Bookmark and share

Web news

Unpublished estimates of climate sensitivity

A news story published by the Research Council of Norway has recently attracted attention by international news media and blogs.
Project results presented indicate that the warming associated by increased atmospheric CO2 concentrations is less than previously assumed. 

The news story was based on research project (CLIMSENS) involving CICERO, and was based on the final report submitted to the research council. The final report contains both published and unpublished material.

Terje Berntsen, a professor at Center for Climate and Environmental Research – Oslo, has lead the project "Constraining total feedback of the climate system by observations and models (CLIMSENS)" which was funded by The Research Council of Norway’s program "Climate change and impacts in Norway (NORKLIMA)".
Climate sensitivity is a measure of how the temperature correlates with changes in atmospheric CO2 concentrations. Typically scientists calculate how much the temperature increases from a doubling of CO2. The IPCC’s best estimate from the panel’s fourth assessment report in 2007 is 3°C. The uncertainty range however, is very high – between 2°C and 4,5°C according to the IPCC report. This has implications for future temperature projections. If the climate sensitivity is low, for example 2°C, we can emit more CO2 before climate change becomes dangerous. In CLIMSENS the Norwegian scientist aimed to reduce this uncertainty range and improve our ability to project future temperature increase.
In the final report from CLIMSENS Berntsen et al has calculated their best estimate of climate sensitivity to be 1.9°C. This result has not yet been published. What has been published is a paper by Aldrin et al describing the method they used. Berntsen at al used statistics to investigate the relationship between global temperatures and changes in radiative forcing. (Radiative forcing is a measure of how much energy is contained in the Earths atmosphere. CO2 is one of many factors). When the scientists added data from the year 2000 to 2010 to their 160 year long time series, the climate sensitivity dropped from 3.7°C to 1.9°C. Berntsen explains this large change in sensitivity by a combination of the substantial increase in radiative forcing and simultaneous modest increse in temperature observed from 2000 to 2010.
1.9°C is a best estimate. The uncertainty range, with 90 per cent confidence, is from 1.2°C to 2.9°C. This is substantially lower than the IPCC conclusions from 2007. These results are not yet published in a scientific journal and are still under peer review, a standard scientific quality control. CICERO will wait with dissemination of these results until they are accepted by a journal.
Many scientists have estimated climate sensitivity since the 2007 IPCC report using both statistical tools and models. Some of the more recent work was summarised by Hans Martin Seip, a professor at CICERO, in our popular science magazine Klima in the January 2012 issue.
Three of the studies in Seip’s article use observations from the last glacial maximum and pre industrial times and compare these to changes in radiative forcing. Schmittner at al arrive at a climate sensitivity of 2.3°C, while Köhler et al arrive 2.4°C. Using the same method Hansen and Sato estimate the climate sensitivity to be 3°C. 
Real Climate, a blog about climate science, calculate that lowering the climate sensitivity from 3°C to 2.3°C means we have 35 years, instead of 24 years, of emissions at current levels before there is more than a 50 per cent chance that temperatures will rise by more than 2°C.
Huber and Knutti have investigated changes in greenhouse gasses, aerosols, sun radiation and energy budgets from 1950 onwards. Their primary objective was to determine what had cause the post 1950 temperature increase. They also estimated the climate sensitivity and arrived at 3.6°C, with a likely range from 1.7°C to 6.5°C.
Real Climate has recently published a more thorough break down of recent research on climate sensitivity.
Aldrin et al, Bayesian estimation of climate sensitivity based on a simple climate model fitted to observations of hemispheric temperatures and global ocean heat content, Environmetrics, 2012.
J.E. Hansen and M. Sato, 2011. Paleoclimate Implications for Human-Made Climate Change arXiv: 1105.0968v2
M. Huber and R. Knutti, 2012. Anthropogenic and natural warming inferred from changes in Earth’s energy balance. Nature Geoscience, 5, 31 – 36.
M. Huber et al, 2011. Constraints on Climate Sensitivity from Radiation Patterns in Climate Models, J. Climate, 24, 1034 – 2052.
R. Knutti and G-K. Plattner, 2012: Comment on "Why Hasn’t Earth Warmed as Much as Expected?" by Schwartz et al. 2010. J. Climate. doi:10.1175/2011JCLI4038.1, in press.
P. Köhler og medarbeidere, 2010. What caused Earth’s temperature variations during the last 800,000 years? Data-based evidence on radiative forcing and constraints on climate sensitivity. Quaternary Science Reviews 29, 129–145 
A. Schmittner og medarbeidere, 2011. Climate Sensitivity Estimated from Temperature Reconstructions of the Last Glacial Maximum, Science 344, 1385 – 1388.
In review:
Skeie et al. A lower and more constrained estimate of climate sensitivity using updated observations and detailed radiative forcing time series, In review in Journal of Climate.

Last updated: 28.01.13

CICERO, P.O. Box. 1129 Blindern,
N-0318 Oslo, NORWAY
Visiting adress: Gaustadalléen 21, 0349 OSLO
Christian Bjørnæs
Web editor:
Eilif Ursin Reed
+47 22 85 87 50