Reaching No Consensus

November 3, 2009

The New York Times wrote today of a recent study in the Proceedings of the National Academy of the Sciences [original study here] on how the Mt. Kilimanjaro ice cap has declined 26% since 2000. The author of the NYT article, Sindya N. Banhoo, goes on to say the following in the second paragraph:

Yet the authors of the study, to be published Tuesday in the Proceedings of the National Academy of Sciences, reached no consensus on whether the melting could be attributed mainly to humanity’s role in warming the global climate.

The pretty green is my contribution. Even after prettying it up, I think the language remains deceptive and fallacious. To that, then, I dedicate this post.

I don’t usually go to the source on news articles, but for some reason I felt the urge to do so in this case. One thing the NYT article doesn’t emphasize — it is mentioned later in the piece, but it isn’t the focus of the story — is in the abstract of the NAS study. To quote the authors:

The Northern Ice Field has persisted at least 11,700 years and survived a widespread drought 4,200 years ago that lasted 300 years. We present additional evidence that the combination of processes driving the current shrinking and thinning of Kilimanjaro’s ice fields is unique within an 11,700-year perspective. If current climatological conditions are sustained, the ice fields atop Kilimanjaro and on its flanks will likely disappear within several decades.

So first, it is important to note that the original NAS study is limited in scope to a discussion of the Mt. Kilimanjaro ice cap. What I think got me to look at the source was the implication by the NY Times that the authors of the study “reached no consensus” on whether the melting could be attributed to humanity’s role in global warming. It would strike me as outrageous if the authors of a narrow study on Mt. Kilimanjaro ice caps did reach such a consensus, so I had to see for myself.

Here’s what the original piece has to say on anthropogenic forcing…

The authors write:

The Quelccaya ice cap (Peru) has been monitored for more than three decades (12) and is rapidly retreating along its margins. However, the net annual mass accumulation on the summit, derived from the 2003 core and annual pit sampling, has not declined, suggesting other mechanisms are more important. Most obvious would be warmer air temperatures, which would result from the vertical amplification predicted by models that include anthropogenic forcing and are observed in the corrected vertical temperature profiles (22).

As I say often, I’m no scientist, so I don’t want to comment on the content of the article. I don’t even really want to comment on common journalistic practice. (Maybe I’ll throw this in the direction of Tom Yulsman or Keith Kloor to see what they have to say. Being exceedingly charitable to Banhoo, the information that “no consensus” was reached comes from a discussion between Banhoo and the authors of the study, not from the study itself. UPDATE: Yulsman responds, below, and then more at his own blog.) I will even confess that I easily could be wrong in interpreting the NAS article. Better to throw all those confusions in the garbage and start by supposing that the authors had written in their original report that they could “reach no consensus.”

Which brings me around to where I’d like to be. I’d like to discuss the ambiguity of implication and the appeal to ignorance.

I do agree that the question of anthropogenic global warming is a natural connection that readers might draw when learning about the content of the study. Indeed, it’s a question that the authors of the study loosely nod at in their discussion section. Is the melting ice cap a result of anthropogenic climate change? Inquiring minds want to know! So it is important for the author of the popular article — Banhoo, in this case — to address that question, and that question specifically.

But the choice of language here is unfortunate, and it is extremely difficult to parse. Nevertheless, it seems to be a common confusion among some crowds, so perhaps it needs closer treatment.

For starters, it gives the impression that the authors sought consensus, but could not reach it. Perhaps they did. We are given no information about what might have thwarted that consensus. Might it have been that there was not enough information? Might it have been that they were engaged in a tangled lover’s triangle and so couldn’t focus on the question at hand? Might it have been that several hundred mischievous kitty cats  had escaped from their cages and dumped alum, Sylvester-style, in their orange juice? There’s that problem of implication here. The researchers fought valiantly and hard. Alas, a conclusion about the effects of anthropogenic warming was not to be had.

But, second, consider the claim on its face. It’s not false. It is a true claim. The statement “The authors of the study reached no consensus on X” is true, where X equals “whether the melting could be attributed mainly to humanity’s role in warming the global climate.

Here are a few other things that the authors of the NAS article did not reach consensus on. The scientists reached no consensus on X, where X equals:

  • whether my son is being cranky with his teachers at this very moment.
  • whether I liked or disliked the beer that I drank last night.
  • whether it will snow in Madagascar on Tuesday.
  • whether it was 45 degrees in Boulder yesterday.
  • whether the severity of H1N1 can be reduced by eating a healthy breakfast.
  • whether blame for the Yankees’ loss in Game 4 of the 2009 World Series should be laid squarely at the feet of AJ Burnett.

