Posts Tagged ‘uncertainty’

Climate change as a matter of risk management requires different choices in communication

June 11, 2018

Rowan Sutton just published a very short article (an “idea”) in ESDD entitled “a simple proposal to improve the contribution of IPCC WG1 to the assessment and communication of climate change risks”. From a risk management point of view a focus solely on the most likely outcome is not recommended, especially when the impacts increase sharply towards one end of the scale.


A common measure of risk is likelihood x impact (Fig 1). It is standard practice in risk assessment to highlight both the most likely impacts and low likelihood high impact scenarios. Such scenarios merit specific attention because the associated costs can be extremely high, so decision makers need to know about them. It follows that WGI has a responsibility to assess and communicate explicitly the scientific evidence concerning potential high impact scenarios, even when the likelihood of occurrence is assessed to be small. In past reports the assessment of key parameters by WG1 has focussed overwhelmingly on likely ranges only. When information has been provided about the tails of distributions only likelihoods have been communicated using terms – following the IPCC’s uncertainty guidance (Mastrandrea et al, 2010) – such as “very unlikely” or “extremely unlikely”: a clear steer that policy makers should largely ignore such possibilities. But this is wrong. Policy makers care about risk not likelihood alone. The IPCC’s uncertainty guidance ignores impact and is symmetric with respect to high or low impact scenarios; this is inappropriate for the communication of risk (Fig 1).

Figure 1: A schematic representation of how climate change risk depends on equilibrium climate sensitivity (ECS).

Some will argue that the WGII report is needed to provide information on impacts. For detailed information this is certainly the case, but the general shape of the damage function for a large basket of impacts (Fig 1) is insensitive to such details, and is all that is needed to justify WGI providing a much more thorough assessment of relevant scenarios. Other critics will suggest that for WGI to identify high impact scenarios explicitly would constitute scaremongering; this concern is no doubt one reason why previous WGI reports have focused so much on the likely range. But it is misguided. Policy makers need to know about high impact scenarios and WGI has a responsibility to contribute its considerable expertise to making the appropriate assessments.

A very similar point has been made by Kerry Emanuel in his post “Tail risk vs Alarmism” on CCNF:

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.


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.

Hans Custers schreef een kort Nederlandstalig blog over Sutton’s artikel.


It’s what we know that’s most important

November 11, 2010

If we’re to be out there, we also have to be smart.

Chris Mooney offers some thoughts and advice based on climate scientists’ efforts to increase their engagement with the public and explain -and if necessary defend- the science.

Facts and framing: Both are important

When it comes to science communication, the facts are the baseline from which one absolutely cannot stray; but at the same time, we have to be aware that people respond most strongly to the frame.

Uncertainty and risk

Remember that the political attack is also largely scientific in nature, at least in terms of its framing. It exaggerates uncertainty about particular scientific studies (…) in order to distract from the big picture.

So any scientist walking into this context had better be ready for one obvious trap: Being lured into talking about uncertainty to the detriment of what we actually know.

This is in sharp contrast to what Judith Curry is pushing for: Framing the issues in terms of uncertainty and stressing what we don’t know. I am in firm agreement with Chris Mooney here. Judith’s strategy is a dead end in terms of increasing the public’s knowledge about climate change.

Conversations about uncertainty invoke a frame which in the public mind is easily confused with doubt. Non-scientists have a very different perception of uncertainty than scientists. Framing what we know and don’t know in terms of risk is much more useful in getting the message across, because it leaves less room for misinterpretation (there is less of a gap in how this term is understood, whereas “uncertain” to a layperson means “I don’t know”).

Let me stress that I’m not advocating to downplay the uncertainties. But emphasizing (let alone exaggerating) them is not the road to increase people’s understanding of the issue, where what we do know is much more important to convey (if you goal is to increase the public’s knowledge of the scientific knowledge). It is thereby useful to distinguish the different levels of confidence of the knowledge: Some aspects are virtually certain, whereas others have a wider confidence interval.

