Climate sensitivity [:]This is the measure used by researchers of how much they expect the world’s average temperature to increase in response to particular increases in levels of carbon dioxide in the atmosphere. According to one table from the unpublished [IPCC] report, which was seen by The Economist, at CO2 concentrations of between 425 parts per million and 485 ppm, temperatures in 2100 would be 1.3-1.7°C above their pre-industrial levels. That seems lower than the IPCC’s previous assessment, made in 2007. Then, it thought concentrations of 445-490 ppm were likely to result in a rise in temperature of 2.0-2.4°C.
(Source the Economist, July 20, 2013, at 71)
The two findings are not strictly comparable. The 2007 report talks about equilibrium temperatures in the very long term (over centuries); the forthcoming one talks about them in 2100. But the practical distinction would not be great so long as concentrations of CO2 and other greenhouse-gas emissions were stable or falling by 2100. It is clear that some IPCC scientists think the projected rise in CO2 levels might not have such a big warming effect as was once thought.
There are several caveats. The table comes from a draft version of the report, and could thus change. It was put together by the IPCC working group on mitigating climate change, rather than the group looking at physical sciences. It derives from a relatively simple model of the climate, rather than the big complex ones usually used by the IPCC. And the literature to back it up has not yet been published.
Still, over the past year, several other papers have suggested that views on climate sensitivity are changing. Both the 2007 IPCC report and a previous draft of the new assessment reflected earlier views on the matter by saying that the standard measure of climate sensitivity (the likely rise in equilibrium temperature in response to a doubling of CO2 concentration) was between 2°C and 4.5°C, with 3°C the most probable figure. In the new draft, the lower end of the range has been reduced to 1.5°C and the “most likely” figure has been scrapped. That seems to reflect a growing sense that climate sensitivity may have been overestimated in the past and that the science is too uncertain to justify a single estimate of future rises.
If this does turn out to be the case, it would have significant implications for policy. Many countries’ climate policies are guided by the IPCC’s findings. They are usually based on the idea (deriving in part from the IPCC) that global temperatures must not be allowed to increase by more than 2°C above pre-industrial levels, and that in order to ensure this CO2 concentrations should not rise above 450 ppm. The draft table casts doubt on how solid the link really is between 450 ppm and a 2°C rise. It remains to be seen whether governments conclude from this that it is safe to let CO2 concentrations climb even further, or whether (as some will doubtless argue) a 2°C rise was too much anyway and it is now possible to aim for less.
Excerpt, Climate science: Sensitive information, Economist, July 20, 2013, at 70