Hard to believe, but all of those statements are also true. The researchers could reach no consensus on any of those claims. Identifying points upon which the committee could not, or did not, reach consensus, is an open-ended way of approaching an enormous range of lingering questions. Indeed, I daresay that that range is infinite, even for true statements. (Don’t get me started on ‘could’.)

Third, some of the clauses X after the bullets are true, some are false, and some are arguable. Taken as a whole, however, all of the above claims are true. That’s kind of confusing, because when you say “The researchers could reach no consensus on X,” there is an implication that clause X is in doubt. Consider the layers: (a) the full claim is true, (b) the claim signals doubt, suggesting falsity of clause X, and (c) the truth or falsity of clause X is independent of whether a small group of researchers could reach consensus on it. There are three layers of confusion, so you could have a claim that is true, where the clause X is true, and nevertheless, the overall suggestion is that clause X is false. It appears from the article that the researchers have doubts either about the case for humanity’s role in warming the global climate or, more narrowly, about the case for humanity’s role in warming the global climate and melting the ice caps of Kilimanjaro. (Just as you shouldn’t get me started on ‘could’, don’t get me started on ‘consensus’.)

Which raises the fourth more important point: the implication that a given claim X is not agreed to by a select body of researchers is not evidence for or against that claim, even though it may appear to be. This is the Argumentum ad Ignorantiam fallacy, masked by the lack of consensus and the “whether” option.

So what gives? Why the mention that the researchers had reached no consensus “on whether the melting could be attributed mainly to humanity’s role in warming the global climate”? Maybe just to address the pressing question. But if so, that’s a pretty poor way of making such a point.

It seems to me that the appropriate position, therefore, is to address the question by leaving it out: either by saying that the issue of global warming was “outside the scope of the study,” or simply to emphasize what the study did reach consensus on.

UPDATE: Yulsman responds, below, and then more at his own blog.


  1. The story leaves much to be desired from a science journalism perspective, perhaps because the author is a freelance health reporter, not a science journalist with experience on this subject. (She is on a reporting fellowship at the N.Y. Times science desk. See: http://sindya.typepad.com/test_kitchen/about.html)

    Revkin should have written this piece, or at least vetted it, but I guess he couldn’t because as I understand it, he’s on his way to Instanbul . . .

    For another approach to this story, see how The Independent in the U.K. handled it: http://www.independent.co.uk/environment/climate-change/climate-change-will-melt-snows-of-kilimanjaro-within-20-years-1813631.html It has some holes too, but at least it doesn’t get hung up on this ridiculous “no consensus” thing.

    The story misses the mark in several ways:

    First, let’s observe that this paper was published in the Proceedings of the National Academy of Sciences. That should indicate that this isn’t a minor finding about some narrowly focused issue. Publication in PNAS suggests that this paper is the culmination of an enormous amount of scientific effort, and represents a very important contribution to this research topic. And that should justify some in-depth treatment.

    Second, let us observe that the overarching subject of this important paper in one of the nation’s most prestigious science journals is one of the most epochal environmental issues humanity has ever confronted.

    For a reporter, these factors should suggest that reading the paper and referring to it specifically in the story IS REALLY GODDAMNED IMPORTANT.

    But unfortunately, for my third bullet point I must observe that the story references almost nothing specific from this voluminous, very important paper on an epochal issue in one of the nation’s most prestigious science journals. My guess is that if the author read the paper, there was much in it she didn’t understand. For whatever reason, she fell back on a simple-minded dueling experts approach in which you interview one or two study authors, and then a scientist who disagrees with the conclusions. Wham bam, you’re done. No analysis, no time and effort invested to interpret what the scientists are actually saying in the paper and why it’s significant. (Why, for goodness sakes, should we bloody well care about the Kilimanjaro ice cap? The author says nothing about how loss of glacial runoff will affect local people who depend on it for drinking water.)

