Future Global Temperature Forecasts Trending Downward?

The Economist has published an intriguing table derived from a draft version of the Intergovernmental Panel on Climate Change's Working Group III that relates future projected global average tempertures to specific levels of carbon dioxide concentrations in the atmosphere. Here's the table:

For the record, current carbon dioxide atmospheric concentrations are just shy of 400 parts per million. This table is related to the important issue of climate sensitivity which is generally defined as how much warming would eventually occur as a result of doubling carbon dioxide concentrations in the atmosphere. With regard to the data in this table, the Economist notes:

According to one table from the unpublished report, which was seen by The Economist (above), at CO2 concentrations of between 425 parts per million and 485 ppm, temperatures in 2100 would be 1.3-1.7°C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.

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.

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.

One implication of these data is that humanity may have more time in which to prepare for whatever global warming may occur. A caveat: The data is still from a draft report and can change.

I have been reporting on the research trend toward lower climate sensitivity estimates for some time. To catch up go to Reason's climate sensitivity tag for more background.