This post examines the scenarios and pathways used over the years by the Intergovernmental Panel on Climate Change (IPCC) to represent possible future emissions, atmospheric concentrations, and temperature changes. In particular, it seeks to determine which, if any, of the current Representative Concentration Pathways (RCPs) can be regarded as “business as usual” for the purposes of estimating the consequences of taking no mitigating action against climate change.
Back in January, fellow CCNF columnist and Texas A&M University colleague Andrew Dessler used the RCP8.5 projections as an estimate of what “unchecked” emissions would do to the climate, in his written testimony to the Senate. I conclude below that this is a slight overestimate, and that (fortunately for us) lack of action probably wouldn’t lead to quite so large an impact. The central estimate I get from what the IPCC uses for business-as-usual emission scenarios and the IPCC uncertainty for climate sensitivity is a rise of 3.0 C over the present century.
A Brief History from AR5 WGII
The report from IPCC Assessment Report #5 (AR5) Working Group II (WGII) provides the background (citations omitted and paragraph breaks added for readability):
“A scenario is a story line or image that describes a potential future, developed to inform decision-making under uncertainty.
“Scenarios have been part of IPCC future climate projections since the [First Assessment Report] (1990), where WGIII generated four scenarios (Bau = business-as-usual, B, C, and D) used by WGI to project climate change.“The IPCC Supplementary Report (IPCC, 1992), a joint effort of WG1 and WGIII, defined six new scenarios (IS92a-f) used in the [Second Assessment Report] (1996).”
Note: Five of those six IS92 scenarios were business-as-usual scenarios, assuming no new control measures to counteract climate change. They varied mainly in their assumptions regarding population growth, cheap fossil fuel availability, economic growth, and technological changes. Two were higher than the central IS92a scenario, while two were lower. The sixth, IS92b, assumed that countries would follow through on their pledges to reduce greenhouse gases.
Back to AR5 WGII:
“For the [Third Assessment Report] (2001), the IPCC Special Report on Emissions Scenarios (SRES) (Nakicenkovic et al., 2000) created many scenarios from four integrated assessment models, out of which a representative range of marker scenarios were selected (A1B, A1T, A1FI, A2, B1, B2). In the SRES, scenarios had socio-economic storylines but climate-mitigation options were not included.
“The SRES scenarios carried over into the [Fourth Assessment Report] (2007) and formed the basis for the large number of ensemble climate simulations, which are still in use for climate-change studies relevant to AR5 WGII.”
Note: All six of these marker scenarios were also business-as-usual scenarios. All assumed that nothing additional would be done to reduce greenhouse gas emissions beyond what would happen anyway through increased efficiency, reductions in air pollution, etc.
Back to AR5 WGII:
“With AR5, the development of scenarios fundamentally changed from the IPCC-led SRES process. An ad hoc group of experts, anticipating AR5, built a new structure for scenarios called Representative Concentration Pathways (RCPs) using updated integrated assessment models… The four RCPs are keyed to a range of trajectories of greenhouse gas concentrations and climate forcing. They are labeled by their approximate radiative forcing (W m^-2) that is reached during or near the end of the 21st century (RCP2.6, RCP4.5, RCP6.0, RCP8.5).”
The Representative Concentration Pathways and Corresponding Baselines
The RCPs, developed for the sake of climate modeling, specify the changing atmospheric concentration of greenhouse gases, aerosols, etc. over time. These were then merged with plausible socio-economic pathways to provide complete scenarios. The primary consideration was that the four RCPs effectively span the range of published scenarios. They were also intended to be roughly evenly-spaced in forcing intensity so as to provide good contrasts between scenarios. (van Vuuren et al. 2011a)
RCP8.5 was developed to represent a high-end emissions scenario. “Compared to the scenario literature RCP8.5 depicts thus a relatively conservative business as usual case with low income, high population and high energy demand due to only modest improvements in energy intensity.” (Riahi et al. 2011) RCP8.5 comes in around the 90th percentile of published business-as-usual (or equivalently, baseline) scenarios, so it is higher than most business-as-usual scenarios. (van Vuuren et al. 2011a)
RCP6.0 is a mitigation scenario, meaning it includes explicit steps to combat greenhouse gas emissions (in this case, through a carbon tax). In the course of developing the scenario, a baseline business-as-usual scenario was developed. The mitigation in RCP6.0 has the effect of reducing anthropogenic forcing by 1.2 W m^-2 below the business-as-usual scenario, which in turn is well below the RCP8.5 business-as-usual scenario (Masui et al. 2011). It turns out that there are only ten or so published mitigation scenarios that come in as high as RCP6.0, but there are many published business-as-usual scenarios that have only 6.0 W m^-2 forcing in the year 2100 (van Vuuren et al. 2011a). RCP6.0 probably implies a warmer climate than those other business-as-usual scenarios because it assumes a rapid rise in emissions followed by a steep drop-off after 2060 when a carbon tax kicks in, making total emissions higher than the lower-end business-as-usual scenarios during most of the 21st century.
