What’s the worst case? A possibilistic approach

Reposted from Climate Etc.

Posted on March 27, 2019 by curryja |

by Judith Curry

Are all of the ‘worst-case’ climate scenarios and outcomes described in assessment reports, journal publications and the media plausible? Are some of these outcomes impossible? On the other hand, are there unexplored worst-case scenarios that we have missed, that could turn out to be real outcomes? Are there too many unknowns for us to have confidence that we have credibly identified the worst case? What threshold of plausibility or credibility should be used when assessing these extreme scenarios for policy making and risk management?

I’m working on a new paper that explores these issues by integrating climate science with perspectives from the philosophy of science and risk management. The objective is to provide a broader framing of the 21st century climate change problem in context of how we assess and reason about worst-case scenarios. The challenge is to articulate an appropriately broad range of future outcomes, including worst-case outcomes, while acknowledging that the worst-case can have different meanings for a scientist than for a decision maker.

This series will be in four parts, with the other three applying these ideas to the worst case scenarios for:

  • emissions/concentration
  • climate sensitivity
  • sea level rise

3. Possibilistic framework

In evaluating future scenarios of climate change outcomes for decision making, we need to assess the nature of the underlying uncertainties. Knight (1921) famously distinguished between the epistemic modes of certainty, risk, and uncertainty as characterizing situations where deterministic, probabilistic or possibilistic foreknowledge is available.

There are some things about climate change that we know for sure. For example, we are certain that increasing atmospheric carbon dioxide will act to warm the planet. As an example of probabilistic understanding of future climate change, for a given increase in sea surface temperatures, we can assign meaningful probabilities for the expected increase in hurricane intensity in response to a specified temperature increase (e.g. Knutson and Tuleya, 2013). There are statements about the future climate to which we cannot reliably assign probabilities. For example, no attempt has been made to assign probabilities or likelihoods to different emissions/concentrations pathways for greenhouse gases in the 21st century (e.g. van Vuuren et al, 2011).

For a given emissions/concentration pathway, does the multi-model ensemble of simulations of the 21st century climate used in the IPCC assessment reports provide meaningful probabilities? Stainforth et al. (2007) provide a convincing argument that model inadequacy and an inadequate number of simulations in the ensemble preclude producing meaningful probabilities from the frequency of model outcomes of future climate states. Nevertheless, as summarized by Parker (2010), it is becoming increasingly common for results from climate model simulations to be transformed into probabilistic projections of future climate, using Bayesian and other techniques.

Where probabilistic prediction fails, foreknowledge is possibilistic – we can judge some future events to be possible, and others to be impossible. The theory of imprecise probabilities (e.g. Levi 1980) can be considered as an intermediate mode between probabilistic and possibilistic prediction. However, imprecise probabilities require credible upper and lower bounds for the future outcomes, including the worst-case.

Possibility theory is an uncertainty theory devoted to the handling of incomplete information that can capture partial ignorance and represent partial beliefs (for an overview, see Dubois and Prade, 2011). The relevance of analyzing uncertainty with possibility theory is better appreciated when evidence about events are unreliable or when prediction or conclusion is difficult to make due to insufficient information. Possibility theory distinguishes what is necessary and possible from what is impossible. Possibility theory has been developed in two main directions: the qualitative and quantitative settings. The qualitative setting is the focus of the analysis presented here.

Possibility theory represents the state of knowledge of an state of affairs or outcome, distinguishing what is plausible from what is less plausible, what is the normal course of things from what is not, what is surprising from what is expected. In possibility theory, the function π(U) distinguishes an event that is possible from one that is impossible:

π(U) = 1: nothing prevents U from occurring; U is a completely possible value

π(U) = 0: U is rejected as impossible

The necessity function N(U) evaluates to what extent the event is certainly implied by the status of our knowledge:

N(U) = 1: U is necessary, certainly true; implies p(U) = 1

N (U) =0 : U is unnecessary; implies p(U) is unconstrained

Possibility theory has seen little application to climate science. Betz (2010) provided a conceptual framework that distinguishes different categories of possibility and necessity to convey our uncertain knowledge about the future, using predictions of future climate change as an example. In this context, Betz defines ‘possibility’ to mean consistency with our relevant background knowledge – referred to by Levi (1980) as ‘serious possibility.’

Betz (2010) classified possible events to fall into two categories: (i) verified possibilities, i.e. statements which are shown to be possible, and (ii) unverified possibilities, i.e. events that are articulated, but neither shown to be possible nor impossible. The epistemic status of verified possibilities is higher than that of unverified possibilities; however, the most informative scenarios for risk management may be the unverified possibilities.

A useful strategy for categorizing ‘degrees of necessity’ is provided by the plausibility measures articulated by Friedman and Halpern (1995) and Huber (2008). Measures of plausibility incorporate the follow notions of uncertainty:

  • Plausibility of an event is inversely related to the degree of surprise associated with the occurrence of the event;
  • Notions of conditional plausibility of an event A, given event B;
  • Hypotheses are confirmed incrementally for an ordered scale of events, supporting notions of partial belief.

Guided by the frameworks established by Betz (2010), Friedman and Halpern (1995) and Huber (2018), future climate outcomes are categorized here in terms of plausibility and degrees of justification (necessity). A high degree of justification (associated with high p value) implies high robustness and relative immunity to falsification or rejection. Different classifications and associated p values can be articulated, but this categorization serves to illustrate applications of the concepts. Below is a classification of future climate outcomes used in this paper:

  • Strongly verified possibility – strongly supported by basic theoretical considerations and empirical evidence (p = 1)
  • Corroborated possibility – empirical evidence for the outcome; it has happened before under comparable conditions (0.8 ≤ p < 1)
  • Verified possibility – generally agreed to be consistent with relevant background theoretical and empirical knowledge (0.5 ≤ p < 0.8)
  • Contingent possibility – outcome is contingent on a model simulation and the plausibility of input values (0.1 ≤ p < 0.5)
  • Borderline impossible – consistency with background knowledge is disputed (0 < p < 0.1)
  • Impossible – inconsistent with relevant background knowledge (p ≤ 0)

The contingent possibility category is related to Shackle’s (1961) notion of conditional possibility, whereby the degree of surprise of a conjunction of two events A and B is equal to the maximum of the degree of surprise of A, and of the degree of surprise of B, should A prove true.