This also depends on the level of knowledge of the public. If your audience doesn’t even grasp the basics and has a very twisted view of what is scientifically known, it’s most useful to keep your message simple and focussed on the broad outline of what we do know. As Neven suggested to Judith Curry, who is invited by the Republicans to testify for the US House of Representatives:

Be sure to tell them AGW is real. Start with that and end with that, please.

Herman Daly makes the point very well that the basics of what we know is most important, “at least as to the thrust and direction of policy”. Consider e.g. this quote that I’ve often used since:

To make the point more simply, if you jump out of an airplane you need a crude parachute more than an accurate altimeter.

If politicians or members of the public aren’t even aware that they’re (very, very slowly) jumping out of an airplane (the situation of which can only very, very slowly be changed), then details about the accuracy of the altimeter are of far less importance than telling them they’re about to go mid-air (albeit in slow motion). Don’t tell them which brand of parachute is better though. And only tell them to grab a parachute after you’ve made sure that they value their life in a somewhat similar way as you do. Otherwise you may be accused of advocacy.


In the end, we’re damned if we do and damned if we don’t use scientific lingo and qualifications/weasel words/passive voice that are the norm in scientific discourse. If we do, the message will get lost on or misinterpreted by the public. If we don’t, we’ll be accused of hiding uncertainty/adhering to dogma/being an advocate or whatever accusation happens to be in vogue. I wrote about this catch 22 before.

It will continue to be a balancing act to be both true to the science and make the message palatable and interesting to the audience. But one thing is for sure: Talk the same way as you do to your fellow scientists and your message will fall on deaf ears. Check out Steve Schneiderand Randy Olson for some very useful perspectives on this balancing act.


Mooney again:

You’re going to be accused of being an advocate no matter what you do. (…) Don’t get angry, and don’t get distracted. Remember what it is that the (…) public, and political leaders, need to know about climate research–and tell them that. Tell them twenty times. And then do it again.

Don’t let your anger or frustration shine through in your communication. It doesn’t go over well (except with people who share your anger or frustration, which seems to be a major factor for blog popularity).

Judith Curry on anthropogenic versus natural causes of global warming

September 20, 2010

As most will know by now, Judith Curry has started her own blog, Climate Etc. In a recent post entitled “doubt” she said some things that highly surprised me. Basically, she claims equal evidence for anthropogenic forcing and natural variability being responsible for “most of the observed increase in global average temperatures since the mid-20th century”:

As an example, my personal weights for the Italian flag are:

1. white 40% [uncertainty and unknowns]

2. green 30%, [evidence for anthropogenic forcing]

3. red 30%. [evidence against anthropogenic forcing]

Before posting this, I had an email conversation with Judith Curry about these issues (so as to minimize any misunderstanding on my part). She wrote back (reprinted with permission):

I think you are misunderstanding what the IPCC actually says.  The statement says MOST (>50%) of the warming can be attributed to  anthropogenic, with an confidence of very likely (>90%).    My balance of 50-50 is a hair outside the IPCC range (which could include 51-49), and 1% difference is in the noise here.   Most can imply 51% or 90%.   I will be discussing the issue of attribution at length in a future point.  But my main point in the doubt post is the size of the white area, which is bigger than 5-10% IMO.

So she would translate the IPCC statement in the Italian flag style as anywhere between

white: 10 -1

green: 46 – 89

red: 44 – 10

Her own estimate is not that far off (in terms of the ratio between green and red) from the most conservative IPCC statement (where “most” means “just a bit more than half”), except having a much greater estimate of the uncertainty (which is her main point). So she seems to interpret red and green as portions attributable to anthropogenic and natural forcing, whereas initially I had interpreted them as evidence for the statement at hand versus evidence against. Both makes sense, but both are different. Also, one could claim that the IPCC statement encapsulates the uncertainty in the fairly wide “most of the warming”, i.e. it could still span a wide range, allowing for plenty of uncertainty. Similar as say, the weather in a week’s time is very uncertain, but I can still say that it’s very likely to be in between zero and thirty degrees C (being late september in Holland). These different interpretations make me doubt the usefulness of the Italian flag symbol to aid in clarifying (dis-)agreements.

