by Judith Curry

In considering ‘worst case’ climate change impacts, we first need to assess the realistic worst case for global carbon emissions.

The recently published U.S. National Climate Assessment shows that we are currently on track for RCP8.5.

The assumptions that underlie the IPCC emissions scenarios are described in the 2000 IPCC Special Report on Emissions Scenarios [link].

RCP8.5 is sometimes referred to as a ‘business as usual’ scenario. It is not. Rather, it is an extreme scenario.

Larry Kummer wrote a 2015 guest post at Climate Etc. entitled A closer look at scenario RCP8.5. LK describes RCP8.5 as a “useful worst case scenario.”

RCP8.5 figures prominently in the most alarming of future climate scenarios. I am wondering if RCP8.5 is too extreme, in the sense that is may be impossible, given constraints on recoverable fossil fuel supply.

In particular, there is a fairly large number of papers arguing that assumptions about coal are incorrect: (list of papers courtesy of LK). The first major study questioning the actual extent of coal reserves: “The Peak in U.S. Coal Production“ by Gregson Vaux, 27 May 2004 More evidence that reserves are overstated: “Coal Of The Future (Supply Prospects for Thermal Coal by 2030-2050)“ by Energy Edge Limited, Prepared for the Institute for Energy of the European Commission’s Joint Research Centre, February 2007 More evidence that reserves are overstated: “Coal: Resources and Future Production“ by Energy Watch Group, March 2007 (47 pages,) The major study showing that coal reserves are overstated: “Coal: Research and Development to Support National Energy Policy“ by the National Academies, June 2007 “Why do climate change scenarios return to coal?” by Justin Ritchie and Hadi Dowlatabadi in Energy, 1 December 2017. For broader fossil fuel supply issues, see Wang et al. (2017); The implications of fossil fuel constraints on climate change projections — A supply side analysis [link]. From the Abstract: Climate projections are based on emission scenarios. The emission scenarios used by the IPCC and by mainstream climate scientists are largely derived from the predicted demand for fossil fuels, and in our view take insufficient consideration of the constrained emissions that are likely due to the depletion of these fuels. This paper, by contrast, takes a supply- side view of CO2 emission, and generates two supply-driven emission scenarios based on a comprehensive investigation of likely long-term pathways of fossil fuel production drawn from peer-reviewed literature published since 2000. The potential rapid increases in the supply of the non-conventional fossil fuels are also investigated. Climate projections calculated in this paper indicate that the future atmospheric CO2 concentration will not exceed 610 ppm in this century; and that the increase in global surface temperature will be lower than 2.6 C compared to pre-industrial level even if there is a significant increase in the production of non-conventional fossil fuels. Our results indicate therefore that the IPCC’s climate projections overestimate the upper-bound of climate change. Furthermore, this paper shows that different production pathways of fossil fuels use, and different climate models, are the two main reasons for the significant differences in current literature on the topic.

Ritchie and Dowlatabadi (2017) explain that RCP8.5 contains a return to coal hypothesis, requiring increased per-capita coal use based on systematic errors in coal production outlooks. Ritchie and Dowlatabadi conclude that RCP8.5 should not be used as a benchmark for future scientific research or policy studies. It looks like substantial technological advances would be needed to tackle the less recoverable resources. Given the climate change and environmental quality issues, how realistic is it to expect these technological advances to be pursued — rather than pursuing greener energy technologies? . So my main questions are: Should RCP8.5 be used as a ‘worst case’ emissions scenario? Is RCP8.5 impossible based upon the knowledge we currently have?

RCP8.5 may be useful for climate research, for considering processes in a substantially altered environment. However, many ‘catastrophic’ impacts of climate change don’t really kick at the lower CO2 concentrations, and RCP8.5 then becomes useful as a ‘scare’ tactic. For policy making, I’m not sure that RCP8.5 is a useful scenario. I would appreciate your thoughts on this.