This possibility scale does not map directly to probabilities; a high value of possibility (p) does not indicate a corresponding high probability value, but rather shows that a probable event is indeed possible and also that an impossible event is not probable.

3.1 Scenario justification

As a practical matter for considering policy-relevant outcomes (scenarios) of future climate change and its impacts, how are we to evaluate whether an outcome is possible or impossible?  In particular, how do we assess the possibility of big surprises or black swans?

If the objective is to capture the full range of policy-relevant outcomes and to broaden the perspective on the concept of scientific justification, then both confirmation and refutation strategies are relevant and complementary. The difference between confirmation and refutation can also be thought of in context of regarding the allocation of burdens of proof (e.g. Curry, 2011c). Consider a contentious outcome (scenario), S. For confirmation, the burden of proof falls on the party that says S is possible. By contrast, for refutation, the party denying that S is possible carries the burden of proof. Hence confirmation and refutation play complementary roles in outcome (scenario) justification.

The problem of generating a plethora of potentially useless future scenarios is avoided by subjecting the scenarios to an assessment as to whether the scenario is deemed possible or impossible, based on our background knowledge. Section 2 addressed how black swan or worst-case scenarios can be created; but how do we approach refuting extreme scenarios or outcomes as impossible or implausible? Extreme scenarios and their outcomes can be evaluated based on the following criteria:

  1. Evaluation of the possibility of each link in the storyline used to create the scenario.
  2. Evaluation of the possibility of the outcome and/or the inferred rate of change, in light of physical or other constraints.

Assessing the strength of background knowledge is an essential element in assessing the possibility or impossibility of extreme scenarios. Extreme scenarios are by definition at the knowledge frontier. Hence the background knowledge against which extreme scenarios and their outcomes are evaluated is continually changing, which argues for frequent re-evaluation of worst-case scenarios and outcomes.

Scenario refutation requires expert judgment, assessed against background knowledge.

This raises several questions: Which experts and how many? By what methods is the expert judgment formulated? What biases enter into the expert judgment?

Expert judgment encompasses a wide variety of techniques, ranging from a single undocumented opinion, to preference surveys, to formal elicitation with external validation (e.g. Oppenheimer et al., 2016). Serious disagreement among experts as to whether a particular scenario (outcome) is possible or impossible justifies a scenario classification of ‘borderline impossible.’

3.3 Worst-case classification

On topics where there is substantial uncertainty and/or a rapidly advancing knowledge frontier, experts disagree on what outcomes they would categorize as a ‘worst case,’ even when considering the same background knowledge and the same input parameters/constraints.

For example, consider the expert elicitation conducted by Horton et al. (2014) on 21st century sea level rise, which reported the results from a broad survey of 90 experts. One question related to the expected 83-percentile of sea level rise for a warming of 4.5oC, in response to RCP8.5. While overall the elicitation provided similar results as cited by the IPCC AR5 (around 1 m), Figure 2 of Horton et al. (2016) shows that 6 of the respondents placed the 83-percentile to be higher than 2.5 m, with the highest estimate exceeding 6 m.

While experts will inevitably disagree on what constitutes a worst case when the knowledge base is uncertain, a classification is presented here that is determined by the extent to which borderline impossible parameters or inputs are employed in developing the scenario. This classification is inspired by the Queen in Alice in Wonderland: “Why, sometimes I’ve believed as many as six impossible things before breakfast.” This scheme articulates three categories of worst-case scenarios:

  • Conceivable worst case: formulated by incorporating all worst-case parameters/inputs (above the 90 or 95-percentile range) into a model; does not survive refutation efforts.
  • Possible worst case: 0 < p < 0.1 (borderline impossible). Includes multiple worst-case parameters/inputs in model-derived scenarios; survives refutation efforts.
  • Plausible worst case: p just above p = 0.1. Includes at most one borderline impossible assumption in model-derived scenarios.

A few comments are in order to avoid oversimplification of this classification for a specific application. Simply counting the number of borderline uncertain parameters/inputs in deriving a scenario can be misleading if these inputs are of little importance in determining the scenario outcome. If these borderline impossible parameters/inputs are independent, then the necessity (and likelihood) of the scenario is reduced relative to the necessity of each individual parameter/outcome. If the collection of borderline impossible parameter/inputs produce nonlinear feedbacks or cascades, then it is conceivable that these parameters/inputs somehow have a cancelling effect on exacerbating the extremity of the outcome. Model sensitivity tests can assess to what extent a collection of borderline impossible parameters/inputs contributes to the extremity of the outcome.

The conceivable worst-case scenario is of academic interest only; the plausible and possible worst-case scenarios are of greater relevance for policy and risk management. In the following three sections, applications of these ideas about worst-case scenarios are applied to emissions/concentrations, climate sensitivity and sea level rise. Apart from their importance in climate science and policy, these three topics are selected to illustrate different types of constraints and uncertainties in assessing worst-case outcomes.

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76 thoughts on “What’s the worst case? A possibilistic approach

  1. “The conceivable worst-case scenario is of academic interest only.”

    I would say it has great journalistic interest for purposes of scaremongering. Take it from someone who has negotiated with Teamsters, you do not counter an irrational case with a rational one. You need a complimentary irrational case. We need an implausible optimistic scenario.

    • No, for “optipistic’, substitute, ‘pessimistic’ model showing the catastrophic results of ADOPTiNG alarmist/green measures to ” fight” global warming – millions starving , freezing, disease running rampant due to suppression of energy to “fight” global warming.

    • Judith Curry is rapidly becoming a one-note, boring, repetitive, somewhat obsessive individual. She could find another subject to flog, or take up crochet.

