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The debate is just beginning — on the Cretaceous!

Filed under: — raypierre @ 23 January 2008

Most of us who are involved in research related to climate change have been asked at one time or another to participate in public debates against skeptics of one sort or another. Some of us have even been cajoled into accepting. In the pre-YouTube days, I did one against the then-head of the American Petroleum institute at the U. of Chicago law school. Gavin did an infamous one against Crichton and company. People are always demanding that Al Gore debate somebody or other. Both Dave Archer and I have been asked to debate Dennis Avery (of “Unstoppable Global Warming” fame) on TV or radio more than once — and declined. It’s a no win situation. If you accept you give the appearance that these skeptics have something to say that’s actually worth debating about — and give their bogus ideas more publicity. If you decline there are all sorts of squawks that “X won’t debate!” or implications that scientists have declared “the debate” (whatever that is supposed to mean) prematurely closed when in fact it is “just beginning.”

Scientists tend to react badly to demands like this in part because the word “debate” is a rather poor description of the way disagreements get hashed out in science. John Ziman has a good discussion of the extent to which scientific questions are ‘debatable’ here (pdf). In a lawyerly debate, it is fair game for each side to pick and choose whatever argument has the most persuasive force with the audience, jury or judge, without any obligation to consider the force of counter-arguments except insofar as they affect one’s defense against the opponent. Science, in contrast, is a deliberative, cooperative, yet still competitive enterprise, where each side is duty bound to fairly consider all arguments and data that bear on the matter at hand. This is not to say that scientific disputes are necessarily dispassionate or orderly. Indeed, I’ve seen near-fistfights break out over things like the Snowball Earth and the interpretation of Neoproterozoic carbon isotope excursions.

The repeated challenges to debate are probably meant to imply that scientists — and their supporters, including Al Gore — are fixed in their ideas, unreceptive to the new and challenging, and unwilling to defend their ideas in public. This picture is hard to square with how scientists actually behave among themselves. It is not that scientists don’t debate, dispute, disagree about matters related to climate. All those things happen, but not on the subjects that skeptics like Inhofe or Fred Singer or Dennis Avery would like to debate (like whether global warming is mainly caused by CO2 or solar variability, or whether the IPCC warming forecasts represent a credible threat.). Those sorts of things are indeed considered settled science by serious climate scientists. Then, too, scientists are justifiably wary of being drawn into staged debates on such diffuse, ill-defined and largely meaningless topics as whether global warming counts as a “crisis.” In the war of the sound bites, the people who feel free to lie and distort can always win. David Mamet made this point eloquently in Bambi vs. Godzilla. A debate like that is not any kind of debate in the sense understood by scientists.

In fact scientists are probing theories and conceptions all the time, trying to break them. The best way to become famous is to overturn established wisdom, so scientists look hard all the time for opportunities to do this. The problem of Hothouse climate states like the Cretaceous and Eocene is a case in point.

The Cretaceous is the time period from 145 million years ago up to the demise of the dinosaurs about 65 million years ago. The Eocene is a more recent period, from 56 million years ago to 34 million years ago. In between is the Paleocene, which is generally somewhat cooler than the late Cretaceous or mid Eocene. It has long been known that the polar climate — particularly the Arctic climate — was very different from today’s. Many lines of evidence indicate temperatures well above freezing, with little or no permanent land ice and infrequent or absent sea ice. Lemurs could live in Spitzbergen, and crocodiles on Hudson Bay, to name a few examples. Most evidence also points to an absence of ice in Antarctica as well. These Hothouse (or Super Greenhouse) climates have much warmer polar regions than is the case for today’s climate, and winters were evidently very mild. These hothouse climates are idealized as having been almost completely free of significant ice sheets on land and sea ice cover in the ocean. Hothouse climates pose a challenge to our understanding of climate in general, but more particularly they serve as a critical clue as to what surprises a high-CO2 world might have in store for us.

This is so because, at present, the only viable theory for Hothouse climates is that they come about as a result of elevated CO2 concentrations, which in turn are due to long term changes in the Earth’s carbon cycle. The CO2 theory has many problems, some of which I’ll discuss below, but no theory without elevated CO2 has been able to even come close to accounting for the Hothouse states. These climates would be just dandy as a natural test of the Earth’s sensitivity to long lived greenhouse gas concentrations were it not for one nasty fact: it is very, very difficult to get an accurate idea of how high the CO2 concentrations were so far back in time (see Crowley and Berner or Broadly Misleading on RC). For example, estimates for the Eocene range from values similar to modern CO2 concentrations all the way up to 15 times pre-industrial CO2. This unpleasantly large range represents uncertainties in the proxies used to estimate CO2 in the distant past. Various general circulation models can achieve largely ice-free polar conditions with CO2 between 4 and 8 times present concentrations, though even at those levels there are difficulties in accounting for the mildness of the winters. And up until recently it was thought that the tropical temperatures in such simulations were far warmer than reality — but more about that anon.

