[Given recent interest in previous comparisons of CMIP5 simulations and observations of global mean surface air temperature, this is now a permanent page which will be incrementally updated as more data accumulates]
The figure below shows a comparison of CMIP5 simulations & observations of global mean surface air temperature, using a 1986-2005 reference period, and is an updated version of Figure 11.25 from IPCC AR5. The HadCRUT4 observations are shown in black with their 5-95% uncertainty. Several other observational datasets are shown in blue.
The grey shading shows the CMIP5 5-95% range for historical (pre-2005) & all future forcing pathways (RCPs, post-2005); the grey lines show the min-max range. The red hatching indicates the IPCC AR5 assessed likely (>66%) range for the 2016-2035 period. The UK Met Office forecast for 2015 is shown by the green error bar.
There are several possible explanations for why the observations are at the lower end of the CMIP5 range. First, there is internal climate variability, which can cause temperatures to temporarily rise faster or slower than expected. Second, the radiative forcings used after 2005 are from the RCPs, rather than as observed. Given that there have been some small volcanic eruptions and a dip in solar activity, this has likely caused some of the apparent discrepancy. Third, the real world may have a climate sensitivity towards the lower end of the CMIP5 range. Last, the exact position of the observations within the CMIP5 range depends slightly on the reference period chosen. A combination of some of these factors is likely responsible.
Note also that as the HadCRUT4 dataset has gaps over the Arctic it is likely to be a slight underestimate of the true recent global temperature anomaly. And, in this version of the figure, the CMIP5 simulations are NOT masked to the HadCRUT4 observational coverage, unlike some previous examples on this blog.
2nd February 2015: Cowtan & Way 2014 data added
26th January 2015: Entire page updated for 2014 temperatures
27th January 2014: Page created.