      Frankly, Judy, I just don’t give a flying frack in space about your attempts to induce panic attacks in us lesser beings. You’ve pounded that drum for such a long time, that the variations are now repetitions of what you’ve cranked out before Now. And I just do NOT CARE. You’re boring.

      • Sara. You clearly haven’t a clue what Curry is on about. Her whole point is that there is likely nothing to worry about.

      • If you don’t care, stop hating Judith so much. She has a lot of valuable things to say, that some of us actually do want to hear.

      • I don’t see your point Sara. Are you upset that she says we know that adding CO2 to the atmosphere will cause warming? It seems to me that Dr. Curry is trying to establish the logical framework for evaluating whether alarmist claims can be “scientifically” rejected as being impossible or borderline impossible. She is doing so on terms that at least in principle could be accepted by other persuadable scientists. Conceding the point about warming serves to disarm those who will stop listening if they think she claims warming from CO2 could be zero or negative.

        Those other scientists may be closet skeptics who, without this kind of esoterica may be afraid to buck the party line. To be sure, it is not bedtime reading, and will never be the sort of thing that the average person will be able to follow or find interesting.

        I suspect that it’s naive to believe that there are many other active climate scientists who are closet skeptics or even persuadable given that their meal ticket depends on not being persuaded. Dr. Curry may be projecting. However, it would still be useful to have rigorous arguments to refute wildly improbable worst-case scenarios. Those who are already openly-skeptical can cite this to bolster the case when arguing at a much simpler level with undecided voters.

        The same folks who can’t follow the arcane details will certainly grasp the idea that “this peer-reviewed study says that your scary headline is borderline impossible”. Or “the worst-case plausible scenario is not much to be concerned about”.

        • A worst case cost/benefit analysis over a 20 to 30 year period is much different from one over a 100 to 300 year period. The latter is wild speculation. Unforeseen demographic, technological and economical changes will kill you.

          Could one foresee/predict the world of the early 21st Century from the vantage of the end of the 19th Century? The crystal ball of a super computer is no better than one consisting of a pen and ink. One can’t even imagine a realistic future, much less calculate one using current knowledge/techniques. Modelturbation, anyone?

          • You can say that, sure. But her approach, if I’m understanding it correctly, is to listen to the wild projections and then follow a rigorous process that can rule out some of the wild ones based on known science rather than based on what the models predicted.

            If this framework gets used to substantiate some of the moderately catastrophic projections that really only depend on trusting model results, then that would certainly be an unintended consequence of her efforts.

          • Dr. Curry’s work will either be ignored, or twisted to fit the narrative of CAGW.

            The only place to beat the charlatans is at the ballot box. Study, logic, analysis, etc. will not sway the political/economic profiteers. The UN, outside of the Security Council, is run by Third World kleptocrats, save-us-all NGOs and profiteering hustlers.

      • Sara,

        Why the hatred?

        From what I just read, JC is possibly the ONLY climate scientist interested in actually getting to a scientific truth.

        The rest just fudge data, extrapolate exponentially and then make absurd and invalid (scientifically speaking) claims of impending catastrophe.

        Judy is an actual climate scientist, who literally walked away from it because of the abject abandonment of scientific principles in the academic realm.

        She has now found a way to put her expertise to a valid use in a way that should help us define the potential for catastrophic consequences of AGW. So far the only thing we have to go on in that respect is rampant conjecture and alarmist speculation.

    • “We need an implausible optimistic scenario” so Pennies instead of Heat Waves from the heavens?
      You could call that a complementary irrational case. Its virtue is that it shows the worthlessness of the whole category, including the option of Climate Catastrophe.

  2. Does she ever play the lottery? And/or have climate parameters ever been programmed into AlphaZero?

  3. Nothing sucks like an ice age.

    I’d rather have Miami under water than New York under a glacier.

      • I wouldn’t want to be T-Rex meat either either – which is what I might have been if I lived in North America back when CO2 was a lot higher than it is now.

        Ice ages are mass extinctions. A few degrees warmer would be nothing of the sort..

      • Rhys,
        You don’t live in the USA, do you? You will never be ‘crocodile meat’ in Miami!
        Alligators are native to Florida and other southern coastal states of the USA.
        Crocodiles are native to Africa.

        • Sorry, J Mac, but there IS DEFINITELY an American crocodile, a rare but very real species of aquatic reptilian. They do not look like alligators, having a longer, somewhat slender snout, and they are much lighter in color than the alligators. They live mostly in the Caribbean and the southern end of Florida. https://myfwc.com/wildlifehabitats/wildlife/american-crocodile/

          It’s a protected species, so leave it alone. The gators bite and likewise do American crocodiles.

          • It is a ‘rare’ species….. and Alligators are umbiquitous throughout Florida.
            The chance of Rhys becoming ‘crocodile meat in Miami’ is less than his being struck by lightning in Miami.

        • Crocodiles are native to Africa

          We have quite a few native to Australia, too. Both saltwater and freshwater ones.

          I think they are a little more widespread than you believe.

          • I did not say crocs were exclusive to Africa. Your statement as to my knowledge (not beliefs) is presumptive… and wrong.

        • I am sure you will find significant differences in being eaten by an alligator rather than a crocodile.

          And Ice Ages will see mass migrations toward equators, just as warming will see migrations poleward.

          Homo Sapiens originated in Equatorial Africa and did so considerably before the end of the last Ice Age. So Ice Ages will not wipe out Homo Sapiens any more than Minoan warming wiped out polar bears.

          Canadians, Scandinavians and Russians may be challenged in an Ice Age, but I doubt those in Dar es Salaam will all drop down dead.

          You Americans are touchy about minor terminology issues but expect British Citizens to tolerate you calling the UK ‘England’, the Queen the ‘Queen of England’ and many more ignorant and inaccurate claptrap besides.

          You also think you can conquer everywhere and are blissfully unaware that 150 million of you will die on day 1 of WWIII if you ever try to conquer Russia…..

          • Rhys Whilst you are right to be annoyed at many Americans ignorant persistence in referring to the UK as England, the rest of your post is bollocks.

