by Judith Curry
Is the RCP8.5 scenario plausible?
3. Scenarios of emissions/concentration
Most worst-case climate outcomes are associated with climate model simulations that are driven by the RCP8.5 representative concentration pathway (or equivalent scenarios in terms of radiative forcing). No attempt has been made to assign probabilities or likelihoods to the various emissions/concentration pathways (e.g. van Vuuren et al. 2011), based on the argument that the pathways are related to future policy decisions and technological possibilities that are considered to be currently unknown.
The RCP8.5 scenario was designed to be a baseline scenario that assumes no greenhouse gas mitigation and no impacts of climate change on society. This scenario family targets a radiative forcing of 8.5 W m-2 from anthropogenic drivers by 2100, which is nominally associated with an atmospheric CO2 concentration of 936 pm (Riahi et al. 2007). Since the scenario outcome is already specified (8.5 W m-2); the salient issue is whether plausible storylines can be formulated to produce the specified outcome associated with RCP8.5.
A number of different pathways can be formulated to reach RCP8.5, using different combinations of economic, technological, demographic, policy, and institutional futures. These scenarios generally include very high population growth, very high energy intensity of the economy, low technology development, and a very high level of coal in the energy mix. Van Vuuren et al. (2011) report that RCP8.5 leads to a forcing level near the 90th percentile for the baseline scenarios, but a literature review at that time was still able to identify around 40 storylines with a similar forcing level.
Storylines for the RCP8.5 scenario and its equivalents have been revised with time as our background knowledge changes. To account for lower estimates of future world population growth and much lower outlooks for emissions of non-CO2 gases, more CO2 must be released to the atmosphere to reach 8.5 W m-2 by 2100 (Riahi et al., 2017). For the forthcoming IPCC AR6, the comparable SSP5-8.5 scenario is associated with an atmospheric CO2 concentration of almost 1100 ppm by 2100 (O’Neill et al. 2016), which is a substantial increase relative to the 936 ppm reported by Riahi et al. (2007).
As summarized by O’Neill et al. (2016) and Kriegler et al. (2017), the SSP5-8.5 baseline scenarios exhibit rapid re-carbonization, with very high levels of fossil fuel use (particularly coal). The plausibility of the RCP8.5-SSP5 family of scenarios is increasingly being questioned. Ritchie and Dowlatabadi (2018) challenge the bullish expectations for coal in the SSP5-8.5 scenarios, which are counter to recent global energy outlooks. They argue that the ‘return to coal’ scenarios exceed today’s knowledge of conventional reserves. Wang et al. (2017) has also argued against the plausibility of the existence of extensive reserves of coal and other easily-recoverable fossil fuels to support such a scenario.
Most importantly, Riahi et al. (2017) found only one single baseline scenario of the full set (SSP5) reaches radiative forcing levels as high as the one from RCP8.5 (compared with 40 cited by van Vuuren et al. 2011). This finding suggests that 8.5 W/m2 can only emerge under a very narrow range of circumstances. Ritchie and Dowlatabadi (2018) notes that further research is needed to determine if plausible high emission reference cases consistent with RCP8.5 could be developed with storylines that do not lead to re-carbonization.
Given the socio-economic nature of most of the assumptions entering into the SSP-RCP storylines, it is difficult to argue that the SSP5-RCP8.5 scenarios are impossible. However, numerous issues have been raised about the plausibility of this scenario family. Given the implausibility of re-carbonization scenarios, current fertility (e.g. Samir and Lutz, 2014) and technology trends, as well as constraints on conventional coal reserves, a categorization of RCP8.5 as ‘borderline impossible’ is justified based on our current background knowledge.
Based on this evidence, Ritchie and Dowlatabadi (2017) conclude that RCP8.5 should not be used as a benchmark for future scientific research or policy studies. Nevertheless, the RCP8.5 family of scenarios continues to be widely used, and features prominently in climate change assessments (e.g. CSSR, 2017).
JC note: next installment is climate sensitivity