Representative Concentration Pathways (RCPs) are scenarios of greenhouse gas and aerosol concentrations, together with land use change, that are consistent with a set of broad climate outcomes. The term pathway emphasizes that the long-term concentration levels are of interest as well as the trajectory taken over time to reach that outcome.
Radiative forcing is the difference between how much energy from sunlight is radiated back to space and how much is absorbed by the Earth. A net gain of energy by Earth will cause warming. Conversely, cooling occurs when Earth loses more energy to space than it receives from the sun. While measurements of radiative forcing give some indication of temperature trends in the lower atmosphere, they do not provide sufficient information to predict overall climate response.
Four RCPs have been developed and numbered according to the projected change in radiative forcing by 2100: +2.6, +4.5, +6.0 and +8.5 watts per square meter. The higher the number, the greater the change in radiative forcing. RCP8.5 is thus the worst-case scenario.
To rephrase the title of this post: How likely is RCP8.5? Answer: it depends who you ask. Last year’s Fourth National Climate Assessment does not assign a “formal likelihood” to any of the RCP scenarios. However, Scientific American just ran a piece “EPA Head Targets “Worst-Case” Climate Scenarios” claiming “experts in climate modeling” estimate there’s a 10% chance RCP8.5 could be surpassed by the end of the century. The author does not include a link or reference to any report or peer-reviewed paper confirming the 10% probability. Upon further research I found Capellan-Perez et al (2016), who provided a 12% probability of reaching RCP8.5 by 2100. On the other hand, we have Ritchie and Dowlatabadi (2017), who say the probability of reaching RCP8.5 by 2100 is “virtually zero”.
Why the difference in expert opinion about the likelihood of the RCP8.5 worst-case scenario? Because climate models generate different futures depending on their assumptions and inputs. For instance, the authors claiming a 12% probability of RCP8.5 assume “fossil fuel resource availability is a key driver of emission pathways” and rely on relatively high estimates of coal reserves. According to this scenario, humans won’t transition to renewables until fossil fuels start running out and by then it will be too late to avoid catastrophic climate change.
The assumption of a coal-dominated near-future is not specific to Capellan-Perez - it’s been standard in RCP8.5 climate models for years. For instance, check out this chart from an influential 2011 paper, in which projected RCP8.5 coal consumption just about equals all other sources of energy use combined:
Are current trends pointing to the long-term dominance of coal? Not according to the following:
And not according to the following papers (list courtesy of LK in Judith Curry’s Is RCP8.5 an impossible scenario?):
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.
My take-away: to the extent that climate change worst-case scenarios are based on inaccurate assumptions about coal reserves, they are not plausible. Show me an RCP8.5 without that assumption and I may change my mind.
Next: How should worst-case scenarios inform government policy?
The Beginner’s Guide to Representative Concentration Pathways by Graham Wayne/Skeptical Science
Capellan-Perez et al “Likelihood of climate change pathways under uncertainty on fossil fuel resource availability”. Energy Environ. Sci. 2016, 9, 2482–2496. DOI: 10.1039/C6EE01008C
Ritchie, J. and H. Dowlatabadi (2017). "Why do climate change scenarios return to coal?" Energy 140: 1276-1291. https://doi.org/10.1016/j.energy.2017.08.083