The scenarios are each built “bottom-up” with an extensive set of detailed assumptions to which no particular forecast probabilities are attached. Each RCP specifies a set of anthropogenic emissions and atmospheric gas concentrations up to the year 2100. They encompass a wide range of global average CO2 concentration in the year 2100, with 421 ppm under RCP2.6 up to 936 ppm in RCP8.5. The practitioner is often left with uncertainty over which RCP to adopt for a specific impact analysis. Radiative forcing from CO2, and hence an induced temperature change, has long been recognized to be a logarithmic function of rising concentration relative to the estimated mid-18th century value of 278 ppm. The forcing contribution at the upper end of the RCP year-2100 CO2 range is therefore nearly triple that at the low end. The span of 515 ppm between RCP2.6 and RCP8.5 is four times the CO2 change which has been experienced to date. These clearly induce a wide range of uncertainty to analysis, and the greenhouse gas projection has a central influence upon quantitative conclusions reached and the consequent recommendations made. While the RCPs provide an ample range over which to conduct scientific investigations, the applied climatology practitioner must invariably question which is most applicable for their investigation and then justify the chosen assumption to decision makers. Without a reasoned explanation for a bounded range little progress can be expected.
So, can guidance be offered to estimate a more constrained range of future CO2 concentration? The RCPs were developed “bottom-up”, and so they can benefit from “top-down” methodological review, which is a best-practice in forecast development. A set of such methods previously demonstrated in a range of forecasting applications have been studied and findings of their applicability will be reported in this presentation. The instrumental measurement record of atmospheric CO2 has fortunately now exceeded 60 years, which provides a robust data set against which to test the analyzed projection methods and develop rigorous forecasts of CO2 levels. Their confidence intervals have been quantified and forecast validation exercises were performed with encouraging findings. A more constrained range of logical projections to future elevated CO2 levels was identified to dissipate some of the uncertainties and focus decision maker discussions to convincing expectations of future impacts.