    Fourth, the author fails to give us any context at all, let alone the very important one that Lonnie Thompson (the paper’s lead author) and Georg Kaser (the skeptic quoted in the story) have had a long-running disagreement about the cause of ice loss on Kilimanjaro. Here are the two competing hypotheses:

    * Local changes in land use, precipitation, cloudiness and humidity are largely to blame (championed by Kaser)
    * Global change driven by humans play at least an important role (championed by Thompson)

    One other piece of context: The issue has become politicized because Gore used Kilimanjaro’s melting ice cap as a kind of poster child for global warming. But the idea that humans are changing the climate does not depend one iota on what is the ultimate cause of the disappearing ice cap on Kilimanjaro.

    And this is where the breathtaking naivete and inadequacy of the Times story really hurts.

    Let’s have a look:

    The lede (first paragraph) of the story tells us the news — that Kilimanjaro’s ice cap has declined 26 percent since 2000. And then the nutgraf (the second paragraph), which is supposed to tell us the significance, proceeds to state something that is at best grossly misleading: the authors of the study “reached no consensus on whether the melting could be attributed mainly to humanity’s role in warming the global climate.”

    Ben, you picked up on the word “consensus,” and approached this paragraph from the point of view of logic. I picked up on the weasel word “mainly,” along with what the author does not say; I’ll analyze the results from a science journalist’s perspective.

    Here is some key language from the discussion section of the paper:

    “Attributing the ice fields’ shrinkage to specific drivers is hampered by the scarcity of ground-based meteorological observations in this region of East Africa, while satellite-borne observations span only three to four decades.”

    Ok, caveat emptor, I get it. We’re hampered by insufficient data.

    Then this:

    ” . . . widespread glacier mass loss, shrinkage, and retreat at high elevations (5,000 m above sea level) in lower latitudes (30° N to 30° S), particularly in the thermally homogeneous tropics, suggests the likelihood of an underlying common driver on which more localized factors such as changes in land use, precipitation, cloudiness, and humidity are superimposed.”

    Translation: Glaciers are melting all over the damn place, including the tropics, suggesting that what’s at work on Kilimanjaro is not merely a local phenomenon — not some result of a change in weather due to deforestation at the base of the mountain, but something common to all these different regions.

    Hmmmm… What could that possibly be?

    “Evidence presented here documents that Kilimanjaro’s remaining summit glaciers are rapidly thinning and laterally shrinking and that the slope glaciers are responding very similarly. Ice cores collected in 2000 provide several lines of evidence suggesting that drier and less cloudy conditions are unlikely to be sufficient to account for the observed ice loss . . .”

    Translation: Local climatic shifts toward drier and less cloudy conditions by themselves can’t explain the observed shrinking of summit and slope glaciers.

    So are they saying that something else must be playing a role? (Doh!)

    “The Quelccaya ice cap (Peru) . . . is rapidly retreating along its margins. However, the net annual mass accumulation on the summit . . . has not declined, suggesting other mechanisms are more important. Most obvious would be warmer air temperatures, which would result from the vertical amplification predicted by models that include anthropogenic forcing and are observed in the corrected vertical temperature profiles.”

    Why can’t these bloody scientists write in English!

    Seriously, it sounds like it’s still snowing like the dickens on the ice cap atop Quelccaya, a really, really tall mountain in Peru. And that means the Quelccaya ice cap can’t be declining mainly because of a drop in precipitation. The other possibility is, uhm, I know! Warming! And wouldn’t you know that this has been predicted by models that include the human impact on climate.

    So warming seems to be the cause of shrinkage on an ice cap similar to Kilimanjaro’s. And local climatic trends can’t explain the trends.

    But we just don’t know. (Anything.) And there’s no consensus. (None.)


    ” . . . the climatological conditions currently driving the loss of Kilimanjaro’s ice fields are clearly unique within an 11,700-year perspective. These observations suggest that warmer near-surface conditions observed in the region, coupled with observed vertical amplification of temperature in lower latitudes (23–25), are playing an important role.”

    What to put in a story? Maybe something like the following (with some dot-connection requiring interview confirmation), along with a crucial sentence or two on the broader context I mentioned earlier:

    The melting of the ice cap could be a result of a combination of local factors (such as changes in land use, precipitation, cloudiness and humidity); and the global factor of human-caused climate change. The authors say a lack of data about the local factors hampers scientists’ ability to determine their relative contribution, but the available evidence suggests that local factors alone cannot explain why Kilimanjaro’s ice cap is disappearing.

    Glaciers all over the world — including glaciers elsewhere in the tropics — are shrinking. In Peru, for example, warmer temperatures seem to be implicated in the shrinkage of an ice cap similar to Kilimanjaro’s atop Quelccaya. This widespread melting of glaciers and ice caps “suggests the likelihood of an underlying common driver,” the authors of the study wrote.