          • Rhys, Hatred of the evil Americans seems to consume you the way I’ve seen drunken Irishmen in Boston talk about the bl**dy Brits who need to get out o’ Ireland. It’s not a good look for you.

            Are you seriously so deranged as to believe that anybody in the US is interested in conquering Russia? What a crock. Stuck in the ’60s much?

            But I’m still not convinced that you’re a real person rather than a Russian troll-bot programmed to sound like Dennis from Monty Python and the Holy Grail. (Now we see the violence inherent in the system, do you see him repressin’ me? That’s what I’m on about…)

            Say, British Citizen, do you not consider yourself a British Subject? The Queen of England may not be amused. Too bad her government and all those representatives and senators in the English Parliament at Westminister are about to feed you to the globalist crocodiles in Brussels. Or are they alligators? hmmm
            Sorry, it’s wicked of me to push your buttons like this, but what can you expect from a Yank?

      • Agreed.

        NY actually has been under a glacier in the past. Returning to that state is a lot more plausible (and potentially quicker) than having Greenland or Antarctica melt.

        It’s too bad folks got off on a “where crocodiles live” tangent. That has nothing to do with anyone’s point.

  4. “A few comments are in order to avoid oversimplification of this classification for a specific application”

    Mumbo Jumbo. I seriously set forth the try to follow this. I did follow it and it left me confused. If this is what the academics try to follow then there is no wonder they are confused.

    Where is the discussion on “No Cause for Alarm”?

    • That was just the first installment. All she has done is to lay out the ground rules. The practical stuff starts with the next installment.

    • Here is a potential discussion on “No cause for alarm”.
      Your kettle boils at 100C at sea level no matter how much you turn up the heat. It has done that for thousands of years. This fact is but one datum point on the graph giving the boiling temperature of water at various altitudes, the mean of which gives the general perception of global temperature.
      This may be explained if you go back to the basic science, thermodynamics and behaviour of water. It is a stable situation, evidenced by the historical global temperature of the earth which has not varied by much more than +/- 3% on the Kelvin scale. (my guess).
      A pesky 1.6Watts/sq.m increase in purported GHE is not going to make any significant difference, even if it were true.
      In trite terms one may say: “The Earth sweats to keep cool, just like you and I.”
      So long as my kettle boils at 100C I have NO cause for alarm.
      Worth discussion methinks.

      PS: A caveat: While water is good at cooling things it is not much good at heating things up; so ice ages are the things to be alarmed about, not warming.
      http://cognog2.com

  5. In climate the concept of worst case scenarios is challenging, if certain amplification/damping parameters (e.g.sensitivity of response to seeohtwo) are themselves subject to uncertainty.

    You may find that there will be a range of worst case scenarios for various combinations of parameter values.

    There is also the question of ‘worst in what sense’?

    For politicians there may be flooding scenarios with widely differing cost outcomes solely dependent on where on e.g. the Missouri/Mississippi/Ohio basin the worst floods occur. The actually weather/climate indices for those different flood scenarios might merely be different timings and geographical locations of particular snow and rainstorms.

    Hurricane scenarios for politicians matter more when landfall occurs. A Cat 5 hurricane which stays offshore before blowing itself out across the Atlantic is less of a political problem than a Cat 4 devastating three major cities.

    A long term drought at slightly lower temperatures may be more damaging to agriculture than a hotter, wetter year.

    A year where annual rainfall is packed into a few storms in one month may be more worst case than identical rainfall spread out over four months.

    Scenarios may be impacted by human water consumption, nothing to with climate at all other than folks wanting to shower often in warm weather.

    I am not convinced at all that it should be scientists who should draw up what ‘worst case scenarios’ look like. Politicians should describe them, then scientists should work on understanding how they might come about.

    When I hear nonsense about the Greenland Ice Sheet melting or half of Antarctica calving, my eyes glaze over. These scenarios are fantasy, not realistic.

    It is realistic to postulate widespread flooding of the Central Valley of California, since that happened in 1861/2. It is also possible to postulate widespread Ice Cover of the Thames in London, as that happened 300+ years ago on a regular basis.

    It is reasonable to postulate magnetic reversals, since they are in the climatological record.

    It is reasonable to postulate failure of major dams/terrorist targeting of such, since WWII saw the allies bombing all kinds of dams and dam failure is not unprecedented.

    It is reasonable to ask what humans could do using weather warfare, since such technology may be secret, but everyone knows it exists.

    If I am a politician, I would want to know what zapping the ionosphere might produce in terms of extreme weather…..

  6. The worse case scenrio would be related to alarmist policy, such as a Green New Deal. Such a policy could kill millions of people due to dramatically increased poverty, causing starvation and disease, combined with the resulting breakdown of law and order across society, which is exactly the scenario we see in Venezuela.

    IMO, The effects of extreme climate policy absolutely needs to be included in any climate change policy discussion and people responsible for introducing such policy should be personally liable for any deaths resulting. This should even consider excess winter deaths caused by energy poverty or energy unreliability as is already occuring today. Bottom line is that legislated mass murder is just as evil as other forms of mass murder such as terrorism, the only difference being the orders of magnitude greater.

  7. The challenge is to articulate an appropriately broad range of future outcomes, including worst-case outcomes, while acknowledging that the worst-case can have different meanings for a scientist than for a decision maker.
    I’d start with the worst case scenario for climate scientist and look at the validity of the Greenhouse Effect having any valididty when applied to the earth’s atmosphere. It just could be that it has no validity at all. Yes this would be disasterous for all those feeding on the doomsday gravy train but absolutely fabulous for those of us that have to pay to keep the train running.

  8. “The challenge is to articulate an appropriately broad range of future outcomes, including worst-case outcomes, while acknowledging that the worst-case can have different meanings for a scientist than for a decision maker.”

    I would start with looking at the possibility that the Green House Effect as it applies to greenhouses is not applicable to atmospheric temperature. Perhaps catastrophic for climate scientists but certainly a good outcome for the world.