In the past few years there has been a real shake-up in the conception of what hothouse climates are like. First, it was found that the Tropical regions in hothouse climates are not tightly thermostatted as had been previously thought. Prior indications of a cool tropics turned out to be an artifact of alteration of the chemistry of marine sediments after they were deposited — a nightmare known as diagenesis to paleoceanographers. The tropics are actually quite a bit warmer than today’s tropics. For example, the Eocene tropical ocean may have been as warm as 35C, as compared to about 29C today. The upward revision of tropical temperatures is quite a good thing for the CO2 theory, since it removes a good part of the “low gradient” problem, wherein models were thought to systematically exaggerate the pole to equator temperature gradient.

So far, so good. But then, just last year through heroic efforts involving a nuclear icebreaker, a conventional icebreaker and an icebreaking drill-ship. a deep-time sediment core was recovered from the Arctic ocean. The results, which came out in a series of papers in Nature (here,here and here) were startling. At times the Arctic was practically a freshwater lake, indicating some quite dramatic changes in the hydrological cycle. And more germane to the matter at hand, in the early Eocene, the Arctic was much warmer than previously thought. According to Sluijs et al ocean temperatures were as high as 23C — rather like Key West today. These temperatures come to you courtesy of a novel biochemical proxy known as Tex86, derived from certain lipids produced by tiny plankton called Crenarchaeota. Tex86 is the new wunderkind of paleoceanography.

Will wonders never cease? Evidently not. Just when the hothouse starts looking really, really hot, along comes a new Science article by Bornemann et al, dealing with climatic conditions in the Turonian (93.5 to 89.3 million years ago). The principal result of this paper is that there appears to have been a 200,000 year period right smack in the middle of one of the warmest periods of the past half billion years, when there were ice sheets (presumably in Antarctica) that were up to 60% the volume of today’s Antarctic ice sheets. How in the world do you get such large ice sheets in a high CO2 climate warm enough for crocodiles to survive in the Arctic at the other side of the planet? And this apparent glaciation is not the result of a global cold snap. As in the Eocene results quoted earlier, the tropical ocean surface temperatures are again on the order of 35C — courtesy once more of the wondrous Tex86 proxy.

How was the ice volume inferred? Primarily by an especially meticulous application of an old technique. When a glacier forms on land, the water it is made of is enriched in the lighter form of oxygen, 16O, which leaves the ocean enriched in the heavy form, 18O. Single-celled shelly creatures called foraminifera (“forams” for short) record this composition, but they are very subject to diagenesis. The key to the new estimate was to take samples from pristine glassy portions of exceptionally well-preserved foram shells. The sample was taken from a core in the Tropical Western Atlantic, so the investigators are able to determine tropical surface temperatures, making use of Tex86 proxies from organisms living near the surface. The ocean water isotopic composition is estimated using both surface-dwelling and deep-dwelling forams.

Since the oxygen isotope composition of forams depends on temperature as well as ocean water composition, the Tex86 proxy was used to correct for the temperature effect in forams living near the surface. There is no independent temperature proxy for the deep ocean, but the investigators assumed (a bit questionably) that deep ocean temperature did not change much over the time period. Be that as it may, the deep ocean oxygen isotope shift (uncorrected for any temperature effect) was similar in magnitude to the estimate from surface forams. Once you have the oxygen isotopic composition of sea water, you can translate that into ice volume by making an estimate of the isotopic composition of glacier ice. All this is easier said than done, but they did it. The glacial interval corresponds to the excursion of delta-18O toward positive values in the figure below, taken from the paper:

There is a useful commentary by Richard Kerr One must exercise the usual caution we urge in connection with radical results, and await confirmation before taking it to the bank. As Kerr points out, there is other data from this time period that doesn’t show the isotope shift.