    What might that common driver be? The authors note that human-caused warming of the climate is expected to be amplified at elevations like those reached by Kilimanjaro. And they conclude by suggesting that warming of this nature is playing an “important role” in the ongoing shrinkage of Kilimanjaro’s ice cap.

    As a reporter on this story, I would have asked Lonnie Thompson, one of the study’s lead authors, whether I had an accurate view of the paper’s findings. Then I would have asked him some targeted but open-ended questions designed to elicit comment on these points. I also would have interviewed Kaser, just as the Times author did. But I would have avoided a simple-minded dueling experts approach (one says one thing, the other the opposite, and that’s it) by pressing Kaser to explain why he doesn’t buy the analysis by Thompson et al. (The bubble thing Kaser talked about in the Times story just doesn’t cut it.) And I would have explained this scientific dispute, and how it has been politicized.

    Lastly, about that weasel word “mainly.” It is a huge red flag that should have gotten an editor’s attention. Maybe it’s technically true that the researchers “reached no consensus on whether the melting could be attributed MAINLY to humanity’s role in warming the climate. But damn it, the authors did suggest that humanity’s role in warming the climate was playing an “IMPORTANT ROLE.”

  2. Is there a link to the paper? I couldn’t find it on the PNAS site. Thanks.

  3. Krikey. Apologies. I’ll include a link to the paper above, but for the time being, here it is:


  4. Correction: For reasons that escape me, I originally and mistakenly claimed that this was a Washington Post article. It’s not. It’s in the NY Times. I’ve gone through and corrected references.

  5. Looks to Eli as if and pretty well covers the situation as in local and global

  6. In response to a Review Comment on the AR4 First Draft, IPCC withdrew the claim (relying on Lonnie Thompson) that the Kilimanjaro ice cap was 11700 years old.

    The reviewer stated:

    Thompson’s dating of Kilimanjaro is very precarious. The assumed accumulation is implausibly low – it’s only 50 m thick (as compared to 160 m at Quelccaya), but is dated to 11700 BP versus start of AD440 at Quelccaya.

    The IPCC section author replied as follows (and later versions did not cite Thompson’s date):

    Noted, I know this point concerning the dating of Kili – we have to decide together shall we keep this reference or not – we cannot discuss the dating problem within the Holocene glacier box

  7. “Withdrawal of a claim” makes that sound ever so much worse.

  8. “Withdrew the claim” suffers similar logical confusions with “No consensus was reached.” I guess if the claim was withdrawn, the ice cap isn’t 11700 years old.

  9. Stephen: I have no idea whether the ice cap on Kilimanjaro is disappearing because of anthropogenic climate change. As far as I can tell, we have continuing scientific dispute here. And as I wrote above, the answer makes no difference to the case for human-caused global warming. My response to Ben’s original post was intended to point out what was wrong with the New York Times story, and how it could have been reported and written better.

    As for the IPCC AR4 discussions you mention, they happened three years ago. This PNAS paper was published two days ago. In it, Thompson et al make their best case. So it will be interesting to see how AR5 handles the issues.

    Some questions for you:

    * How well do you think the New York Times handled the story?

    * In your opinion, what relevance does Kilimanjaro’s shrinking ice cap have to the overall global warming attribution question?

    * To what do you attribute widespread melting of Earth’s cryosphere, including continuing declines in Arctic sea ice (see http://www.cejournal.net/?p=2204), recently documented declines in Antarctic and Greenland ice sheets (see http://www.nature.com/nature/journal/vaop/ncurrent/full/nature08471.html), and ongoing retreat of glaciers around the world as referenced in the PNAS paper?

    * Do you believe science journalists are giving too much emphasis to the peer-reviewed science that overwhelmingly attributes these changes to anthropogenic influence? And if so, what justification do you offer for a different balance in our stories?

  10. The ice cap is not melting because the temperature is below freezing. It is wasting from lack of supply from snowfall which is a totally different phenomenon. Basing a media release on a single scientific paper is a dangerous procedure because we have a bent science which relies on publish or perish and grant begging from a warmist bias.

    • ?

    • Huh? Even though Thompson states that he found evidence of melting and refreezing in the top 65cm of his cores, and no evidence of that process at work anywhere else in the recovered history of the ice sheet?