  9. “There are some things about climate change that we know for sure. For example, we are certain that increasing atmospheric carbon dioxide will act to warm the planet. “

    Really? It would seem to me that we are certain that CO2 absorbs energy over certain portions of the electromagnetic spectrum and radiates that energy over other portions of the spectrum. Whether this increases the temperature of the planet is uncertain – at least to this observer.

    • Also, we are NOT certain its effects are all bad. There are undoubtedly good effects of higher CO2. We can already see evidence of them in the satellite photos that show the planet getting greener, higher crop yields, etc.

  10. The risk to society is not in real climate change, but it is in the political threats to ordinary life.
    Those who have been using hoked-up horror stories about climate have been the threat.
    Only someone verging on madness would boast that they have the power to set the temperature of the nearest planet.
    But like the financial markets, politics has become highly speculative and compulsive.
    Both the financial and political markets can be described as becoming unstable.
    Ending action.

    • There are many points where I can insert this comment, but I’ll do so here.’

      Stirring up fears of horrible consequences of theoretical (and not measured) temperature changes in order to advance a totalitarian political agenda is nothing more nor less than shouting Fire! in a crowded theater. It’s an act intended to incite panic among otherwise rational people in an effort to make them take actions which will cause them – and all around them – tremendous harm. It is a deliberate terrorist act, and should be treated as such whenever it is performed.

      Last time I checked, deliberate acts of terrorism are not greeted warmly anywhere in the civilized world.

      This is no more complicated than that.

      • How about the positive press of mailing oneself a white powder at work with a threatening message?

  11. Step One would be to show that it is an increase in the level of CO2 that causes an increase in temperature and not that increasing temperature causes an increase in the level of CO2. One also needs a conference with the plants to see what their attitude is.

  12. For policy making, model predictions/projections over periods longer than about 20-30 year time frames are worse than useless; they will cause one to make needless investments in CO2 reductions if the long-term future is different from that predicted by the results of UN IPCC climate models fed into speculative econometric models. Additionally, Nobel Economics Prize winner, Nordhaus, has estimated that the costs of speculative future economic damages is rather modest in terms of expanded future economies.

    Wouldn’t it be better if we ran models annually based on what we have experienced over the last year, added to our prior knowledge? In that manner we would be able to change course over the ensuing 20-30 year period, before negative impacts could be felt. We wouldn’t be forced to alter our current society, economy and energy systems based on speculative 100 to 300 year predictions by climate and economic models.

  13. Judith was in the alarmist camp for a long time, and it appears that her mindset was heavilt formed by it.

    Before all the screaming began, a warm period was called a “climate optimum.” The Medieval climate optimum, the Roman climate Optimum, Minoan, Holocene, Eocene, etc. Warm periods nurtured more quantity and variety of Life. Therefore, any attempt to cool “the planet” is a direct attack on the biosphere. I see no hint of this idea in the article.

    I see no acknowledgement of the meaning of CO2 and photosynthesis, either. Any attempt to keep CO2 below 1100 ppm is also a direct attack on the biosphere, except soil sequestration.

    I believe rising CO2 is also a direct benefit to human and animal health and that this is a major reason we love soda.

    Esther Cook,
    M.S. Animal Physiology

      • In my lifetime – over 70 years now (a personal record) – there hasn’t been, really, any cause for concern, either. More CO2, whether it causes warming or not, would be a good thing for plants and thus for animals too overall.

        I still think natural variation overwhelms any minor effect CO2 may have, witness just the last 12K years or so (and even longer ago) – CO2 is pretty much an NPC…

      • We’re not worried about hundreds of years. [Or even a hundred years. Or even 50 years. Talk about 20 to 30 years and you might have an audience other than the excitable.]

    • ladylifegrows … at 6:28 pm

      Before all the screaming began, a warm period was called a “climate optimum.”

      Good one!

    • Good points. The fact is we know from history there is no such thing as a worst case climate scenario that is worth worrying about. We can adapt to anything we’ve detected.

      There are far worse problems that have real worst case scenarios such as rather large meteors crashing into the planet, a Carrington event, nuclear war, etc. How about we spend our money where it might do some good.

  14. The way to simplify this is to figure out how much a given volume of CO2 raises the temperature of the atmsophere.

    The is the most basic question in climate science, and there is still no answer to this question even after all the decades of study.

    Before accurate predictions can be made, this number must be known.

    Every new study of ECS lowers the temperature. The official low side now is at 1.5C and probably going lower considering other studies actually do put it lower.

    You’re blowing smoke if you make predictions and don’t know this number. And nobody at this moment knows what the correct figure is.

    • It appears that the ECS of the average of the CMIP6 models produced for the UN IPCC’s AR6 will larger than that of the CMIP5 models. Go figure.

  15. Popcorn. I sense Dr. Curry is going to set forth a logical explanation that refutes the CC scaremongering that is prevalent with the politicians and supported by the MSM. A concern is that the MSM won’t listen. Or more to the point, that the MSM is programmed to counter the logic with more scaremongering and unsupported denial.

  16. Does exploring the probability of worst cases include another step down towards the next glaciation?

    Each optimum has been lower than its predecessor; each pessimism lower than its predecessor. I fail to see any reason why the current rise would exceed the MWP and why the subsequent decline at the end of this rise wouldn’t go lower than the LIA.

    That said, it doesn’t stop me from rooting for global warming.

  17. Might be time to recall the immortal Yogi Berra: “Predicting is very hard, especially about the future”. (Actually it originated in Danish, possibly with Niels Bohr.)

    We seem to be discussing the policy implications of the worst case CO2 scenario. I propose that there are no policy implications since CO2 at this time, at these levels, is not in control of climate and, furthermore, we are not in control of CO2. The natural experiment was run in 1929-1931. And there is the exponential decay of its GHG effect after 50% of its effect in the first 20 ppm. And there is no other theoretical handle for us to alter climate and its eight major (known) inputs.