Judith also claims equal evidence / equal portions of attribution for her “litmus test question”:

Will the climate of the 21st century will be dominated by anthropogenic warming (green) or natural variability (solar, volcanoes, natural internal oscillations)?

which is the question with the greatest policy relevance, IMO.  My scores on this one are

green 25%

white 50%,

red 25%.

This is astounding. I interpret this as claiming equal evidence pointing to natural variability being dominant over the next 90 years as compared to anthropogenic forcing. Or alternatively, an equal portion of 21st century climate change being attributable to human induced warming as to natural variability.

I’ve read quite a few “skeptical” papers that attempt to blame the warming on natural processes, and even if you’d take them as face value (even though in most cases they haven’t stood up to scrutiny), their collective body of evidence is a molehill compared to the mountain of evidence pointing to the dominance of anthropogenic forcing.

In a comment, Judith points to a blog post at Skeptical Science, with graphs of several natural forcings (solar) and cycles (PDO, ENSO) for the past 25-30 years. They exerted a slight cooling effect over this period. In contrast, CO2 exerted a strong warming effect, and indeed, the temperature has gone up. That hardly qualifies as equal amounts of evidence pointing to the warming being natural versus anthropogenic.

For future warming, her pronouncement is even stranger. In all plausible scenario’s, greenhouse gas concentrations will continue to rise for at least another few decades; in a business as usual scenario for many more to come. What natural forcing or variability could plausibly rival this relentlessly rising anthropogenic forcing in magnitude? Is there evidence at all for that being plausible? If so, is that evidence really as large as the evidence showing that greenhouse gas forcing will exceed the likely bounds of natural variability (if it hasn’t already)?

Take a look at how the global average temperature has varied over the past 130 years (yearly averages with 11 year running mean in bold):

An indication of unforced natural variability is given by the year to year variation, amounting to 0.1 to 0.2 deg C on a yearly basis. As the Skeptical Science article shows, natural forcings can not explain the most recent warming, because they don’t exhibit a trend of the needed direction or magnitude.

Now let’s stipulate that it’s all due to longer term natural variation (as opposed to a forcing) of an unspecified kind. What would that mean for the planetary energy balance? If internal variations would have been responsible for most of the planetary warming, the earth would be emitting more energy to outer space than it receives, resulting in a negative radiative imbalance at the top of the atmosphere (or the energy would have to come from other parts of the earth’s system).

Neither is the case. It’s actually opposite: There is a positive radiation imbalance and other reservoirs (e.g. oceanscryosphere) are also gaining more energy.

And then we haven’t even looked into the future yet. I recently posted this graph of two scenario’s that plausibly bracket a business as usual trajectory (red) and a stringent emission reduction trajectory (blue). The measured temperature increase up to now (which according to most scientists is strongly impacted by anthropogenic emissions) is given by the black line.

Judith’s estimates for the 21st century come down to a chance of warming of 0.25 + 0.75/2 = 0.625 versus a chance of cooling of 0.375, assuming natural factors having a 50% chance of either warming or cooling. So she deems the chance of warming to be roughly twice that of cooling, presumably even in a BAU scenario. Too bad that’s too long of a timeframe to place a climate bet on.

Look at how much the red -and even the blue- projections will likely deviate from the recent temperatures. I sure hope that there will be some magical counteracting cooling effect, but I haven’t seen any plausible evidence for such.

Postscript: From our email conversation, I understand her point of view better than before. But unlike the statement about the 20th century warming, which is not as far off from the IPCC as I had initially thought (except for the amount of uncertainty), her statement about the projected 21st century warming is hard for me to square with the mainstream scientific view, for which there is lots of evidence.

