The Plausibility of RCP8.5 – part II

A while ago I wrote a post about the plausibility of RCP8.5. It was essentially pointing out that there are a range of emission pathways, and hence cumulative emissions, that could lead to an RCP8.5 concentration pathway. Some of them are low enough that we would need to start substantial emission reductions quite soon if we wanted a high chance of avoiding an RCP8.5 concentration pathway.

I’ve since found the paper that presented this and thought I would slightly update my post. The figure on the right is essentially the key one. The top panel shows the emission pathways that can lead to the 4 different concentration pathways (RCP2.6, RCP4.5, RCP6 and RCP8.5). The bottom panel shows the cumulative emissions from 2006-2100 for each of these concentration pathways, and also shows the historical cumulative emissions up till 2005.

The key thing to note is that there is a range of possible emission pathways for each concentration pathway, and that the range of cumulative emissions that can produce a specific concentration pathway is quite high (represented by the black dots in the lower panel). In the case of RCP8.5, the mean from 2006-2100 is 1734GtC, but the 1\sigma range is ± 209 GtC (i.e., there’s a ~66% chance of it being between 1525 GtC and 1943 GtC). Just for comparison, we’re currently emitting about 10 GtC per year.

Something else to note is that for the higher concentration pathways (RCP6 and RCP8.5) the estimates from Integrated Assessment Models (IAMs) tend to be higher than the estimate from Earth System Global Circulation Models (ES-GCMs). In other words, the ES-GCM results suggest that avoiding RCP8.5 would require emitting less than would be suggested by IAMs. This is all summarised in the Table below.

Just to be clear, I don’t think it is particularly likely that we will follow an RCP8.5 concentration pathway. However, I don’t think it’s as unlikely as some like to suggest. Based on the Figure above, we could end up on an RCP8.5 concentration pathway if we double our emissions from ~10GtC/year today to ~20GtC/year by 2050. Given that we expect the global economy to keep growing and that there has been a relationship between emissions and GDP, avoiding doubling our emissions in the next few decades will require decoupling emission growth from GDP growth (well, assuming that the global economy does indeed continue to grow). There are hints that this is starting to happen (we’ve seen economic growth over a period where emissions have grown little) but I do think we should be careful of assuming that this will continue.

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39 Responses to The Plausibility of RCP8.5 – part II

  1. verytallguy says:

    Something I’ve never been clear on is if the emissions pathways include carbon cycle feedbacks or not.

    From my read of your post, it seems that they do, but with two differing methodologies, either some kind of empirical model reflected in IAMs or in a physical model reflected in earth system GCMs.

    Do I have that right?

  2. vtg,
    What I’m not sure about is if they include things like permafrost release. My understanding is that they don’t – the land uptake seems to be vegetation and soil. If it doesn’t consider things like permafrost release, the associated emission pathways could be even lower than suggested by the ES-GCMs.

  3. John Hartz says:

    ATTP: While you communicate in science-speak, David Wallace-Wells communicates in prose. Have you read his new book, The Uninhabitable Earth?

  4. JH,
    I haven’t read it yet. I may do so, if I get a chance to.

  5. John Hartz says:

    ATTP: In the context of your OP, the following paragraph from David Wallace-Wells’ book caught my eye,

    In reading about warming, you will often come across analogies from the planetary record: the last time the planet was this much warmer, the logic runs, sea levels were here. These conditions are not coincidences. The sea level was there largely because the planet was that much warmer, and the geologic record is the best model we have for understanding the very complicated climate system and gauging just how much damage will come from turning up the temperature by two or four or six degrees. Which is why it is especially concerning that recent research into the deep history of the planet suggests that our current climate models may be underestimating the amount of warming we are due for in 2100 by as much as half. In other words, temperatures could rise, ultimately, by as much as double what the IPCC predicts. Hit our Paris emissions targets and we may still get four degrees of warming, meaning a green Sahara and the planet’s tropical forests transformed into fire-dominated savanna. The authors of one recent paper suggested the warming could be more dramatic still—slashing our emissions could still bring us to four or five degrees Celsius, a scenario they said would pose severe risks to the habitability of the entire planet. “Hothouse Earth,” they called it.

  6. Willard says:

    i like this version better

  7. John Hartz says:

    ATTP: Although I have Wallace-Wells’ book, I have also not yet read it, I have, however read many reviews of it and many excerpts from it.

    The excerpt I posted above is from the article, When will global warming make the earth uninhabitable? by David Wallace-Wells, CQ-UK, Mar 9, 2019

  8. John Hartz says:

    ATTP: David Wallace-Wells’ book includes 235 pages of text and 65 pages of notes documenting his sources of information. It is also completely devoid of graphics.