RCP4.5 is a typical mitigation scenario, and like RCP6.0 it has a corresponding baseline scenario to which mitigation policies are applied. The business-as-usual scenario corresponding to RCP4.6 ends with radiative forcing of 6.9 W m^-2 in the year 2100 (Thomson et al. 2011).
RCP2.6 is an extremely aggressive mitigation scenario. Indeed, it was only approved as a scenario after it was confirmed that more than one model was capable of producing radiative forcing that low under reasonable assumptions. As with RCP6.0 and RCP4.5, it was derived with respect to a baseline scenario. This baseline scenario produces radiative forcing of 7.2 W m^-2 in the year 2100 (van Vuuren et al. 2011b).
What we see so far is that the only business-as-usual scenario among the RCPs is RCP8.5, a high-end business-as-usual scenario. The other business-as-usual scenarios produced in the course of developing the other three RCPs have radiative forcing in the year 2100 of between 6.9 W m^-2 and 7.2 W m^2, about midway between RCP8.5 and RCP 6.0.
AR5 WGIII’s Survey of Published Baseline Scenarios
Meanwhile, AR5 WGIII (Adaptation and Mitigation) needed to estimate the costs of mitigation, and that can’t be done without having a reference point of zero mitigation. For that, a plausible range of baseline scenarios is needed; a single high-end baseline scenario wouldn’t do. AR5 WGIII uses the ensemble of business-as-usual scenarios that have been published by independent investigators. While those are associated with a wide range of total emissions, most are clustered between the RCP8.5 and RCP6.0 scenarios by the year 2100, with the exception of scenarios that explicitly assume very rapid gains of technological efficiency.
AR5 WGIII cautions that the distribution of baseline scenarios doesn’t represent a true probability distribution of uncertainty in future emissions, for several reasons. Nonetheless, from my point of view it seems that RCP6.0 can crudely represent a very likely (95% probability) lower bound on business-as-usual radiative forcing by the year 2100 and RCP8.5 can crudely represent a likely (90% probability) upper bound on business-as-usual radiative forcing by the year 2100. At intermediate years, such as 2050, RCP4.5 is higher than RCP6.0 and is a better very likely lower bound.
AR5 WGIII uses the MAGICC intermediate-complexity model to convert the scenarios to probability distributions of temperature changes, using the climate sensitivity expressed in AR4 (which has a higher lower bound than AR5). The chart below compares the probability distribution of temperatures produced by MAGICC to the ensemble of temperatures produced by the CMIP5 global climate models.
The Upshot: Business-As-Usual Climate
Given all the considerations discussed above, I propose that a good central estimate of the business-as-usual global warming in 2081-2100 relative to 1986-2005 (based on consensus opinions of future development scenarios and climate sensitivity) would be 3.0 C, with a 5%-95% range (encompassing uncertainty due only to known unknowns) of 1.8 C to 4.3 C. Since 1985-2005 was about 0.6 C warmer than preindustrial, give or take a tenth of a degree, the total business-as-usual global warming magnitude is estimated as 3.6 C, with a 5%-95% range of 2.3 C to 5.0 C. If you prefer to think in Fahrenheit, as most members of the United States public do, that’s 5.4 F over a century (probable range: 3.2 F to 7.8 F) and 6.5 F since preindustrial (probable range: 4.1 F to 9.0 F).
Your own opinion of climate sensitivity, likely world economic development, etc., may differ from the middle-of-the-road consensus values, especially if you think you’re smarter than a typical climate scientist. If so, adjust your prediction for business-as-usual global warming accordingly. And if you want to correct for the fact that 1986-2005 to 2081-2100 represents 95 years rather than 100 years, add 5% to get true century-long temperature change projections.
If one uses RCP8.5, as Prof. Dessler did, to estimate temperature change, one gets 4.7 F to 8.6 F over this century. My corresponding range, 3.2 F to 7.8 F, is lower because it is centered around the bulk of business-as-usual scenarios and it is broader because it includes the uncertainty associated with the choice of scenario. My central value, 5.4 F over the century, is near the low end of the RCP8.5 range.
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