  11. In case anyone’s interested, I’ve combined the PNAS article, the New York Times story, and a piece in Time, into an “assignment” for the readers of my blog, CEJournal, as well as the students in my graduate level Science Writing class at the University of Colorado. I’m asking everyone to read the materials and engage in a discussion about them over at CEJournal. So have a look: http://www.cejournal.net/?p=2223

  12. This somewhat larger-picture paper (also just published in PNAS) seems to have gotten no publicity at all despite its obviously greater significance:

    Recent changes in a remote Arctic lake are unique within the past 200,000 years

    Abstract: The Arctic is currently undergoing dramatic environmental transformations, but it remains largely unknown how these changes compare with long-term natural variability. Here we present a lake sediment sequence from the Canadian Arctic that records warm periods of the past 200,000 years, including the 20th century. This record provides a perspective on recent changes in the Arctic and predates by approximately 80,000 years the oldest stratigraphically intact ice core recovered from the Greenland Ice Sheet. The early Holocene and the warmest part of the Last Interglacial (Marine Isotope Stage or MIS 5e) were the only periods of the past 200,000 years with summer temperatures comparable to or exceeding today’s at this site. Paleoecological and geochemical data indicate that the past three interglacial periods were characterized by similar trajectories in temperature, lake biology, and lakewater pH, all of which tracked orbitally-driven solar insolation. In recent decades, however, the study site has deviated from this recurring natural pattern and has entered an environmental regime that is unique within the past 200 millennia.

  13. Thanks for this tip Steve. I hadn’t seen that paper. I’ll try to post something on it at CEJournal.

  14. Very interesting article, Steve. Odd that it wasn’t picked up. What’s most interesting is that researchers reached no consensus on whether the arctic was melting due to overindulgence in Halloween candy. For some reason, that also wasn’t picked up.

  15. It’s always been my policy to not leave candy laying around, Ben. Just ask my dentist. 🙂

    I think one obvious reason it wasn’t picked up is because its results are so very expected. Of much more concern to me is the near-ignoring by the media of the substantial body of research that’s developed in the last couple of years finding major ice sheet loss associated with the CO2 range we’re now entering. You’d think that these results combined with recent observations that 350 ppm was enough to send the GIS and WAIS onto a steady melt path would be cause for considerable alrem, if not a blind panic. The risk level of what we’re doing is extreme, and hardly anyone knows it.

  16. Ben Hale
    “I do agree that the question of anthropogenic global warming is a natural connection that readers might draw when learning about the content of the study. Indeed, it’s a question that the authors of the study loosely nod at in their discussion section. Is the melting ice cap a result of anthropogenic climate change? Inquiring minds want to know!”

    Here’s an excellent article which canvases this question (you will also find this link in the comments of the link to the Yusman blog in your post above). You need to read the whole article to get the full nuisance.

    Main take home points: –

    1. The shrinkage has been taking place since at least as far back as 1880. By 1953 for example the ice area had shrunk by 66% of what it was in 1880.

    2. If I can quote directly, “The observations described above point to a combination of factors other than warming air—chiefly a drying of the surrounding air that reduced accumulation and increased ablation—as responsible for the decline of the ice on Kilimanjaro since the first observations in the 1880s. The mass balance is dominated by sublimation, which requires much more energy per unit mass than melting; this energy is supplied by solar radiation.
    These processes are fairly insensitive to temperature and hence to global warming”.
    3.”An additional clue about the pacing of ice loss comes from the water levels in nearby Lake Victoria. Long-term records and proxy evidence of lake levels indicate a substantial decline in regional precipitation at the end of the 19th century after some considerably wetter decades. Overall, the historical records available suggest that the large ice cap described by Victorian-era explorers was more likely the product of an unusually wet period than of cooler global temperatures”.

  17. But that raises the question of the extent to which the regional change in precipitation may have anthropogenic causation beyond land use change.

  18. In the end, what difference does it really make whether Kilimanjaro’s ice cap is:

    * melting in large measure due to warmer temperatures?

    * melting because of changes in cloudiness, humidity, etc. brought on by land-use change?

    * melting because of changes in cloudiness, humidity, etc. brought on by anthropogenic climate change?

    * melting because of some combination of all of the above?

    Does the solution to this mystery have any bearing on the question of whether humans are causing extensive melting of the cryosphere?

  19. Not in the slightest, as Lonnie Thompson pointed out.

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