MIT Scientist: An Obligation to Take on ‘Tail Risk’ vs. Alarmism

IPCC report cover

Respected MIT scientist Kerry Emanuel takes on the sometimes dicey issue of ‘tail risk’ and explains why he thinks it important for the science community to discuss it … notwithstanding inevitable risks of doing so. (Reposted with permission.)

The American Association for the Advancement of Science just published a statement on climate risk on which I am a co-author. This statement has several aims, one of which is to highlight the importance of societal risk in the low-probability tail of the climate change probability distribution. I would like to take this opportunity to explain why we think it necessary to talk about tail risk, and the road blocks we scientists face in doing so.

Tail risk is a concept that everyone is familiar with at some level. To take a rather obvious example, suppose an 8-year-old girl comes to a busy street which she must cross to catch her school bus. Unsure what to do, she asks an adult bystander for advice. The adult replies that, most probably, she will make it across the street unharmed.

Credit: Image by Will Mego via Flickr.

Any other reasonable adult listening to such advice would regard it as radically incomplete. Surely, no one would encourage the girl to cross the street if there were a 1% chance that she would be run over. The most probably outcome is, in this example, largely irrelevant. But here there is very little downside to walking the girl up the street to where there is a traffic light.

In assessing risk, one has to estimate the probability distribution of the event (car colliding with girl), convolve that with an outcome function (girl likely dies if hit), and account for the cost of mitigation (take 5 minutes to walk to a traffic light). In the realm of climate change, climate scientists are the ones charged with estimating the event risk, while other disciplines (e.g. economics, engineering) must be brought to bear on estimating outcome, and the costs of mitigating the risk or adapting to it.

In assessing the event risk component of climate change, we have, I would argue, a strong professional obligation to estimate and portray the entire probability distribution to the best of our ability. This means talking not just about the most probable middle of the distribution, but also the lower probability high-end risk tail, because the outcome function is very high there. For example, here is an estimate of the probability distribution of global mean temperature resulting from a doubling of CO2 relative to its pre-industrial value, made from 100,000 simulations using an integrated assessment model. (We use this here as an illustration; it should not be regarded as the most up-to-date estimate of global temperature increase probabilities.)

Figure from Chris Hope, University of Cambridge.

More or less in agreement with the most recent IPCC report, the most probable “middle” of the distribution runs from about 1.5 degrees C to about 4.5 degrees C, while there is a roughly 5% probability of temperature increases being less than about 1.8 degrees C and more than about 4.6 degrees C. But, given the corresponding distributions of rainfall, storms, sea level rise, etc., the 5% high-end may be so consequential, in terms of outcome, as to be justifiably called catastrophic. It is vitally important that we convey this tail risk as well as the most probable outcomes.

But there are strong cultural biases running against any discussion of this kind of tail risk, at least in the realm of climate science. The legitimate fear that the public will interpret any discussion whatsoever of tail risk as a deliberate attempt to scare people into action, or to achieve some other ulterior or nefarious goal, is enough to make almost all scientists shy away from any talk of tail risk and stick to the safe high ground of the middle of the probability distribution. The accusation of “alarmism” is quite effective in making scientists skittish in conveying tail risk, and talking about the tail of the distribution is a sure recipe to be so labelled.

Predictably, the AAAS statement evoked just such responses. For example, in her climate blog, Judith Curry states that ” …..these particular experts seem more alarmed than the expert authors of the IPCC report (well, the WG1 anyways), citing many very low probability events as something to be alarmed about……When scientists become alarmists, I don’t think it helps public opinion.” And this, from Roger Pielke (Sr): “This AAAS report is an embarrassment to the scientific community”.

Judy Curry is right that the IPCC working group 1 (WG1) almost entirely avoids the issue of tail risk (which is one reason that the AAAS felt compelled to do so), and Drs. Pielke and Curry speak for most scientists in expressing the fear of embarrassment in any discussion of low probability events. After all, by their very definition, such risks are unlikely to be the outcome. If we want to be admired by our descendants, the best strategy is to stick with the peak of the probability distribution, and with high probability, we can then ridicule those “alarmists” who warned of the tail risks, just as the adult who advises the girl to cross the street will, in all likelihood, be able after the fact to chastise the one who counseled against it.

And yet. Does the dictum to tell “the truth, the whole truth, and nothing but the truth” not apply to climate scientists? If we omit discussion of tail risk, are we really telling the whole truth?

So far it has been difficult to quantify tail risk beyond that implied by figures such as the one above, which resulted from running an integrated assessment model many times with many combinations of parameters varied across plausible ranges. We have also tried to use paleoclimate data and the observed response of climate to large volcanic eruptions to narrow down the probability distribution. A wild card in climate risk assessment is the problem of abrupt, irreversible climate change, which evidence in ice cores and deep sea sediments suggests are general features of past climate variations. We also have to be mindful that the graph above and many risk assessment studies use the canonical doubling of CO2 as a benchmark, whereas we are currently on track to triple CO2 content by the end of this century. (As a rough measure of global temperature change under triple CO2, multiply the values on the horizontal axis of the figure by 1.5.) Unless we find a way to extract carbon from the atmosphere, the climate risks would become alarmingly high (and not just in the tails) in the 22nd Century, even if we stopped emissions by the end of this century.