Randy Olson on lemmings and leaders

August 20, 2010

Great article by Randy Olson at the Benshi, contemplating two polarized options of dealing with the climate crisis: like lemmings or like leaders.

Lemmings would tend to wait for the problem to become so massive that there’s just no other option than to deal with it, as in ‘we need a catastrophy before we start taking this problem seriously’.


leadership is what ought to be expected of a species of primates whose birth canals have had to widen over the ages to make space for enlarged crania.

He also discusses the following important point, based on the following suggestion he received from a friend:

“Accept arguments on the basis of evidence alone (not on the basis of who presents them).”

to which he counters:

That sounds great and admirable, at least in principle. But it’s not realistic in the complicated world of science-based issues. (…)

This is why we have leaders. It’s called civilization. At some point we put our trust in those with that stuff called “knowledge.” Like the IPCC. It’s not perfect, but it’s our best shot at avoiding the lemmings scenario.

For the complicated world of science-based issues, the lay person needs shortcuts to evaluate the trustworthiness of the information. One of my older posts that I like most deals with exactly that question. For health issues, it’s not much different.

And to plug another, unrelated but also very good article, Stephan Lewandowsky wrote a guest post over at Skeptical Science on short term uncertainty in the weather versus long term certainty in the climate, with the price winning quote:

There is uncertainty [about climate change], but only in the way that there is uncertainty about what happens when you drive into a brick wall at 80 km/h. You might just get away with a few bruises and a concussion, but it is far more likely that you would break a leg or worse.

No one in their right mind would drive into a brick wall because the outcome is “uncertain.”

Reflections on climate discussions in the blogosphere between Keith, Lucia and me: The spectrum of opinions, uncertainty, risk and inertia

June 11, 2010

Keith Kloor has a post up that is an almost literal transcription of a conversation we had between him, myself and Lucia.

It’s a good initiative I find, to attempt to ‘bridge the climate divide’, as his post is titled. It’s an important theme for me as well. I’ve tried to find common ground with others before, e.g. Tom Fuller and Roger Pielke Jr.  Not that I see such a huge divide between Lucia and myself at all, but that’s also what makes such a conversation both possible, useful and enjoyable. A conversation between more extreme or more excitable voices on either side may quickly become an exercise in mudslinging; there has to be some common ground in order to have a conversation.

The crux of what I had to say is this (quoting myself):

So you have a large amount of inertia in the energy system, in the carbon cycle and in the climate system, which means if you start taking actions, it’s decades into the future until they start taking effect.

If you combine that inertia in those different systems, with uncertainty of the precise effect, and with some knowledge that it could go pretty wrong with a business as usual scenario, then you have to take proactive steps, and that’s where the urgency comes from.

In my view, it’s similar to a chainsmoker who gets told by a physician, “hey, you should really be careful, you should stop smoking if you care about your heath.” And the person says, “hey I can still bike to the town and I feel fine and my grandmother lived until she was 96 and died in a car accident.”

You can postpone dealing with smoking until you’re in the intensive care unit. But that’s a little late. That’s the line of argument in which I see the urgency of climate global warming.

I plan to go into these issues in more detail at some point.

Thanks Keith and Lucia!

Lucia’s report is here. She makes the interesting observation that

It seemed that Bart and I may disagree less when on Skype than when posting comments at Roger Pielke Jr.’s blog. That’s an interesting thing in and off itself.

Though perhaps we have slightly different recollections of our (few) discussions at Roger’s. We never were very antagonistic as far I recall, though on the impersonal internet it’s always easier (tempting even?) to be more antagonistic than one is in “real” life (insofar as Skype is real and not internet; ah well, you get the point).

What do we know?