    Based on what Wallace-Wells has said about the book in various interviews, I believe he would like to have it today’s version Rachel Carlson’s The Silent Spring. I, for one, hope that it does.

  9. David B. Benson says:

    Of course it would boost the economy to start on doubling the world’s supply of 3 trillion trees. Use lo-tech labor to avoid fossil fuels. Help avoid high carbon dioxide futures.

  10. Ken Fabian says:

    There is no doubt we have powerful and influential mainstream business and political leaders that support unconstrained use of fossil fuels, and will continue to use the tools for influencing public opinion and government policy they have at their disposal – including PR, Advertising, Lobbying, Strategic Donating, Post Politics Inducements, Tactical Lawfare and Tankthink – towards inducing that outcome. They seek to influence both the range of options democratic processes put to their constituents on this as well as the choices they make at polling booths.

    These people may or may not believe the misinformation they peddle about more use of fossil fuels being good and that the restricting and stopping the use of fossil fuels being bad but I don’t doubt those with the influence to do so are prepared to continue to undermine the foundations of democratic processes to get their way. With such determination to allow and support the commercial activities that will lead to extreme emissions scenarios they will remain within realms of real possibility.

  11. John Hartz says:

    Ken Fabian: To add insult to injury, the Koch brothers and their ilk are pretty much dictating US environmental policy under the Trump Regime. The US is a Plutocracy, not a Democracy.

  12. Steven Mosher says:

    “ATTP: David Wallace-Wells’ book includes 235 pages of text and 65 pages of notes documenting his sources of information. It is also completely devoid of graphics.”

    thats a shit ton of tweets broken up into 280 character blurbs, unroll please

  13. Steven Mosher says:

    “itted to J. Climate).
    We have shown that there is significant model spread
    in the diagnosed compatible emissions, dominated by
    projections of land carbon changes, due in part to the
    diverse response of land carbon cycle models to changes
    in CO2 and climate and widely different treatments of
    land-use change. We recommend that particular effort is
    required to better evaluate and improve terrestrial carbon cycle stocks in ES-GCMs. Anav et al. (2013) show
    a very wide range of vegetation and soil carbon stores
    simulated and, although there is not a one-to-one relation between present stocks and future changes, it is
    clearly a priority for ES-GCMs to better represent the
    magnitude of carbon amounts before we can have confidence in projections of future changes.
    We find that land carbon storage may increase or decrease in future dependent on scenario and the treatment
    of future land-use change, although most models simulate an increase for most scenarios. The spread in land
    carbon uptake among models is as high as across the
    RCP scenarios. Models largely agree that ocean carbon
    storage will increase under all scenarios, with higher
    atmospheric CO2 driving greater ocean carbon uptake.
    Projections of ocean carbon changes show much greater
    agreement than projections of land carbon changes.
    Overall, uncertainty in concentration scenario is the
    major cause of uncertainty in emissions (and airborne
    fraction) and not uncertainty in climate–carbon cycle

  14. Dave_Geologist says:

    Hit our Paris emissions targets and we may still get four degrees of warming,
    John, I would rephrase that to “we will still get four degrees of warming”. Just not by 2100. The Earth System Sensitivity feedbacks, which are what the geological record informs us about, will take longer to kick in. But kick in they will, as sure as night follows day. They’re not in most GCMs, so when the GCM temperatures flatten off, real-world temperatures will keep on rising, for centuries.

  15. paulski0 says:

    I think there are a couple of underappreciated factors with regards the RCPs:

    1) The logarithmic relationship between CO2 and forcing means differences in emissions and concentrations which seem very large to us now are actually not very important in non(or minimal)-mitigation scenarios. For example, the entire human industrial CO2 rise so far is about 130ppm, which corresponds to forcing of just over 2W/m2, approximately 2/3 of the total WMGHG forcing since pre-industrial. The same concentration difference between 870ppm and 1000ppm amounts to only a 0.75W/m2 forcing. Not negligible, but would only be about 10% of the total WMGHG forcing at that point.

    Even without assuming higher-end carbon cycle feedbacks, the low-end ~20PgC 2100 emissions scenario shown on the above plot could still get to 7.5W/m2 total net forcing at 2100 pretty easily. Again, not a negligible difference from 8.5, but not very important in the grand scheme of things.

    Also means things like permafrost loss probably aren’t very important in minimal-mitigation scenarios, at least in terms of radiative forcing growth over the next century or so.

    2) Time doesn’t stop at 2100. Seems obvious, but a lot of the people (meaning pretty much all of them) arguing against plausibility of reaching RCP8.5, or that we shouldn’t consider RCP8.5 in impact studies, really miss this point. The hypothetical 7.5W/m2 scenario mentioned above would likely hit 8.5 within a couple of decades of the 22nd Century.