There are two additional things I myself noticed, which seem inconsistent. The first is that in order to get reasonable numbers for ice volume, the investigators needed to assume that Antarctic ice in the Cretaceous period had the same isotopic lightness as Antarctic ice today. Most theories of fractionation would have Antarctic ice being less fractionated in a warm climate, however. Perhaps the high Equator to Antarctic gradient helps keep the Antarctic ice light, but this is something that needs to be checked. What’s even more troubling to me is that the bottom-dwelling forams (uncorrected for temperature) indicate the same ocean water isotopic shift as the temperature-corrected surface dwelling forams. However, if Antarctica glaciates, the deep ocean should be filled with cold Antarctic bottom water, which should produce an additional positive isotopic shift in the uncorrected bottom dwelling forams. That this shift isn’t seen suggests that something is amiss to me.

Still, this paper already has a lot of modelers scratching their heads. To give an example of the magnitude of the problem, I reproduce below a figure from one of Rob DeConto’s old simulations (Nature 421, (2003) ), showing the glacier distribution in Antarctica as a function of CO2, as CO2 is steadily decreased. These are done for orbital parameters favorable to Antarctic glaciation; the simulations don’t use Late Cretaceous geography, but they do give a good idea of how hard it is to get a big glacier in Antarctica with anything much above twice the preindustrial CO2.

It is salutary to keep in mind that in many past cases where data conflicted with robust modeling results, it turned out to be the models that were right and the data that was wrong. This was the case for the early satellite reconstructions of twentieth century lower tropospheric temperature, which showed a spurious cooling. It was also the case for early reconstructions of tropical climate during the Last Glacial Maximum, which failed to show the cooling we now know to prevail in that region during glacial times.

So, what does all this mean for CO2 and anthropogenic global warming? Does it mean we don’t know beans about climate, so let’s have a party and why worry? No, actually. All this hothouse strangeness gives us a great deal more to worry about. The tropics is not strongly thermostatted, and there appear to be feedbacks in the system that can amplify polar warming more than previously thought possible. Perhaps due to clouds? Matt Huber, one of the foremost Eocene modellers, stated in a recent seminar at the University of Chicago that he could get closest to reproducing the Eocene hothouse by assuming that the Earth’s real climate sensitivity is at the high end of the IPCC range — around 4C per doubling of CO2. Or, perhaps there are mode switches in the climate system we know nothing about, which we are risking triggering by increasing CO2. Without understanding the Hothouse climates, it’s impossible to say how close we are to the danger zone.

But what of this new riddle of Cretaceous ice? An optimist might say that the result shows that you can keep a lot of ice in Antarctica even in a very warm climate. On the other hand, the conditions allowing the ice to exist in a warm climate are evidently very fragile, since it was there (assuming the result holds up) for only 200,000 years — the wink of an eye, in geological terms. That could mean that the factors governing whether Antarctic ice stays or goes in a warm climate are more subtle than we thought, offering more possibilities for surprise transitions. Or it may turn out that Cretaceous CO2 is really only twice the pre-industrial level, but that there’s some whopping positive feedback which bumps up tropical temperatures to 35C. In a scenario like that, the strange and unexplained resistance of Antarctica to warming might save some Antarctic ice, but that would be cold comfort, since the rest of the world would be toast.

Or, it may turn out that the processes determining the glaciation and deglaciation of a partly ice covered Antarctica have nothing to tell us about the present situation starting with a large Antarctic ice sheet. I’d be surprised if this turned out to be the case, but it could happen. One thing is for sure — if the result survives, it will provide an important and challenging test for the next generation of ice sheet models.

Could it be that the glaciation is telling us that we are completely barking up the wrong tree with the CO2 theory of hothouse climates? Perhaps, but somebody will have to pony up a quantifiable alternative before that avenue can be pursued. And whatever the alternative is, the challenge of simultaneously explaining the coexistence of a super-hot tropics with Antarctic glaciation — and also explaining why this happened for only 200,000 years — is apt to be as big as any challenge posed to the CO2 theory. One could probably get a climate something like the suggested one by combining moderately elevated CO2 with making a lot of low clouds over Antarctica while making the rest of the world essentially cloud free (or somehow making the high cloud greenhouse effect dominant in the rest of the world), but that’s quite a stretch. If somebody comes up with a way of doing that which can be expressed in a sound mathematical formulation, I’ll be the first to want to have a look at it. Cosmic ray enthusiasts could have a field day with this, but I doubt they’d have much success.

However you slice it, it starts to look like the Eocene and Cretaceous are tugging at our sleeve, whispering to us “There are things going on with climate you don’t begin to understand. Proceed with caution.”