October 26, 2009

– The direction of the (expected) changes is clear

      – Globe is warming

      – It’s due to us

      – It’s bad news

– Carbon is forever; Aerosols are not

– Uncertainty + Inertia = Danger

That is the short version of what scientists know about climate change.

And a normative statement: 

– Science should inform policy measures. We are used to that regarding human health; we should also get used to it regarding climate change.

update: See here for a more elaborate description of the scientific consensus on climate change.

The NIPCC report: don’t be fooled

June 13, 2009

(Nederlandse samenvatting hier)              (For a sneak preview, see the bottom line below)

The new ammunition put forward by “skeptics” seems to be the Heartland InstitutesNIPCC report 2009 (“Climate change reconsidered”). It is made to resemble, at least in format and in name, the IPCC report. According to Dutch “skeptic” (and contributor to the report) Hans Labohm it completely shatters the AGW (anthropogenic global warming) theory (e.g. here, in Dutch). That’s a very bold assertion, which should be backed up by very strong evidence for it to be taken seriously. Let’s take a look at the executive summary…

Second opinion
The preface starts as follows: “Before facing major surgery, wouldn’t you want a second opinion?”
Now that’s funny. I recently described the IPCC process using the same analogy: If you get a second opinion on your health condition, and it confirms what your specialist said in the first place, your trust in the diagnosis probably increases. Now imagine that you collect the interpretations of medical professionals all over the world, and by and large they their conclusions converge to the same broad picture. This happens to be how the IPCC comes to its conclusions.
Their opening statement is actually a strong argument for going with the consensus position on a complex topic. Yet they use it to argue in the opposite direction; very peculiar.

It continues: “When a nation faces an important decision that risks its economic future, or perhaps the fate of the ecology, it should do the same.” (i.e. getting a second opinion)
Huh? Risking our economic future? If they’re talking about the costs of emission reduction, they are seriously exaggerating. Who is being alarmist here? There will be winners and losers, yes, but that’s something entirely different. Everybody has a choice to join the winners or the losers. Different from the horse races, it’s easy this time to predict who (in the long run) will be the winners and who will be the losers. Take your pick.

The usual stuff
The previous NIPCC report has already been commented on by RealClimate, and it doesn’t seem like there’s much news under the sun this time. The same old and tired arguments feature in the current report. The RealClimate article has many links that debunk the various talking points, and I’m not going to repeat all of them here. A presentation from the lead author, Fred Singer, has been briefly discussed at RealClimate as well. It’s a good example of yet another groundhog day. For those who have followed the staged ‘climate debate’, the list of authors is revealing: Many of the usual suspects, with a history so to speak.

There are the usual, to be expected arguments, like that it’s all the sun’s fault. And logical fallacies, like ‘the climate changed before without human activity being involved, so therefore it must be natural now as well’. Try that line of argument in a court of law against a pyromaniac, by saying that forest fires have always happened naturally. It won’t fly, and it reveals that this report is not about science. The good thing is, with such erroneous lines of reasoning, no specialized knowledge is needed to see that.

Degrees of uncertainty
What I didn’t expect, however, was to see otherwise interesting research be put in a context as if it somehow “falsifies the AGW theory”. In many cases, it hardly has any relevance to the attribution of current climate change, or to future projections.

Ironically, their main argument against climate modeling is its associated uncertainty (mistaking it for knowing nothing, and ignoring that uncertainty goes both ways). That doesn’t stop them from putting forward hypothetical feedbacks that have no evidence whatsoever of operating on a globally significant scale. By the way, climate modeling is mocked in the report as merely being “the opinions of scientists transformed by mathematics and obscured by complex writing”. Doesn’t sound like they know what a climate model really is.

