On the Post Mortem Relevance of RCP8.5

A lot of propaganda has been circulating in recent weeks concerning the IPCC's decision to remove RCP8.5 from use in the upcoming AR7 report. The decision is a good one, and I want to show why below with a simple toy model I created. I think it's fair to say that RCP8.5 is no longer a plausible future, given current emission reductions and the shrinking prices of renewable energy sources.  For some very insightful comments from others about the death of RCP8.5, see Zeke Hausfather's post at Climate Brink (Michael Mann's comments on it here) and the wonderful information at Climate Action Tracker. But the the usual suspects among contrarian political activists have been portraying the IPCC's decision as a win for "skeptics" and/or as an admission that the IPCC's projections have been fraudulent, such that over a decade of research that used RCP8.5 can now be safely discarded. Nothing could be further from the truth; it's actually an indication that renewable energy sources are outcompeting their fossil fuel counterparts and that we've made some real progress in limiting our emissions. It also suggests that there's high proportion of bad faith actors, shills and grifters who are either incompetently or deceptively misrepresenting what RCP8.5 was and why it has now been discarded.

The RCPs were designed as representative concentration pathways that described the concentrations resulting from possible courses of action we might take. They were not predictions or projections; they are better thought of as conditions (or possible courses of action) under which projections can be made. RCP8.5 was a scenario in which our carbon emissions resulted in 8.5 W/m² radiative forcing by 2100. In 2011, when RCP8.5 was defined, it represented a "worst case scenario," representative of the ~90th percentile of no policy outcomes by 2100. In other words, of the possible concentration outcomes of "no climate policy" courses of action, RCP8.5 was designed to represent forcings greater than about 90% of those outcomes. Following 2011, this scenario has sometimes been wrongly described as "business as usual," and I generally agree with many who have said this is a mischaracterization of RCP8.5. There are many possible concentration outcomes of no climate policy courses of action, and RCP8.5 represented roughly the worst 10% of those outcomes. And what is generally perceived as "usual" for us changes as new policies are introduced. What was "usual" in 2011 is no longer "usual" today. I would like to show here that even in 2011, we were not on track to achieve 8.5 W/m² radiative forcing by 2100, and getting to those concentrations would have required a ramping up of carbon emissions from the trends found in previous 30 years of emissions. Here I'm not so much saying that in 2011 a radiative forcing of 8.5 W/m² wasn't plausibly attainable by 2100; rather, I'm suggesting that we weren't likely on a pace that would attain it.

At the same time, RCP8.5 does represent well the political rhetoric of a lot contrarian propaganda, especially the rhetoric coming out of the CO2 Coalition. William Happer (founder of the CO2 Coalition) and Gregory Wrightstone (its executive director), along with many of their published political tracts, claim that the planet is starved of CO2 and we're in a CO2 famine. They claim that we need to increase our carbon emissions to give the planet more "plant food," and the "mild warming" resulting from this (if any at all) would be beneficial to humanity. Wrightstone even published a graphic in his book Inconvenient Facts suggesting that CO2 is dangerously low, and we need to get above 2000 ppm CO2 just to get our CO2 "tank" above a quarter full. As I also show here, here and here, CO2 Coalition dogma is that fossil fuel emissions are saving the planet from a possible CO2 famine and mass extinction. For them, we need more of CO2, not less of it. In other words, RCP8.5 conservatively describes the path of action that the CO2 Coalition wants. What I want to argue here is that post mortem RCP8.5 continues to have relevancy in showing the likely results of the future that those espousing this kind of contrarian rhetoric want.

From Wrightstone's Book Inconvenient Facts

I set up a toy model based on the observation that emissions increased at rates of 0.14 GtC/yr during the 30 years from 1983-2012 but only by 0.022 GtC/yr from 2012-2024. That change in emission rates is directly attributable to efforts to reduce our carbon emissions. I extend those trends linearly through 2100 to represent a "business as usual" path (1983-2012), a "current climate policy" path (2012-2024). To get a "what CO2 Coalition wants" path, I set emissions to follow an increasing rate of of 0.18 GtC/yr between 2012 and 2100 (that's about 13% higher than actual trend). This trend is almost sufficient to reach the "1/8 mark" represented in Wrightstone's graphic (<1000 ppm). In other words, this is a conservative representation of what the CO2 Coalition claims to be advocating for, whether or not it's actually plausible to achieve. Below I show what results from these three linear projections. In the table at the bottom of this post, I share the resulting annual and cumulative emissions in 2100 due to each scenario. It's of course an over simplification that the emissions from a growing population and technology use over 88 years following 2012 would result in emissions following strict linear trends, but I'm just using this as a simple model to illustrate a point. 