  16. verytallguy says:

    Lost amidst the Brexit futore, what at first sight at least looks like a very welcome contribution to avoiding RCP8.5 announced by UK govt today.

  17. paulski0 says:


    There have been some interesting, and kind of unexpected, recent developments in non-governmental mitigation policies recently, driven by activist shareholders of major companies. Glencore have agreed to cap coal production, and Amazon have been pushed to shape up

    Particularly interesting stuff on the Glencore one: it was apparently pushed through by an investor collective called Climate Action 100+, which claims to be formed of a little over 300 investors with $33 trillion worth of assets under management and are dedicated to pushing the largest companies in the world to meet the Paris targets.

  18. Thomas William Fuller says:

    Representative Concentration Pathways replace the IPCC’s Special Report on Emission Scenarios (SRES). There are four RCPs named after the additional forcing in watts per square meter they are anticipated to have by 2100 compared to pre-industrial forcings. The four are 2.6, 4.5, 6.0 and 8.5.

    Each RCP was developed by a different team. They also used different models. They cannot be compared to each other head to head.

    The first important thing to know about RCPs comes to us directly from the IPCC. Writing in a document titled, ‘IPCC Scenario Process for AR5’ they say “The goal of working with scenarios is not to predict the future but to better understand uncertainties and alternative futures, in order to consider how robust different decisions or options may be under a wide range of possible futures”. The IPCC decided to act only as a catalyst for the process–in essence they commissioned independent scientific teams to generate the RCPs.

    “As stand alone products, the RCPs have limited usefulness to other research communities. First and foremost, they were selected with the sole purpose of providing data to climate models, taking into consideration the limitations in climate models differentiating levels of radiative forcing. They lack associated socioeconomic and ecological data. They were developed using idealized assumptions about policy instruments and the timing of participation by the international community.”

    RCP 8.5 comes up with these assumptions to get them to their goal–not vice versa:

    “RCP 8.5 – High emissions This RCP is consistent with a future with no policy changes to reduce emissions. It was developed by the International Institute for Applied System Analysis in Austria and is characterised by increasing greenhouse gas emissions that lead to high greenhouse gas concentrations over time. Comparable SRES scenario A1 F1 This future is consistent with:  Three times today’s CO2 emissions by 2100  Rapid increase in methane emissions  Increased use of croplands and grassland which is driven by an increase in population  A world population of 12 billion by 2100  Lower rate of technology development  Heavy reliance on fossil fuels  High energy intensity  No implementation of climate policies.”

    Is it possible? Sure. Is it likely? I don’t know. Is it a prediction of the future?

    Hell no.

    “Therefore, there is a need to develop socioeconomic and climate impact scenarios that draw on the RCPs and associated climate change projections in the scenario process. Referencing the RCP and climate change projections has two potential benefits; they would facilitate comparison across research results in the CM, IAM, and IAV communities, and facilitate use of new climate modeling results in conjunction with IAV research.

    “The parallel phase has several components. Within CMIP5, CM teams are using the RCPs as an input for model ensemble projections of future climate change. These projections will form the backbone of the IPCC’s Working Group I assessment of future climate change in the 5th Assessment Report (AR5). The IAM community has begun exploring new socioeconomic scenarios and producing so-called RCP replications that study the range of socioeconomic scenarios leading to the various RCP radiative forcing levels. In the meantime, IAV analyses based on existing emission scenarios (SRES) and climate projections (CMIP3) continue.

    “In the integration phase, consistent climate and socioeconomic scenarios will inform IAM and IAV studies. For example, IAV researchers can use the new scenarios to project impacts, to explore the extent to which adaptation and mitigation could reduce projected impacts, and to estimate the costs of action and inaction. Also, mitigation researchers can use the global scenarios as “boundary conditions” to assess the cost and effectiveness of local mitigation measures, such as land-use planning in cities or changes in regional energy systems.

    “These scenarios need to supply quantitative and qualitative narrative descriptions of potential socioeconomic and ecosystem reference conditions that underlie challenges to mitigation and adaptation. And they have to be flexible enough to provide a framework for comparison within which regional or local studies of adaptation and vulnerability could build their own narratives. The defining socioeconomic conditions of these scenarios have been designated Shared Socioeconomic reference Pathways (SSPs).”

    Source: A framework for a new generation of socioeconomic scenarios for climate change impact, adaptation, vulnerability, and mitigation research; Arnell, Kram, Carter (

  19. Tom,
    Did you have a point? I don’t recall anyone here claiming that RCPs sre predictions of the future.

  20. Of course, if climate sensitivity were to provide a nasty surprise, Tom’s protesting about “but RCP8.5” would be even more moot.