Do we not have a professional obligation to talk about the whole probability distribution, given the tough consequences at the tail of the distribution? I think we do, in spite of the fact that we open ourselves to the accusation of alarmism and thereby risk reducing our credibility. A case could be made that we should keep quiet about tail risk and preserve our credibility as a hedge against the possibility that someday the ability to speak with credibility will be absolutely critical to avoid disaster. What do you, the reader, think?

Dr. Kerry Emanuel is the Cecil and Ida Green professor of atmospheric science at the Massachusetts Institute of Technology, where he has been on the faculty since 1981. Professor Emanuel’s research interests focus on tropical meteorology and climate, with a specialty in hurricane physics. He is the author of two books aimed at a general audience: Divine Wind: The History and Science of Hurricanes, and What We Know about Climate Change.

Reprinted with permission of Climate Change National Forum. Original article published March 31, 2014.

Bookmark the permalink.

11 Responses to MIT Scientist: An Obligation to Take on ‘Tail Risk’ vs. Alarmism

  1. ben oko says:

    I wholeheartedly agree that tail risk needs to be part of the discussion. Discrediting folks as “alarmists” may be convenient for deniers but the whole picture needs to be before the public. And the more scared worriers like myself become the more likely we are to do something.

  2. georgia gillespie says:

    the general public is very accustomed to “most possible” and “least possible”. i live in the snow capital of the U.S. , syracuse, n.y. we get forecasts listing snow depth from “4 in. to 10 in.”…depending… certainly, scientist can do this with no panic from the public. the problem lies with the ratings/money-motivated media; they are the exploiters, creating sensationalism and the resultant fear by stating only “the most”. tail risks should absolutely be presented. and when the expected happens, scientists should not be victims of their own long-standing timidity. they should go on media, strongly clarifying the tails…and change an unfamiliar term to a familiar—-most/least as an example.

  3. Lewis Gannett says:

    How would discussion of tail risk jeopardize credibility? One loses credibility by being wrong. But when will we know that bad, low-probability outcomes won’t happen, proving concern about them wrong? Not for some time; not perhaps for a long time. Why would scientists lose credibility during that period of uncertainty? If they’re “not yet wrong,” who could rationally question their credibility? Conversely, why would scientists *conserve* credibility via *not* discussing bad possibilities during a period of outcome uncertainty? The notion that avoidance of tail-risk discussion now will preserve options for future emergency action, is preposterous and dangerous. It’s yet another absurd excuse for doing little to prepare for what could be the direst trouble humans have ever faced. Are some factions in the climate debate *actually arguing* that we shouldn’t discuss and prepare for bad possibilities until they’re visible on the horizon? Something is seriously wrong with that attitude. Since when did the adage “prepare for the worst” go out of style?

  4. gws says:

    Thank you very much for this article!
    I do strongly agree that the probability distribution should be discussed and the tail end not ignored, and even highlighted as the pathway to be avoided, or call it “insured against”.
    That said, conveying risk as the convolution of the “probability distribution … with an outcome function” (as per definition) to the public is very hard. Most people tend to focus on either the first or the second. An example is the strong focus on the outcome function of nuclear disasters as an argument against the use of nuclear power. One can give an hour long talk explaining risk without being able to convey the simple concept of the convolution.
    So I am thankful for the girl-crossing-the-street metaphor, because only these kinds of metaphors, and I have used similar ones, are useful to convey the concept to a more often than not math-limited general audience.

    Hoping to see more of that.

  5. Lawrence Jones says:

    While Vice President, Cheney famously said “If there’s a one percent chance that Pakistani scientists are helping al-Qaeda build or develop a nuclear weapon, we have to treat it as a certain in terms of our response. It’s not about our analysis. It’s about our response.”

    One can disagree with the particulars, but the one percent doctrine is a fair example of the precautionary principle. It is also the reason homeowners buy fire insurance.

    If there is a five percent chance that today’s excess greenhouse gases can trigger an abrupt climate change, we have to treat it as a certainty in terms of our response. Explaining that is worthy task.

  6. Evan Kuchera says:

    I think your framing of how tail risk should be discussed is both correct and would be well received. But you have to be aware that there are a lot of tail risks in this life (asteroids, communicable diseases, nuclear war, volcanoes, etc) and there is no expertise a climate scientist can provide to sort out the right way to manage the competition for mitigation resources amongst all of them. Most laymen see the timescales on climate change tail risks and take a wait and see attitude. A climate scientist would be fine with that values-based reaction to the scientific data; a climate alarmist would insist that the tail risk in climate change must be addressed now.