The IPCC has also noted that the land and ocean sinks become less efficient under high emission scenarios, meaning that the airborne fraction of our emissions increases under higher emission rates. I used the IPCC's assessment for the airborne fraction of our emissions for RCP7 for my 2012-2024 and 1983-2012 scenarios (56%). I used the airborne fraction from RCP8.5 for the CO2 Coalition scenario (62%). The resulting concentrations in 2100 are also on the table below.

For the radiative forcings for these changes in concentrations, I used 1) the IPCC's most recent assessment for the forcings for doubling CO2 at 3.93 W/m² and 2) a preindustrial CO2 concentration of 278 ppm. The 2025 Indicators of Global Climate Change assessed total radiative forcing from human activities for 2024 at 3.10 W/m², and CO2 radiative forcings were about 78% of that total (2.4 W/m²). This may be oversimplified, but for the sake of this post I'm just assuming that same proportion would continue through 2100, meaning that total radiative forcings would be 1.28x the radiative forcing from CO2 alone. This is is what I used to assess the effective RCP value associated with each of these scenarios. The 1983-2012 scenario results in 7.5 W/m². The 2012-2024 scenario results in 6.4 W/m². The CO2 Coalition's top scenario results in 8.5 W/m² (effectively RCP8.5).

This suggests to me that when RCP scenarios were brand new in 2012, we were on a path best represented by RCP7.5 (if there were one). We would likely have to ramp up our emissions to achieve about 890 ppm CO2 by 2100 to achieve RCP8.5, consistent with what the CO2 Coalition wants (and we'd still have less than 1/8 in the "tank" by 2100). To calculate the temperature response associated with each scenario, I used a TCR of 1.9°C. Here's the resulting warming by 2100 for each scenario, with HadCRUT5 plotted for comparison.

The table below summarizes all these results. It shows that my "no climate policy" path (1983-2024) would lead to about 3.6°C warming. The desired CO2 Coalition path would lead to about 4.1°C warming, and the "current climate policy" path (2012-2024) would result in 3.1°C warming. The latter overestimates future warming because there are ongoing efforts to reduce emissions. We should not assume that the 0.02 GtC/yr trend of increasing emissions will continue through 2100. At some point in the near future, we're likely to achieve peak emissions, and the resulting cumulative emissions by 2100 will be much lower. This means that we're likely on a path for less than 3°C warming. 

Projection	Emission	Airborne	2100 CO2	2100 Emissions		2100 Radiative Forcing		2100 Global
Scenario	Trend	Fraction	Amount	Annual	Cumulative	CO2	Total	Warming
2012-2024	0.02 GtC/yr	56%	671 ppm	13.3 GtC	1701 GtC	5.0 W/m²	6.4 W/m²	3.1°C
1983-2012	0.14 GtC/yr	56%	781 ppm	23.1 GtC	2117 GtC	5.9 W/m²	7.5 W/m²	3.6°C
CO2 Coalition	0.18 GtC/yr	62%	890 ppm	27.1 GtC	2317 GtC	6.6 W/m²	8.5 W/m²	4.1°C

The results published by the Climate Action Tracker are broadly consistent with my results above. Their scenario resulting from emissions prior to the signing of the Paris Agreement results in about 3.6°C warming in agreement with my simple, toy model. Their "policies and action" scenario results in about 2.6°C warming. In my toy model, that warming is associated with a cumulative emissions of 1360 GtC and resulting CO2 concentration of 580 ppm (effectively 5.4 W/m²). This suggests they expect continued implementation of climate policies to result in about 0.5°C less warming by 2100 than is indicated by my linear trend of increasing emissions from 2012-2024.

This of course is progress, but it is not good news. We need to keep warming from reaching 2°C above the 1850-1900 mean. And this is what the contrarian rhetoric fails to appreciate. The fact that we're not following RCP8.5 does not mean that we don't have a problem. The problem is just as real with or without RCP8.5, because the path we're on is bad, even if there's another path we're not following that's even worse. And we continue to need drastic reductions in our emissions if we expect to stay below 2°C warming.