The final version of the IPCC AR5 WG1 assessment on the physical basis for climate change has now been published. The AR5 includes, for the first time, a specific chapter and assessment on ‘near-term’ climate change, which covers the period up to 2050, but with a specific focus on the 2016-2035 period.
UPDATE (11/02/14): The text below has been edited to reflect that the final AR5 WG1 report has now been published and to add more details.
The near-term period is interesting because the projections can be verified rather soon and because understanding the changes over this period may be relevant for adaptation decision making. This period is also relatively insensitive to the particular emissions scenario, although aerosol emissions decline quite rapidly in all RCPs which may be slightly unrealistic. However, the near-term is made complicated because of the role of climate variability.
The IPCC has made a probabilistic assessment of how global temperatures are projected to evolve over the next 20 or so years, which is valid for all RCPs, but with a few caveats such as no future large volcanic eruptions. The assessment has also been discussed in a recent UK Parliament Committee hearing.
The synthesis figure (Fig. 11.25) is shown below, and assesses near-term global surface temperature changes. The top ‘spaghetti’ panel shows the raw CMIP5 output using one ensemble member per model. This is designed to illustrate the role of variability in the climate and that the emissions scenarios (colours) show little difference in this period.
The middle panel shows the overall assessment, including the 5-95% uncertainty ranges from the raw simulations using two reference periods – 1986-2005 (light grey) & 2006-2012 (dark grey). The observations and their uncertainty are also shown. The red hatched region shows an indicative ‘likely’ (>66%) forecast for global mean surface temperature during 2016-2035 and the black bar shows the assessed likely range for the mean of 2016-2035 which is 0.3-0.7°C above the mean of 1986-2005. This assessment is made with ‘medium confidence’.
The bottom panel summarises some of the evidence used in the overall assessment which includes estimates from: (i) a range of multi-model estimates, (ii) GCM simulations constrained by observations, (iii) initialised predictions, (iv) plausible future rates of change. The assessment also considers the sensitivity of the projections to the choice of reference period. The full description and discussion of this figure (Section 126.96.36.199), along with the rationale for the assessed range is also worth reading.
Note that the observations for 2012 are just outside the 5-95% range from the simulations (using the 1986-2005 reference period), differing from some previous analyses shown here on Climate Lab Book. This small change in the percentile is partly because in this version: (i) the simulations are not masked in the same way as the observations, (ii) a different reference period is used, and (iii) all RCP scenarios are used. Other reference periods would give slightly different percentiles. Figure 11.9 is a similar figure using RCP4.5 only, with some additional focus on the next 10 years.
A key development in AR5 compared to AR4 is that these projections are more easily tested in the next few years, and this is a very welcome and positive addition. The full chapter discusses changes in other climate variables and the ability of the current GCMs to make hindcasts.