    As Puls said, “Scientifically it is sheer absurdity to think we can get a nice climate by turning a CO2 adjustment knob. Many confuse environmental protection with climate protection. it’s impossible to protect the climate, but we can protect the environment and our drinking water. On the debate concerning alternative energies, which is sensible, it is often driven by the irrational climate debate. One has nothing to do with the other.”

    Just one possible correction: The fourth power output of IR in response to temperature change takes place on land. Over 70% of the earth’s surface, downwelling IR warms the top several microns of the seas, which evaporates, rises, and deposits its heat higher in the troposphere. Stratospheric CO2 then radiates IR out to space. A natural refrigeration cycle.

    So absent the possibility of policy, or indeed any human activity, predictably altering the climate in any direction, although this discussion has more than theoretical interest and may aid our adaptive responses, there is no practical impact on the climate, and no reason to spend time and resources on the question. Except for fun, which certainly counts. And Lord knows theoretical math has had many unforeseen practical consequences.
    Actually there is at least one human activity that can predictably alter climate and that’s global nuclear war. But it won’t warm the planet. Anything else?

  18. Just to be picky we might acknowledge that it’s logically impossible to prove a negative, and then go on to acknowledge that the common use of the word impossible is loose enough to merge with the inconceivable. But then, one man’s inconceivable is another man’s possible, so… ah, the hell with it.
    Oops. Forget to mention this: “For example, we are certain that increasing atmospheric carbon dioxide will act to warm the planet.”
    That is certainly not true. Not in the last 550 million years at least, and not during WWII and post-war reconstruction, when all that CO2 produced the alarms about the oncoming Ice Age. And in 1929-1931, human CO2 production declined by 30%, atmospheric CO2 was unchanged, and temps continued to rise to 1942.
    So we may be certain that CO2 may potentially raise global temperature, absent countervailing forces, but not that it will. Because of the exponential decline of the GHG effect of CO2, the next doubling to 800 ppm will increase its GHG effect by 1.4%.

  19. I just realized I misinterpreted. Just because CO2 acts to warm the planet doesn’t mean it necessarily does.
    I take it back. Just because that 1.4% increase in GHG effect at the next doubling to 800ppm is overwhelmed by the other eight forces on climate doesn’t mean it’s not acting.

  20. Why are skeptics still talking about emission/concentration scenarios”

    It has nothing to do with empirical science research, which requires valid data, evidence, testable hypothesis and validation for it to be a success.

    I don’t talk about this anymore as it is a waste of my time.

  21. It’s coming up on ‘earth hour’ here in the Great NorthWet.
    Time to light up the house to the maximum extent possible…. and pour some amber liquids to celebrate reliable, dispatchable, low cost hydro and fossil fuel electrical generation!

  22. What is going on here may be what I ran into a few times on other subjects. Those paying you often want something concrete to act on, not just statistically possible to variable degrees dependent on assumptions, but rather certain to such an actionable degree. It is then easy to float into the role of policy maker, especially tempting for those not resistant to political correctness and even liking it. Too many examples.

  23. “There are some things about climate change that we know for sure. For example, we are certain that increasing atmospheric carbon dioxide will act to warm the planet. “ Not so fast Judith.

    “We” do know some things, but that is not one of those things.

    We know that CO2 will retain incrementally less and less heat as the concentration increases. Retarding cooling is different than warming the planet. Climate sensitivity (or heat retention as function of doubling of CO2 concentration) is a demonstrable log curve which reaches an asymptote after which additional CO2 causes no additional heat retention. The available energy bands become saturated and can absorb no more. Also, additional CO2 added to the atmosphere will be absorbed by the oceans in about 50 to 1 ratio to maintain Henry’s Law, temperature being the same. Also concentration of CO2 in the troposphere will increase concentration of CO2 in the atmospheric layers above the troposphere, which will increase IR emissions into outer space.
    If you put hot coffee in a vacuum thermos bottle, the temperature of the coffee drops sharply at first because the liquid efficiently conducts heat to the inner liner of the thermos. Then the rate of temperature decline slows to a steady slope and continues on that slope until the coffee is the same temperature as the air surrounding the outside of the thermos.
    Now, if instead we use CO2 instead of a vacuum as the insulation, what happens?
    Again the temperature of the coffee drops quickly as above for the same reason, the liquid coffee efficiently conducts heat to the inner liner of the thermos. But now with CO2 instead of vacuum, the gas molecules come in contact with the warm inner wall of the thermos and the CO2 molecules gain energy/increase their activity and pressure. Also, the inner wall is radiating heat as it did with the vacuum but, in addition to radiation from the inner wall in contact with the coffee to the oustide wall in contact with the air, now some of that radiant heat will be absorbed by the CO2 molecules, and more heat will be conducted by CO2 colliding with the innner wall of the thermos. In both cases the CO2 gains energy/increases activity and pressure in the insulating space. And we must add the heat transference by convection of the CO2 gas in the insulating space. The slope of cooling is increased with the CO2 gas as insulator compared to the vacuum insulation. The CO2 is more efficient than the vacuum at transmitting the heat between the two inner walls of the thermos. Heat transmission in the vacuum between the two inner walls is limited to only the radiation between the two walls, because there is no conducting gas, no convection, no collisions. Replacing the vacuum with CO2 increases the rate of temperature loss, i.e. the coffee cools faster.
    Now, if we increase the concentration of the CO2 in the insulation space, what happens? The rate of cooling increases as the CO2 concentration increases but the rate of increase in cooling diminishes incrementally as CO2 concentration increases. Eventually, more CO2 concentration does not further increase the rate of cooling. The molecular physics are the same as CO2 in the atmosphere. The available energy bands in the gas molecule become saturated, and after that the rate of cooling will be constant until equilibrium temperature with the outside is reached. Changing the mix of the gases in the thermos only alters the slope (the rate of cooling), it does not change the amount of heat that will be lost, i.e. the temperature endpoint. The fully energized CO2 molecules will be colliding with the inner walls faster as concentration increases which results in faster conduction of heat between the inner walls as well as emitting IR radiation to the outer inside wall of the thermos, and convection currents are carrying denser gas flows which means more collisions with the outside wall, all of which speed the cooling process.
    Back radiation from the outer wall to the inner wall, or from CO2 to the inner wall, and collisions by CO2 molecules with the inner wall of thermos which is in contact with the coffee cannot increase the temperature of the coffee in the thermos…there is no warming of the coffee, ever. (If we reverse the temperature difference so that the temperature outside the thermos is warmer than the coffee, such as iced coffee in the thermos, in that case the gases in the thermos are retarding the warming of the iced coffee.)
    At any event occurring prior to equilibrium temperature, the CO2 molecules received their maximum energy from the wamer lining of the thermos, resulting in cooling of that lining, and transmission of the heat/energy to the cooler side. In all events, CO2 causes no warming, it only retards cooling.
    The physics is not controlled by the gases or the concentrations of the gases. The physics is controlled by the temperature difference, that is, the temperature of the coffee versus the temperature of the air (or other media) outside the outer wall of the thermos, i.e. the temperature differential, and primarily by additional incoming energy if any. Likewise, the physics of the earth is not mediated by the atmospheric gases or by the relative concentrations of the atmospheric gases. More or less of a particular gas in the atmosphere only affects the rate of cooling or the rate of warming, the gases do not affect the temperature endpoint, on average. (Obviously, the instantaneous temperature at two different, specific spots on the earth is affected by the gases, primarily water vapor, and also clouds and albedo are important. For example, standing in the dry Arizona desert just after sunset versus standing in humid New Orleans just after sunset, there will be a big difference in the rate of cooling at the two places.) More CO2 in the atmosphere will only change the rate of warming or rate of cooling in the troposphere and rate of warming or rate of cooling in stratosphere. Additional heat retained temporarily near the surface will be offset by additional cooling in the upper atmosphere by increased IR emissions into outer space.

