Climate sensitivity is defined as the increase in global temperature for a doubling of pre-industrial CO2. Pre-industrial CO2 was 280 parts per million (ppm), and CO2 is now at 395 ppm. So doubling pre-industrial would be at 560 ppm. We are now 115/280 = 41% of the way there.
Recent coverage by Andrew Revkin of the NY Times blog http://dotearth.blogs.nytimes.com has brought up comments by people that the climate sensitivity has dropped, and the highest ranges of possible temperature shifts near 4.5 degrees is very unlikely.
Corrected Temperature Rise
The first thing wrong with the dropping of the climate sensitivity is based on the flatness of the temperature record using the extreme peak jump in 1998 caused by ENSO as a starting point, which is totally unacceptable statistically and even by common sense. ENSO is the El Niño Southern Oscillation in the equatorial Pacific Ocean. But there is a new twist to this. Work has been done to remove the volcanic ash contribution to the temperature record, as well as the ENSO contribution. Here is what the actual record (light pink) and the corrected curve (dark red) look like:
Figure 1 from Comparing climate projections to observations up to 2011
Stefan Rahmstorf et al 2012 Environ. Res. Lett. 7 044035 doi:10.1088/1748-9326/7/4/044035. Here is their caption:
Comparing adjusted with unadjusted data shows how the adjustment largely removes e.g. the cold phase in 1992/1993 following the Pinatubo eruption, the exceptionally high 1998 temperature maximum related to the preceding extreme El Niño event, and La Niña-related cold in 2008 and 2011.
The dashed red line shows that mean global temperature has been rising steadily, once local time fluctuations have been taken into account.