The report goes on to describe many hypothetical feedbacks in the climate system. Of course, they are all negative: They counteract the initial warming, independent of the cause for the warming. Their combined effect, is the hope, should be evidence that the climate sensitivity is an order of magnitude (!) smaller than the commonly accepted range (between 1.5 and 4.5 degrees C for a doubling of CO2). Not just 50%, no, a factor of 10, I kid you not. My alarm bells go off. Let’s see what the implications of such low climate sensitivity would be. Any climate forcing (whether natural or human induced) would be so strongly damped as to hardly have any effect on global temperatures. But then how come the globe is warming, and has warmed and cooled in the past? A logical consequence of their theory (negligible climate sensitivity) is that it’s hardly possible for the earth’s climate to change. Indeed, there is no physics-based climate model that can satisfactorily model both the current and past climates with such low climate sensitivity.

Many of the proposed feedbacks involve the cooling effects of aerosols. They suggest that these cooling effects are larger than reported by the IPCC. That is contradicted by climate models providing a very decent match to the observed cooling following a major volcanic eruption (emitting sulfate aerosol in the stratosphere). Moreover, some have argued that a strong aerosol radiative forcing means that the climate sensitivity has to be large in order to still be able to explain the temperature trend of the last 100 years, so they seem to be shooting in their own foot.

They come up with all kinds of hypothetical feedback mechanisms involving more natural aerosol emissions in response to global warming: Dimethylsulfide from marine phytoplankton (although a very intriguing possibility, this has never been confirmed to be a significant feedback mechanism, and there is ample evidence to the contrary, which is omitted from the report), biological aerosols (idem), carbonyl sulfide (idem), nitrous oxide (idem), and iodocompounds (idem), about which they write the following:
“Iodocompounds—created by marine algae— function as cloud condensation nuclei, which help create new clouds that reflect more incoming solar radiation back to space and thereby cool the planet.”
Nou breekt mijn klomp (“Now my clogg breaks”), as I would say in Dutch. This route to atmospheric particle formation may be important at coastal sites with exposed seaweed, but its global importance is questionable to say the very least; at present it could best be considered an interesting thought experiment. Moreover, freshly nucleated particles have to grow by about a factor of 100,000 in mass before they start affecting climate, and a lot can happen to them before they reach the necessary size.

All very interesting research topics, but to claim that they are somehow evidence for negligible climate sensitivity is an extreme example of over-interpretation. In these active areas of research, where no firm conclusions have been reached yet on global significance, they selectively cite only those articles that they can somehow spin to support their desired conclusion. I feel that I’ve read enough of this report to know what it’s worth.

Bottom line
This report exhumes a very strong and unfounded faith in negative feedbacks from nature, which are hypothetical with sometimes sketchy, often contradictory, and sometimes no evidence of actually operating at a globally significant scale. This highlights an inconsistent view of uncertainty, and an unwillingness to weigh the evidence: “If it causes cooling, the uncertainty (or lack of evidence) doesn’t matter; if it causes warming, it’s too uncertain (and no evidence strong enough) to matter”.

How would you know?
Let’s apply some of my own recommendations for non-specialists on judging sources:
– The report clearly misses the forest for the trees.
– It gives a hidden argument for going with the consensus (“second opinion”), but somehow twists that around.
– It’s characterization of the IPCC process has the smell of a conspiracy to it and is full of strawmen arguments.
– To their credit (and my surprise), I couldn’t find any obvious confusion of timescales, such as confusing weather and climate.
– It contains some embarrassing mistakes in basic logic.
– The two way cause-effect relationship between temperature and CO2 is not properly recognized.
– Their strong claim of shaking the foundations of climate science is extremely unlikely; They don’t provide compelling evidence for such an extraordinary claim; They vastly overestimate the likelihood of cooling effects (feedbacks), and underestimate, deny or ignore warming effects.
– They grossly exaggerate the economic risks of emission reduction, and downplay the risk of unmitigated climate change.
– Some of the authors have historical credentials in a relevant discipline, more than a few have not. The list of signatories at the end is very thin on relevant expertise.
– The Heartland Institute is a conservative think-tank and not a reliable source of scientific information.

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