    • Bud Bromley accurately stated what is really going on. The bottom line is the radiative greenhouse forcing of CO2 (if any) is so far down in the signal to noise level that it is simply not measurable. And given the fundamental law of conservation of mass and energy, no heat is being produced by the greenhouse gas effect but rather the CO2 may create a delay in cooling. In other words we could increase the atmospheric concentration of CO2 to 1,500 ppm (which is not uncommon for greenhouses producing superb crops,) and the “warming” effect to the Earth would still be unmeasurable. A review of the annual average worldwide temperatures over the last forty years based on the UAH6 data set show a +/- 0.45 C variation. That is remarkable given the fact the Earth’s heater is 93 million miles away and a product of various astrophysical and orbital components. However, one can clearly see the decreases in average temperature resulting from volcanic eruptions generating tremendous ash released into the atmosphere and corresponding increases in temperature from El Nino events. I have produced a chart illustrating such at: https://greatclimatedebate.com/yearly-temperature-variation-and-atmospheric-co2-levels-1979-2018/. In other words the Earth has a remarkable natural thermostat based on positive and negative feedback with the largest greenhouse gas being water vapor (clouds.) and GGE created warmth cannot be measured given natural variations. In terms of Dr. Curry’s motivations, it is probably useful to review the https://www.cfanclimate.net/company website as there are probably opportunities to generate revenues through consulting and the like.

  24. The worst case (and sadly, most likely) scenario is a planetary totalitarian green new deal with billions of humans starving to death and facing a global ecological disaster.

    The dangerous psychopaths (Malthusians, Eugenists, Marxists, Eco-nazis, etc.) that push this holocaustic agenda are the only actual threat of our time and this threat must be addressed.

  25. Re Sara a long way above. I don’t find Judith Curry in the slightest boring but I do have to concentrate.

  26. ‘I’m working on a new paper that explores these issues by integrating climate science with perspectives from the philosophy of science and risk</bold) management.'

    I'm not sure she really is addressing risk management (second paragraph), but is focusing more on probabilistic assessments. I hope she is aware that:
    risk=probability of an accident or disaster happening * consequences of that occurrence.

    This is certainly the definition used within probabilistic studies for the international safety standard IEC 61508 safety of control systems. The hazard can have multiple dimensions: human deaths/injuries, financial losses, status loss, environmental damage, etc.

    A study like this on a global scale is going to be a big challenge.

  27. In the last days of Pompeii we observe a doomsday scenario for those living in and around the volcano who could have been saved if only advanced scientific knowledge could have been available so that all or at least those who heeded the warning could have uprooted and relocated their lives.

    When it comes to our planet until such time as we have colonized space for the “don’t you want to get away crowd” all this doomsday hand wringing is a waste of whatever life we have left. So rather than passing laws to limit fossil fuel, World Governments need to come together and put their energy, fossil fuel and otherwise, and their heads together toward accelerating our space conquest and colonization problem.

    All this Global Warm…aaahh climate change and it is all your fault crap is a bit of a moot point. With all the money in the world governments cannot set the climate. Besides, plague, famine even a meteor extinction event has a higher degree of probability than CO2 Extinction. The end of days is coming and nobody knows when and can only guess at how but my money is on good ol sol in about 4.5 billion more years.

    Take a chill pill all you descendants of the flower children. Take in a concert, get high, feel the grass between your feet, swim naked in lake but most of all live and let live. Your Government cannot save you but maybe they can invest in your relocation.

  28. I don’t know why we avoid the elephant in the room which is the United Nations. They are eager to implement Agenda 21 which can not succeed without controlling earth’s population. Cheap energy is the reason the earth’s population has exploded. Food production and transportation allows the world to be fed. To be successful with Agenda 21, the UN needs to control the world and they will need contributions from the free world to achieve this control. Currently, fascist nations are being enriched by the fossil fuel industry and they are unlikely to submit to the UN. So, the UN must do an end run around the fascist nations. Promote energy independence of the free world to keep capital at home where it can be used for world sustainability.

    If the UN was concerned about humanity they would promote cheap energy for the third world, not expensive green energy. 4 million people die every year from smoke related illnesses.

