The new RCPs are not projections, probabilities, prophecies or pathways – they might possibly be potentialities.
Guest essay by Barry Brill
The IPCC begins with science:
“In sum, a strategy must recognise what is possible. In climate research and modelling, we should recognise that we are dealing with a coupled non-linear chaotic system, and therefore that the long-term prediction of future climate states is not possible.”
For its third and fourth assessment reports, it relied upon SRES scenarios or storylines in lieu of predictions:
‘Futurology’ – the study of postulating possible, probable, and preferable futures and the worldviews and myths that underlie them. There is a debate as to whether this discipline is an art or a science” says Wikipedia.
The fifth assessment has moved on to pure hypotheticals in the form of Representative Concentration Pathways (RCPs):
“Four RCPs were selected and defined by their total radiative forcing (cumulative measure of human emissions of GHGs from all sources expressed in Watts/m2) pathway and level by 2100. The RCPs were chosen to represent a broad range of climate outcomes, based on a literature review and are neither forecasts nor policy recommendations”.
RCPs start with some conjectures about 2100 outcomes, and eventually aim to work out “pathways” back to the present day. The storylines have not yet been written, but the IPCC is co-ordinating extensive modelling work to develop “Shared Socioeconomic Pathways (SSPs)” with suitable narratives tying the SSPs to the RCPs.
This “working backwards” stratagem has previously worked quite well for the IPCC, where the Summary for Policyholders (SPM) is published first and then the peer-reviewed science is amended to fit. Now the climate outcomes are written first, and the ‘futurology‘ is later devised to fit.
Unlike the SRES of AR4 there is no suggestion that the RCPs are “projections” – which the dictionary describes as as “an estimate of what might happen in the future based on what is happening now” and “an estimate of future possibilities based on a current trend”.
So RCP climate outcomes in 2100 are not forecasts or predictions or projections. Nor are they (yet) potential pathways.
What can they be? Could they be described as prophecies or soothsayings – or maybe even wishlists?
It must be remembered that the IPCC has been careful not to claim that any of the four RCPs are probable, or even slightly more likely than an infinity of alternative future possibilities. In a SMP that offers percentage likelihoods in most paragraphs, confidence levels for RCPs are conspicuously absent.
The “representatives” were evidently chosen quite arbitrarily and aimed only at ensuring a good spread. Back in 2000, the SRES Scenarios “represented the full range of driving forces and emissions excluding only the outlying “surprise” or “disaster” scenarios in the literature”. The range of the RCPs is much wider, covering the entire span of the scenario literature, inclusive of outliers. In addition, each RCP includes provision for Land-use and Land-cover Change (LULUC).
The categories are “representative” of a huge range, bounded only by the imaginations of those who have contributed something to the literature over the years. The “representatives” have nothing at all to do with real possibilities or statistics or science.
Likelihoods can’t be improved by taking a mean or a median. Each RCP is heterogeneous and was developed by a separate independent consultancy. Each of them already has assumptions about socioeconomic, technology and biophysical futures that differ from the others, and the gap will widen as each team invents and combines different demographic, policy and energy storylines to improve plausibility for its own predetermined outcome.
Any policymaker (or journalist, for that matter) who wishes to rely upon any hypothetical AR5 climate outcome, will first have to choose a RCP.
On perusing the characteristics of the RCPs (see Table 1) one senses that the 4.5RCP comprises the “base case” – which is accompanied by higher (6.0) and lower (2.6) bands, in the usual way.
The inclusion of “surprise” and “disaster” outliers demanded an add-on in the form of the remarkably large 8.5RCP to cover the extremes – those low-probability high-impact events set out in the Table 12.4 presented to the Royal Society.
Table 1: Characteristics and Effects of the WG1AR5 RCPs
*Anomalies are calculated with respect to 1986-2005.
**There is no close SRES equivalent to any RCP, but these old narratives provide starting-points for the new evolving storylines.
In the new system the IPCC no longer claims any special insight into the likelihood of high future temperatures. While it opines on the impact of certain cumulative emission levels, those levels are purely hypothetical and unrelated to real world happenings.
This is a step in the right direction. Hyperbolic media articles can no longer claim that doomsday outcomes are ‘projected‘ by the IPCC, even as the extreme end of a range. The 8.5 RCP is unmistakably a “what-if?” or “let’s pretend” speculation.
However, as the narratives are filled out by the IPCC’s joint modeling committee (the JIIC), reverse-engineered pathways will emerge and there will undoubtedly be claims that those pointing to the high-end concentrations are more plausible.
To set up the skewing and spinning process, the IPCC has bestowed a brief description on each RCP:
2.6 sees radiative forcing peak at 3.1W/m2 around 2050 and reduce over the second half of the century. That reduction requires new carbon sinks (either the biosphere or geo-engineering) and therefore new technologies.
4.5 assumes total radiative forcing is stabilized before 2100 by a range of technologies and strategies for reducing greenhouse gas emissions.
6.0 assumes total radiative forcing is stabilized after 2100 without overshoot by a range of technologies and strategies for reducing greenhouse gas emissions.
8.5 is characterized by ever-increasing greenhouse gas emissions and represents those scenarios in the literature leading to high concentration levels (“tipping points”).
IPCC commentators are already referring to 4.5 and 6.0 as “carbon-constrained” and 8.5 as the sole “business as usual” (BAU) case. This unvarnished propaganda is quite likely to appeal to the less discerning members of the alarmist media.
Let us assume our descendants in 2100 find that radiative forcing is at 2.6 or less. What are some of the pathways they might see leading back to 2013?
(i) One or more of the CMIP5 key assumptions were wrong;
(ii) The transient climate response (TCR) was close to 1°C;
(iii) Western fertility rates transferred to Africa and South Asia;
(iv) BRIC economies went into a Japan-type long recession;
(v) Doubling world food supply reduced CO2 concentration rates;
(vi) Energy intensity in the developing world declined in line with income;
(vii) Coal usage ceased as natural gas became ubiquitous and cheap;
(viii) A new energy technology (eg Thorium reactors) became dominant;
(ix) A new CCS or other geo-engineering technology removed CO2 from the atmosphere.
Of this selection, I would have thought the ninth was the least probable. But it suits the IPCC agenda to spin 2.6 as “the geo-engineering case”.
Whatever else the new SPCs may deliver, they have already obscured the reduction in IPCC futures caused by the acceptance of lower climate sensitivity ranges. Not for the first time, the IPCC has found a trick which hides the decline.