    Additionally, tail risk works both ways. If reality ends up in the other tail, with minor and possibly even beneficial changes to the climate, any effort to mitigate could end up being a net negative. Statements like “Waiting to take action will inevitably increase costs, escalate risk, and foreclose options to address the risk.” are not correct. Waiting to take action may be a poor choice in a risk management framework, but just because you are gambling doesn’t mean you won’t win.

  7. John Vonderlin says:

    Wow. One hundred thousand simulations made by an integrated assessment model, why that’s just like reality, isn’t it? Without arguing how poorly models have done so far in predicting the complex, chaotic dynamics of Earth’s climate, I’d rather focus on the moral bankruptcy of your metaphor and especially of accompanying it with pictures of a young girl facing traffic. I’m reminded of a parody magazine cover I once saw with a puppy with a gun to its head with the headline, “Buy this Magazine or We’ll Shoot This Dog.” To sink to your level of argumentation, I’d simply point out that on our local news channel tonight there was a story about a young child being run over while crossing the street, both with the light and in the crosswalk. Perhaps, some know-it-all with his GIGO computer simulations had assured them they would be just fine if they didn’t cross in the middle of the block. Perhaps, with the false security generated by an authority figure’s assertions that they’d be fine if they just did as they were told, they let their guard down. I wonder how that would fit in your fat tail? Uncertainty is a scientific truism that should be proselytized from every street corner. It is something the common man can understand just fine, without child-killing metaphors, whatever you smarmy elitists believe.

  8. RobertInAz says:

    There is a difference between this explanation and:
    ” Earth’s climate is on a path to warm beyond the range of what has been experienced over the past millions of years.” from the AAAS statement.

    Scanning the full document – probabilities were few and far between. Fortunately, it stayed out of the policy realm.

    I disagree with this statement: “But there are strong cultural biases running against any discussion of this kind of tail risk, at least in the realm of climate science. ”

    I believe the cultural bias (or resistance) is the the policy prescriptions that inevitably follow the tail risk description.

  9. Of course scientists must discuss tail risk.

    Worrying about cries of “alarmism” from the denialists would be self censorship. Indeed, the true aim of denialist propaganda is silencing:

    quoted by Jim Hoggan from:

    The stakes could hardly be greater. The risks in the tail are mass extinction. These risks must be confronted regardless of the “formal” probability of their occurrence.

    Elizabeth Kolbert hints, in The Sixth Extinction,
    how falling ocean pH levels could cause a collapse of ocean ecosystems.

    It is not out of the realm of possibility that such a collapse could cause a mass extinction similar to the end-Permian extinction. See When Life Almost Died:

    There is no way to “mitigate” the effects of a dead ocean and the resulting breathable atmosphere. This is the ultimate tail risk: a mass extinction whose severity lies somewhere between the end Cretaceous and end Permian extinctions.

    The Paleocene-Eocene Thermal Maximum
    does not provide a completely comforting proxy for the effects of the present rapid infusion of CO2 into the atmosphere. Indeed, CO2 is entering the atmosphere at a rate 3 or 4 orders of magnitude greater than at any time in the (recent?) past. There is no time for ocean plants and animals to adapt to through evolution.

    But let us say, just for the sake of argument, that the oceans, miraculously, will not collapse as the result of the massive and continuing CO2 pulse. In other words, suppose the effects of increased CO2 is “only” limited to the land. Is this reason for complacency? Surely not. For example, see this terrifying Ted Talk by David Roberts:

    In some ways it is easier for non-scientists such as Elizabeth Kolbert and David Roberts (and myself :-) to discuss the risks in the tail. Scientists are typically reluctant to say *anything* that can not be proven conclusively. That’s understandable, but in this case it’s important for scientists to say as much as they possibly can while still remaining true to their professional codes of conduct.

    Edward K. Ream

    P.S. Since this is my first post here, let me introduce myself. I am a computer engineer: my lifelong project is Leo:

    I have been studying science for several decades. Steven Jay Gould’s books showed my that evolution is the most successful, most revolutionary scientific theory ever created. I have enjoyed reading Science and Nature every week for the last five or six years.

    I am a financial contributor to Peter Sinclair’s Climate Denial Crock of the Week and This is Not Cool. Peter can vouch for my bona fides. I am also a financial contributor to the Dark Snow Project:


    • “There is no way to “mitigate” the effects of a dead ocean and the resulting breathable atmosphere.”

      Oops, I meant to say *unbreathable* atmosphere.


  10. Nancy says:

    What is the scenario for an Earth without the Arctic ice cap? At the rate the ice is melting, it does not appear there is any stopping this scenario from happening in our lifetime. I think we need to talk about how our weather patterns will change and how that will change food production.

    It’s funny how humans think they rule the Earth. They forget that Mother Nature bats last.