pielke, a climate policy expert at the University of Colorado at Boulder, US. "But also the uncomfortable reality - for climate modellers - that finite research dollars invested in ever more sophisticated climate models offer very little marginal benefit to decision makers."
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