| | | | | Would an entirely different methodology to the one developed for UKCP09 give different results? There is currently no independent approach which considers the full set of uncertainties covered in the UKCP09 methodology (i.e. large-scale effects of physical atmosphere and ocean processes, sulphur and feedbacks, combined with downscaling to regional scales). Also, there is no method which seeks to provide projections for the wide range of variables included in UKCP09, including consideration of joint changes for multiple combinations of these variables at different future periods. However, methods have been published which consider subsets of these types of uncertainty, applied to projections of key variables such as mean temperature and precipitation. A well-documented example is ASK (after authors Allen, Stott and Kettleborough) which projects future changes in surface temperature at global or sub-continental scales (e.g. northern Europe), with associated uncertainties, by assuming a linear relationship between errors in simulations of past and future climate changes. We applied the ASK technique to the simulations from 17 variants of the Met Office Hadley Centre global model, and derived the projections shown in Figure 26 (blue lines). We compared these with UKCP09 (red lines), omitting the sampling or carbon cycle and downscaling uncertainties from the UKCP09 projections, since these are not considered in ASK. The (50% probability level) of changes in temperature over northern Europe by the 2080s from the ASK method is about 4% higher than that using UKCP09, and gives a 25% narrower spread (10 to 90%). It is reassuring that two methods based on different principles and assumptions should give relatively similar projections in practice, when compared on a like-for-like basis. This further supports our own assessment that the UKCP09 projections are likely to be relatively robust to the assumptions in the methodology used, and hence that the spread of outcomes is consistent with current evidence. | |
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| Figure 25: Comparison of probabilistic climate projections (10, 50 and 90% probability levels) for changes in 10-year annual mean temperature under the UKCP09 Medium emissions scenario. Changes shown are for Northern Europe, relative to the average climate of 1906–2005. Results using the UKCP09 methodology are in red, those from the ASK method in blue. Observations are shown by the black line. | |
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