In an epic climbdown, following a long campaign by sceptics to point out the routine misuse of the RCP8.5 IPCC Representative Concentration Pathway – and particularly owing to the dogged and determined efforts of Roger Pielke Jr. – the alarmist main stream media has finally conceded that scientists wrongly applied this worst case scenario in their research, often mis-labeling it as “business as usual”. Huge numbers of ‘impact studies’ have used this highly unlikely concentration pathway which have then been reported by the media without caveats (often because the researchers themselves have not bothered to provide perspective or have even hyped their own findings), giving the impression that climate change will be very bad or ‘worse than we thought’. So when the BBC’s Matt McGrath pens an article which is surprisingly contrite and factual, you know you are winning the war against climate crisis alarmism.

Now, there is huge confusion regarding the Representative Concentration Pathways – even among scientists themselves – and I’m not going to pretend that I understand all the nuances and intricacies involved in their development and use as tools for climate modelers, but some things seem clear – to me at least. The first is that even though they describe atmospheric concentrations of greenhouse gases by 2100, in ppm, they are, by necessity, intimately connected with emissions of GHGs. There are other factors which affect concentrations of course – like climate and carbon cycle feedbacks – but it seems pretty obvious that the quantity of GHGs which humans put into the atmosphere is a huge factor in determining the resulting concentrations of GHGs which accumulate in the atmosphere. Richard Betts on Twitter seems somewhat determined to underplay this fact in stressing the ‘huge difference’ between emissions and concentrations.

haha , very good 😂 ….. but NO! This is the whole point – I'm not saying that at all There is a huge difference between emissions and concentrations This is at the heart of the confusion here — Richard Betts (@richardabetts) January 30, 2020

What it appears that Richard is trying to say (or at least give the impression) is that emissions are not a primary part of RCPs – which is of course pants.

.@hausfath & @Peters_Glen correctly highlight that RCP8.5 *emissions* are not "business-as-usual" BUT it's important to note that in Earth System models, the RCP8.5 CO2 *concentration pathway* can still arise from a lower emissions scenario (eg. SRES A1B) if feedbacks are strong pic.twitter.com/LTfcaPE927 — Richard Betts (@richardabetts) January 30, 2020

This is a very important point (which Zeke and Glen do make, but it seems to get overlooked) Any particular emissions scenario can give rise to many different future concentration pathways, some rising faster and some slower, depending on the strength of carbon cycle feedbacks — Richard Betts (@richardabetts) January 30, 2020

Most climate modelling studies that use RCP8.5 are using the scenario in the form defined in terms of concentrations (the amount of CO2 & other GHGs bulding up in the atmosphere), not the form defined in terms of emissions (the amount humans are releasing). This is a key point. — Richard Betts (@richardabetts) January 30, 2020

RCP stands for Representative Concentration Pathway. These scenarios were originally defined in terms of concentrations, for use as input to climate models. Some standard emissions scenarios were developed to go along with them for studies which required this instead / as well — Richard Betts (@richardabetts) January 30, 2020

On that last point, that the scenarios were originally defined purely as concentrations for use as input to climate models, not even the IPCC appears to agree:

The Representative Concentration Pathways (RCPs) describe four different 21st century pathways of greenhouse gas (GHG) emissions and atmospheric concentrations, air pollutant emissions and land use. The RCPs have been developed using Integrated Assessment Models (IAMs) as input to a wide range of climate model simulations to project their consequences for the climate system.

Zeke Haufather and Glen Peters in their Nature comment published yesterday on the misuse of RCP8.5 say:

In the lead-up to the 2014 IPCC Fifth Assessment Report (AR5), researchers developed four scenarios for what might happen to greenhouse-gas emissions and climate warming by 2100. They gave these scenarios a catchy title: Representative Concentration Pathways (RCPs).

It is a fact that there are many different scenarios that are compatible with very high GHG concentrations in 2100 and the resultant 8.5W/m² radiative forcing at the surface; indeed, a veritable plethora of them as Zeke and Glen point out:

The plethora of future emissions scenarios poses a challenge to users of climate data — from policymakers to investors14. More than 1,200 mitigation scenarios were assessed in AR5 in 2014. Another 400 scenarios were used in the IPCC’s 2018 special report on 1.5 °C of warming.

