[Last updated: 26th January 2016]
The figures below show two comparisons of CMIP5 simulations & observations of global mean surface air temperature. They are updated versions of Figure 11.25a,b from IPCC AR5.
The first panel shows the raw ‘spaghetti’ projections, with different observational datasets in black and the different emission scenarios (RCPs) shown in colours.
The second panel shows the assessment for global temperatures in the 2016-2035 period. The HadCRUT4.4 observations are shown in black with their 5-95% uncertainty. Several other observational datasets are shown in blue. The light 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 dark grey shading shows the projections using a 2006-2012 reference period. The red hatching shows the IPCC AR5 indicative likely (>66%) range for the 2016-2035 period.
The observations for 2015 fall just below the median of the model simulations, and were warmed slightly (<0.1K) by the ongoing El Nino event in the Pacific. 2015 is the first year which is clearly more than 1°C above a 1850-1900 (pseudo-pre-industrial) baseline.
There are several possible explanations for why the earlier 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. Next, the exact position of the observations within the CMIP5 range depends slightly on the reference period chosen. Lastly, this is not an apples-with-apples comparison because it is comparing air temperatures everywhere (simulations) with blended and sparse observations of air temperature and sea temperatures. A combination of some of these factors is likely responsible.
26th January 2016: Entire page updated for 2015 global temperatures.
2nd September 2015: Updated figure to use HadCRUT4.4 up to July 2015 and added link to Cowtan et al. (2015).
5th June 2015: Updated using data from HadCRUT4.3 up to April 2015, and the new NOAA dataset.
2nd February 2015: Cowtan & Way 2014 data added.
26th January 2015: Entire page updated for 2014 temperatures.
27th January 2014: Page created.