Figure A6.21 Relative importance of different uncertainties, and their evolution. The graphic shows (a) Decadal mean surface temperature change (°C) from the historical record, with climate model estimates of uncertainty for the historical period, along with future climate projections and uncertainty. Values are normalized by means from 1961 to 1980. Natural variability derives from model interannual variability, and is assumed constant with time. Emission uncertainty is estimated as the model mean difference in projections from different scenarios. Climate response uncertainty is based on climate model spread, along with added uncertainties from the carbon cycle, as well as rough estimates of additional uncertainty from poorly modelled processes. (b) Climate response uncertainty can appear to increase when a new process is discovered to be relevant, but such increases reflect a quantification of previously unmeasured uncertainty, or (c) can decrease with additional model improvements and observational constraints. The given uncertainty range of 90% means that the temperature is estimated to be in that range, with a probability of 90%. From Cubasch et al. (2013: FAQ 1.1 Figure 1).