However, what is certain is that RCP8.5 assumes high end emissions scenarios (nightmarishly unrealistic scenarios in fact), combined with little or no mitigation. That is why it was originally labeled ‘baseline’ by the IPCC, which somehow got translated to “business as usual”. You simply cannot get to very high GHG concentrations (1000ppm or more) in 2100 and a resulting 8.5W/m² radiative forcing at the surface using CMIP5 models without assuming very high emissions – even with strong climate and carbon feedbacks.

This paper summarizes the main characteristics of the RCP8.5 scenario. The RCP8.5 combines assumptions about high population and relatively slow income growth with modest rates of technological change and energy intensity improvements, leading in the long term to high energy demand and GHG emissions in absence of climate change policies. Compared to the total set of Representative Concentration Pathways (RCPs), RCP8.5 thus corresponds to the pathway with the highest greenhouse gas emissions.

The Representative Concentration Pathway (RCP) 8.5 corresponds to a high greenhouse gas emissions pathway compared to the scenario literature (Fisher et al. 2007; IPCC 2008), and hence also to the upper bound of the RCPs. RCP8.5 is a so-called ‘baseline’ scenario that does not include any specific climate mitigation target. The greenhouse gas emissions and concentrations in this scenario increase considerably over time, leading to a radiative forcing of 8.5 W/m2 at the end of the century.

[https://link.springer.com/article/10.1007%2Fs10584-011-0149-y]

Emissions scenarios (and climate change mitigation policies) are thus intimately connected with Representative Concentration Pathways and it’s not helpful that an award winning climate science communicator takes to Twitter to give the opposite impression. Why is Richard doing it? I don’t know, but damage limitation from the fallout of RCP8.5 being exposed as basically crap – not because of unrealistic assumptions about GHG concentrations but because of unrealistic socio-economic assumptions regarding the extreme use of coal in particular – springs to mind.

We can get another clue as to what might be going on by this tweet from JPascal van Ypersele, IPCC vice-chair for AR5, in response to Richard’s thread on RCP8.5:

I cannot agree more with @richardabetts: RCP pathways, which I helped to exist, are CONCENTRATION pathways, and several EMISSION scenarios are compatible with each of them, taking into account carbon feedbacks. RCP8.5, while not BaU, is important to consider for risk management https://t.co/viK3R95C0v — JPascal van Ypersele (@JPvanYpersele) January 30, 2020

Having been roundly criticised for misusing RCP8.5 as “business as usual”, climate scientists now appear to be changing tack and diverting criticism by claiming that high end scenarios (even nightmarishly unrealistic ones) are essential when considering risk management. Glen Peters and Zeke Hausfather echo this sentiment in the BBC article:

Does this mean that our projections about future temperature rises are wrong?

Not necessarily.

However, the authors are at pains to point out that the lower temperatures aren’t guaranteed.

That’s because scientists are still uncertain as to how sensitive temperatures are to CO2. New models are being used for the next set of major projections due from the IPCC next year. Those models are said to show that temperatures are more sensitive to CO2 than previously thought.

There is also the question of climate feedbacks. Although emissions from human activities might level off over this century, warming could see more permafrost melt which will push more methane and CO2 into the air, putting upward pressure on temperatures.

“I don’t think we can rule out a world of four degrees or above, because of these uncertainties in climate sensitivity and the uncertainties in carbon cycle feedbacks,” said Zeke Hausfather.

“So even under a lower emission scenario, you could have higher sea level rise, higher warming impacts, if climate sensitivity ends up being on the high end.”

“If you think of climate as a problem of risk management, you don’t necessarily just want to plan for the most likely outcome, You want to plan for sort of the tail risk, the relatively low probability but high impact scenario.”

You can immediately see where this is going, can’t you? Having finally been publicly exposed for unnecessarily scaring the pants off of people by abusing an extremely unlikely worst case emissions scenario for years, scientists are now seeking to divert attention from their failings by resurrecting the discredited precautionary principle and relying upon equally highly unlikely hyper-sensitive AR6 climate models and theoretical carbon cycle feedbacks to claim that ‘risk management’ demands a robust real world policy response to fantasy projected climate warmings. Business as usual, in other words, in the whacky world of policy advocating alarmist climate science.

Addendum

Zeke Hausfather further enlightens us on the likelihood (or not) of RCP8.5/SSP5-8.5: