The fierce debate over future scenarios (RCP8.5)

The never ending climate-debating-cycle: Cherry pick data. Insult your colleagues. Get offended by angry replies. Spectators: Pick a fighter! Bring out your popcorn. Repeat.

This is for instance played out in endless threads on Twitter about the use of the future high-CO2-emission-scenario called RCP8.5. The probably loudest voice comes from the political scientist Roger Pielke Jr. Some months ago he wrote a discussion article with a title that shows ”the tone” in this debate: How Climate Scenarios Lost Touch With Reality.

Recently some climate scientists replied: Climate Scenarios and Reality. Which in turn resulted in a frantic burst of tweets from Pielke Jr in which he wanted to explain (often in a disparage way) why climate scientists ”are mad at him”. He is claiming that the scenario-use (or misuse) is one of the biggest science-failures on this side of year 2000 and that it threatens the integrity of the whole climate science community. Sort of.

This is not a new battle, a quick search on the internet reveals that it has been going on for years, involving pretty much the same players. It is difficult not to think that Pielke Jr somehow enjoys these confrontations. He paints a picture of himself being some kind of innocent truthteller in a tainted world, but that’s really not the full picture …:

I have mostly been interested in the physics behind the climate change and have stayed away from the future scenarios etc. as it inevetibly involves economics, politics and opinions. This is indeed a complicated area and I will not even pretend to know all the details. However, it sure looks like Pielke has drawn hasty conclusions based on poor data before. So I took a look at some papers Pielke Jr had published on the ”scenario stuff”. Based on that, in my probably biased view, it is surprising that he does not show some humility about the difficulties rather than declaring his colleagues for fools.

See Carbon Brief for an Explainer on the RCP8.5-scenario. Note that the scenarios presented by IPCC are not predictions of what will happen, but projections of the future under specified conditions. It is a huge difference.

In 2008 Pielke et al wrote a comment in Nature on the future scenarios used by IPCC: Dangerous assumptions. The message was that the scenarios (SRES) were overly optimictic regarding the rate of decarbonization, and potentially seriously underestimated the challenges. They stated that the IPCC’s projections included a too high a degree of spontaneous decarbonization, i.e. assuming that much of this would happen even without climate policy. Pielke et al showed that the observations from 2000-2005 confirmed their worries; The Energy Intensity (EI) and Carbon Intensity (CI) were way off from what the scenarios assumed (meaning that the decarbonization did not proceed as fast as the scenarios assumed):

EI and CI are highly correlated to the economic growth (GDP). Pielke et al points out that we have likely only just begun to experience the surge in global energy use due to the economic development of the poorer parts of the world (hence the headline of their article). Hold that information for later.

IPCC changed scenarios and in the AR5-report four different Representative Concentration Pathways (RCP:s) were used. The high-end-scenario was named RCP8.5. The figure comes from the anticipated forcing (in W/m2) from greenhouse gases in year 2100.

In 2015 Pielke Jr again stated that the new scenarios still used a too aggressive spontaneous decarbonization rate and that the observed EI and CI -data proved it was too optimistic: Assumptions of Spontaneous Decarbonization in the IPCC AR5 Baseline Scenarios (pdf)

And here in plain text:

In the same year, 2015, the Paris Agreement was negotiated and since then the coal consumtion have more or less collapsed, about 75% of the coal plants that were planned or being built then have now been cancelled. (I haven’t got a clue on the cause and effect here though …). Fossil free energy sources have also become much cheaper and as a consequence the rate of CO2-emissions has fallen during the last few years.

As I understand it, the RCP8.5-scenario was never meant to be a business-as-usual-scenario (i.e. what will happen without policy), but rather a worst-case high-end scenario. For many years however, the emissions did follow the path of RCP8.5 (China was for example building a new coal plant every third day) and I guess this is why it often has been treated/referred to as a BAU/no policy-scenario.

The named changes in the energy sector have also made the likelihood of future outcomes to change. As far as I can tell, nowadays it is hard to find any scientist that thinks the RCP8.5-scenario is likely (but by no means impossible). IPCC states in the recent AR6-report: “the likelihood of high emission scenarios such as RCP8.5 or SSP5-8.5 is considered low in light of recent developments in the energy sector.

But despite that, some articles still refers to RCP8.5 as the BAU-scenario. It was not that long ago I commented on a paper that only presented the RCP8.5-outcome and put the results from a more realistic scenario in its Appendix. In this respect, Pielke is right and the climate scientists admits in their reply (see link above) that the ”BAU-scenario” sometimes is used sloppy. But they also think that Pielke’s criticism is taken out of proportion.

Pielke et al‘s discussion piece mentioned above, builds on what was published in one of his scientific articles from late 2020 (i.e. before the latest IPCC AR6 was released): IPCC baseline scenarios have over-projected CO2 emissions and economic growth. They found that the high-emission-scenario was implausable, mainly due to a lower economic growth than what had been assumed in the scenarios (which also affects the EI and CI).

CO2-emissions= population x GDP per capita x Energy Intensity Carbon Intensity

They then calculated what it would take for the economy to catch up with the assumed GDP-growth and thereby making the high-end scenario plausible. They judged that this catch-up-rate was too high to be likely in the real world.

I’m not an economist and they are probably right, but I can’t help noticing that just five years earlier, Pielke claimed the opposite (more or less) by referring to the same parameters, i.e. higher GDP/EI/CI-values than predicted made the model-scenarios being too optimistic in its decarbonization-rates. The numbers supports their conclusions both then and now, but to me, the fact that you can come to opposite conclusions in analyses – based on the same measures – just a few years apart indicates that GDP must be a terrible parameter to rely on for predicting the future (although I understand that these calculations are needed).

The uncertainties is also confirmed in a paper by Nordhaus et al (winner of the so called Nobel prize in economy (which actually stems from a donation from the Swedish National Bank, not from Alfred Nobel testament as the ”real” Nobel prizes)): Uncertainty in forecasts of long-run economic growth, published in PNAS 2018.

They states that there is a 35% probability that the worst-case-scenario RCP8.5 will come true because of the huge uncertainties in determination of a future GDP and the subsequent emissions. Two other studies came to the same conclusion (more or less): Müller et al 2019, and Startz 2020. If true, these uncertainties also means that the GDP-catch-up-rate calculated by Pielke et al is plausibel.

Pielke et al do comment on this though:

However, we argue that this magnitude of catch-up may be unlikely, in light of: headwinds such as aging and debt, the likelihood of unanticipated economic crises, the fact that past economic forecasts have tended to over-project, the aftermath of the current pandemic, and economic impacts of climate change unaccounted-for in the baseline scenarios.”

Appearantly, Pielke no longer believes the future economic growth of poorer countries will ”take off” as he did back in 2008 (see above). Their arguments are probably likely but are only assumptions, and far from being actual facts (and no, this is not equivalent to the uncertainties in the ”physics climate models”).

Days after IPCC AR6 WG1-report was released Pielke Jr ruled that it was biased towards the worst-case scenario by counting how many times it was mentioned in the text compared to the other scenarios. Everyone who have read the report knows that’s not true. He also regulary posts search results from Google Scholar on how many published articles that includes the RCP8.5 word. He really seems obsessed with this RCP8.5-scenario, but in my view these ”investigations” are as pointless as judging Pielke’s paper being alarmistic because he uses ”RCP8.5” much more often than the other scenarios (which he does).

Look, I get it, RCP8.5 is unlikely, all scientists seems to agree on that. But it hasn’t always been. It is easy to be the wise guy in retrospect. And considering what Pielke appearantly thought about the scenarios earlier, I think his now snorky self confidence in calling his colleagues fools for using the RCP8.5 is both headstrong and unfair (yes, that’s my opinion, not a fact).

-Yes, there are still scientists that call the RCP8.5-scenario (or SSP5-8.5) BAU.
-Yes, there are journalists that could ask more critical questions.
-Yes, there are plenty of reasons for using RCP8.5 even though it is not a likely outcome.
-Yes, there are some substance in Pielke’s criticism.

-No, this does not mean that the whole scientific community only focus on the RCP8.5-scenario (read the IPCC-report).
-No, everything that journalist writes are not wrong or alarmistic.
-No, it hasn’t been wrong to use RCP8.5 in risk assessments.
-No, warming doesn’t suddenly stop after 2100 if we continue releasing CO2.

And by all means, emissions are only one of the uncertainties in predicting the future. The other two major factors are climate sensitivity (temperature change after a doubling of CO2) and the carbon cycle – these are not even discussed in Pielke’s paper mentioned above. For example, in a warmer world it is expected that the absorption of CO2 from the atmosphere will be lower than today, meaning more of our emissions will stay in the atmosphere.

Many groups have estimated a future warming with the current policies:

Pic from Breakthrough/Zeke Hausfather

Most points to about 3ºC increase by 2100, but if the climate sensitivity-uncertainty is included, warming to +4ºC could not be ruled out (in line with the SSP4-6.0 scenario). According to the climate scientist Zeke Hausfather the stated policies would lead to 2.4ºC (1.7ºC to 4ºC) most in line the scenario SSP2-4.5, but the world will continue to warm after that point as these emission pathways do not get us to net-zero.

In a recent UN-report, the now stated policies will take us to 2.7ºC (no range given). According to the declarations from all contries, the CO2-emissions will increase with 16% to 2030.

And for sure, the stated policies are still only figures on a paper. We are no way near the goals of the Paris Agreement. And we do not know if another Trump- or Bolsonaro-like leader that doesn’t give a fuck about science appears.

Regarding the never-ending fuzz about the RCP8.5-scenario, I think it’s like what we in Sweden call a storm in a glass of water. There is still probablilities for 4ºC warming even without RCP8.5. And the likely consequences are severe already at 2-3ºC. As seen in the latest IPCC-report, science is adapting to the latest information, the full range of SSP5-8.5 was not even depicted in the diagram in the SPM. Give it some time for God’s sake.

RealClimate: #NotAllModels

And yes, as we are seeing progress in the decarbonization the economy, the usual suspects will claim that the problem never existed or that the market fixed it by itself, just like that (we’ve seen this before).


I wonder what Lomborg thinks now that even Pielke Jr indirectly think his work is a bit ludicrous. I mean, Lomborg states that 3.75ºC by 2100 is the optimum temperature to aim for…


Practised my English, pardon for misstakes 🙂

[Update 27th Oct: Pielke Jr commented my post on Twitter, linking to his article from early 2021: Distorting the view of our climate future: The misuse and abuse of climate pathways and scenarios. Only had time for a quick look on a chapter about the likelihood of RCP8.5 and that naming it BAU is wrong. Think I will stick to natursal science…. ]


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