Addressing Climate Change Uncertainties Arising from Greenhouse Gas Scenarios—Can a More Bounded Range of Future CO2 Concentration be Identified?

From local/regional studies to the level of national climate assessments, a wide range of climate change impact analyses must necessarily employ projections of future atmospheric greenhouse gas concentrations to arrive at quantitative conclusions. Four such projections were defined in the 5^th IPCC assessment report for use by the scientific community and subsequently by practitioners seeking to inform decision makers about future impacts. The scenarios are identified by their total radiative forcing in the year 2100 relative to 1750, spanning a range from 2.6 W/m² for representative concentration pathway RCP2.6 to 8.5 W/m² for RCP8.5. The RCP scenarios each embody different evolving mixes of climate drivers reflected from policies to represent a mitigation scenario (RCP2.6), a couple of stabilization scenarios (RCP4.5, RCP6.0), and a scenario of very high greenhouse gas emissions (RCP8.5).

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-18^th 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.