    To achieve world sustainability, the UN must achieve control of the world. Annual funding of $200 Billion is believed sufficient to sway third world countries to adopt sustainability principles. This money will act as a carrot for the third world to comply with UN demands. The funding will come from the industrialized world, through carbon taxes, from countries with a vested interest in the third world having expensive energy. Currently manufacturing is moving to countries with cheap energy and unregulated environmental controls causing more pollution than where the manufacturing originated. The swayed politicians will benefit the most as they are the most important asset to the UN. Corruption will be king.
    Fossil fuels are by far the greatest factor contributing to the population explosion of earth. Not only can food be grown and brought to market in great quantities, but cheap and abundant fuel allows us to survive winters. Eliminate all fossil fuel tomorrow and earth’s population would plummet. Winter cold and starvation would kill most of us.
    Another roadblock to controlling the world is the river of capital flowing to fascist nations. Fascist Nations like Russia, Saudi Arabia, Iran, Iraqi, Venezuela etc hold the majority of petroleum reserves and are being enriched by the world’s addiction. It’s unlikely these enriched fascist states would submit to the New World Order; run by the UN. Canada is the only free G20 country with sizable petroleum reserves; unfortunately Canada must be sacrificed for the greater good. Doing the math, cheap energy will lead to the demise of the planet. This sounds so dark, but the UN actually wants to cutoff the supply of cheap energy for their goal of controlling the world and its population.

    • Chris Wells, your perception is very insightful and right on the mark. You have correctly identified the principal roadblock slowing down if not inhibiting the UN’s Agenda 21 (30) plan. “Currently, fascist nations are being enriched by the fossil fuel industry and they are unlikely to submit to the UN. So, the UN must do an end run around the fascist nations.”

      The important takeaway from Chris’s post is:
      “Fossil fuels are by far the greatest factor contributing to the population explosion of earth. Not only can food be grown and brought to market in great quantities, but cheap and abundant fuel allows us to survive winters. Eliminate all fossil fuel tomorrow and earth’s population would plummet. ”

      Carbon dioxide as a “greenhouse gas” warming the earth was and still is a diabolical and evil deceptive plot by the “elites” attempting to march us towards a worldwide governing system under the UN with a vastly reduced population (> order of magnitude.) Neither the public or its elected leaders have the ability to judge if the AGW/climate change deception is in fact truth or a deception. See the abstract section of the great climate debate which explains this history for politicians and the people at large: https://greatclimatedebate.com/prosecutorial-abstract/

      Hopefully President Trump’s Presidential Commision on Climate Security will cut through the chaotic confusion and bluff on alarmists, skeptics and scientists and this will lead to the EPA’s declassification of carbon dioxide as a pollutant. Other industrialized national will follow this lead.

      The world is in need of of more inexpensive energy. This can only come from hydrocarbon fuel based energy generation, nuclear, and hydro playing a bit part.

  29. “There are some things about climate change that we know for sure. For example, we are certain that increasing atmospheric carbon dioxide will act to warm the planet.”
    And why, strictly speaking, should we be sure of this? Is there a physical theory that describes the dependence of the gas temperature on the absorbed infrared radiation? On the contrary, we know that approximately 99.96% of dry air is a substance that is transparent to IR radiation. However, these substances, in accordance with their heat capacity, absorb almost all the heat in the atmosphere.
    Has a reliable experiment been carried out confirming the ability of carbon dioxide to absorb excess heat and increase air temperature accordingly? I do not know about such an experiment. But all visitors to this site are aware of the experiment by Anthony Watts, refuting this view.
    In fact, all climate models include a concept of the relationship between CO2 concentration and temperature. In fact, the causal relationship between these quantities is not scientifically proven. Only parallelism between statistically averaged parameters in the considered period of time was found, and the observed deviations from these averaged values indicate unaccounted factors, which themselves may be decisive.
    There is no certainty, there are reasonable doubts.

    • As I pointed out above in my abject apology to Judith Curry, it is certainly true that CO2 will act, but uncertain if if will have any effect.

  30. Thanks for writing this out, Dr. Curry.

    I’m not sold on some of the distinctions, but it’s nice to clearly and concisely see what academics are working with as they attempt some of these quantizations.

  31. Bud Bromley accurately stated what is really going on. The bottom line is the radiative greenhouse forcing of CO2 (if any) is so far down in the signal to noise level that it is simply not measurable. And given the fundamental law of conservation of mass and energy, no heat is being produced by the greenhouse gas effect but rather the CO2 may create a delay in cooling. In other words we could increase the atmospheric concentration of CO2 to 1,500 ppm (which is not uncommon for greenhouses producing superb crops,) and the “warming” effect to the Earth would still be unmeasurable. A review of the annual average worldwide temperatures over the last forty years based on the UAH6 data set show a +/- 0.45 C variation. That is remarkable given the fact the Earth’s heater is 93 million miles away and a product of various astrophysical and orbital components. However, one can clearly see the decreases in average temperature resulting from volcanic eruptions generating tremendous ash released into the atmosphere and corresponding increases in temperature from El Nino events. I have produced a chart illustrating such at: https://greatclimatedebate.com/yearly-temperature-variation-and-atmospheric-co2-levels-1979-2018/. In other words the Earth has a remarkable natural thermostat based on positive and negative feedback with the largest greenhouse gas being water vapor (clouds.) and GGE created warmth cannot be measured given natural variations. In terms of Dr. Curry’s motivations, it is probably useful to review the https://www.cfanclimate.net/company website as there are probably opportunities to generate revenues through consulting and the like.

    • Motivations? You really think motivations matter?
      Deconstruction is a valuable and appropriate tool for the discovery of sources of error, but that it has no place in searching out the sources of truth. That latter belongs to Scientific Method. The rule is that if you’re right, you’re right, no matter where you get your income. And if you’re wrong, you’re wrong, no matter how sweet a guy you are, no matter how good your intentions.
      I fund people whom I agree with (read: who agree with me) and no one thinks worse of them for accepting my money.
      If I’ve misinterpreted your comment, please accept my apology.

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