    Speaking of which, I see that Tom’s co-author on “Climategate: The CRUtape Letters” is saying that we should plan on climate sensitivity being greater than 3°C… 🤔

  21. Thomas William Fuller says:

    RNS, see recent discussions on uncertainty and risk. Hope for the best, plan for the worst may not sound too scientific, but it works. I think sensitivity is 2.1C but I have long advocated planning for 2.5C. See how that works?

  22. Tom,

    I think sensitivity is 2.1C but I have long advocated planning for 2.5C. See how that works?

    No I don’t see how that works. This is neither hoping for the best nor planning for the worst.

  23. Tom,
    What’s that got to do with my site?

  24. Oh, thanks. Yes. I think do I see.

    So 2.5°C sensitivity is the worst case scenario. Got it.

  25. dhogaza says:


    “Did you have a point? I don’t recall anyone here claiming that RCPs are predictions of the future.”

    It was kind of Tom to teach us that scenarios regarding future emissions are based on assumptions, too. I don’t think anyone here could figure that out on their own.

  26. Everett F Sargent says:

    “I don’t think anyone here could figure that out on their own.”

    But someone is of the opinion that we can’t.

    Like these opinions …

    “They cannot be compared to each other head to head.”
    “Maybe they used these good folks:”

    “It is not the end result of a scientific look at the fuels we will burn, the emissions they will cause, the sensitivity of the atmosphere and the results of the interactions between those and other factors.”
    “It is not meant to be used as a prediction to form other scientific work or policy. But that is how they are being used.”

    The Various Misuses of RCP 8.5

  27. Dave_Geologist says:

    2.5C. See how that works? … No I don’t see how that works.

    It works like this ATTP:

    Old-Earth Creationist: “I’m not a science denier. I accept that the Earth is 4.5 billion years old, and that evolution occurs within Kinds without Divine Intervention”.

    Intelligent Designer: “I’m not a science denier. I accept that the Earth is 4.5 billion years old, and that evolution occurs, but it needs Designer Intervention because otherwise It’s All Too Difficult“.

  28. Everett F Sargent says:

    JC is at it again with respect to SSR5-RCP8.5 …
    ” … as well as constraints on conventional coal reserves, a categorization of RCP8.5 as ‘borderline impossible’ is justified based on MY current background knowledge”

    I replaced an ‘our’ with a ‘MY’ given the rather clear cherry picking of the existing literature. It only takes 5-10 minutes of checking JC’s references plus some knowledge of FF reserve estimates (primarily coal per BGR (IMHO Ritchie/Dowlatabadi are no more energy resources experts than I am (find a copy of Dowlatabadi’s cv RE: coal energy resources)).

    Not to suggest that there may be other paths to SSR5-RCP8.5 besides re-ignition of coal, just that JC’s commentary suggests no FF resources by 2100AD.

  29. Everett F Sargent says:

    “FF reserve estimates” should read “FF resource estimates”

  30. Everett F Sargent says:

    Been waiting on this one to ne published in its final peer reviewed format (published 2019-04-12).

    Global emissions pathways under different socioeconomic scenarios for use in CMIP6: a dataset of harmonized emissions trajectories through the end of the century

    Also the SSP5 naming convention might change somewhat (e. g. SSP5-RCP8.5 becomes SSP5-Baseline and SSP3-RCP7.0 becomes SSP3-Baseline)

    Database maintained here …
    Welcome to the SSP Database (Shared Socioeconomic Pathways) – Version 2.0
    (must register 1st)

    Here is an example graphic for CO2,d&scenarios=R4467,R4468

  31. Everett F Sargent says:

    Thought I’d link to ATTP’s and mt tweets on aCO2 concentrations …

    It is the entire carbon cycle that matters, not just the anthropogenic emissions component. A BAU scenario is largely a continued FF emissions scenario (currently at ~81% of total TPES) through to 2100,. Historically, the FF emissions from oil,,gas and coal have never been fixed fractions of time.

  32. John Hartz says:

    Everett F Sargent:

    Historically, the FF emissions from oil,,gas and coal have never been fixed fractions of time.

    Do you mean to say, “… fixed fractions over time”?

  33. Everett F Sargent says:


    Yes (but I myself don’t see the difference between ‘of time’ and ‘over time’ as in either case both are calculated on a (usually) per annum basis).

  34. John Hartz says:

    Everett F Sargent:

    Are you a Brit?

  35. Everett F Sargent says:

    No. At the moment, I am rather embarrassed to say that I am an American.

  36. John Hartz says:

    Everett F Sargent: Same here. If I weren’t 75 yrs old, I would emigrate.

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