We already knew hothouse climates were interesting, but darned if they don’t just keep getting more and more interesting. It puts me somewhat in mind of the old Yiddish curse– “May you live in interesting times.” But, to paraphrase Maurice Sendak — Let the Wild Rumpus Continue!

308 Responses to “The debate is just beginning — on the Cretaceous!”

  1. 1
    Danny Bloom says:

    Very interesting post, Raypierre, and yes, we ARE living in interesting times. In this regard, I recently set up a Climate Clock, to keep track of PPM stats, and feedback welcome: One of the scientists at the Troll Station in Antartica told me privately that the reporter in question was warned that there is a diference between NOAA stats and current stats and the reporter should be careful what he reports.

  2. 2
    Andrew says:

    Assuming an earth without ice sheets: if the planet experienced a long period of time (hundreds of thousands of years) with elevated CO2 levels; would ocean bottom water warm to near surface water temperatures? I’m thinking of a passage I read in an oceanography text discussing how the bottom water of the Gulf of Mexico is currently very cold due to past inflows of glacial melt from the Mississippi and sills that keep it from mixing with the Caribbean. I was left with the question of whether or not a couple of missed ice ages would allow this water to warm to Caribbean levels, as well as wondering to what extent this pool of cold water effects the current climate of the Gulf. What ranges of global temperature sensitivities to increased CO2 result if ocean temps, top to bottom, were assumed to be equal to a global annual average? I.e. If the ocean sink for heat were removed? Isn’t the current ocean heat sink a vestige of the last and previous ice ages? Is the lack of such a sink a factor in past “hot house” climates?

  3. 3
    Patrick 027 says:

    Very Interesting, Thanks!

  4. 4
    Steve Reynolds says:

    …bumps up tropical temperatures to 35C. In a scenario like that, the strange and unexplained resistance of Antarctica to warming might save some Antarctic ice, but that would be cold comfort, since the rest of the world would be toast.

    That does not sound like a very scientific statement. What do you mean by ‘toast’, and what evidence is there to support that condition?

  5. 5
    Thomas Lee Elifritz says:

    One word – impact.

  6. 6
    David Pearson says:

    Surely Cretaceous and Eocene paleogeography is important here …. in other words the absence of a circum Antarctic current before the Drake Passage opened and also before Australia separated from Antarctica, plus the presence of a circum equatorial current able to travel between the Americas as well as through the remains of the Tethys Ocean north of Africa. Doesn’t this mean we have to frame our thinking about the role of GHG concentrations in terms of a quite different energy distribution system from that of today ?

    [Response: Could well be. There’s no good way to figure ocean heat transports in your head, so I wouldn’t hazard a guess. Note that DeConto’s calculations are done with a mixed layer ocean. i don’t know if anybody has yet followed up a similar coupled ice-sheet/GCM calculation using a fully coupled model. I’m edging toward that myself. I’ll note that the ocean was held out as a great savior for lots of other hothouse paleoclimate problems (notably the low gradient problem), but hasn’t proved to be as key a player as had been hoped. The obvious thing to do would be to do a fully coupled CSM run with asynchronously coupled land ice sheets and Turonian geography. Given how long it takes for ocean models to equilibrate, and given that one needs multiple runs to get the glaciers to settle down, this could take a while. –raypierre]

  7. 7
    Chris Colose says:

    Very nice piece Dr. Pierrehumbert,

    I did e-mail André Bornemann just a few hours after the online version of the journal came out, and I was very interested in it. Dr. Bornemann did say that he was a Cretaceous-ice skeptic, and still was not even convinced after his Science paper, which I found interesting.

    As you bring up we do have the paradox of ice, tropical-like SST’s, the paleobiological record, and of course high CO2, and then we have this study and other suggestions that Antarctica ice sheet was hanging around back then. Moreover, sea-level change near this time is on the the scale of tens of meters in a very short time. From our current knowledge there is only one mechanism known that is able explain such rapid fluctuations: glacio-eustasy.

    One thing that the Bornemann study did suggest, and I brought it up here before, is getting better understanding of paleoelevation at the time (e.g., how much elevation is necessary to nucleate and ice and glaciers on Antarctica). At this point, I think models will need to tell us a lot more (At least in the near future) than what we might get from Cretaceous rocks at the time.

  8. 8
    Chris Colose says:


    it wasn’t a scientific fact, it was a scenario…if the globe warms a lot, and antarctica doesn’t change much, that means from basic math that somewhere else must get much warmer. Call it “hot” “toast” “fried chicken” or what have you, but it probably won’t be good

    If the cretaceous ice turned out to be true, than I would think that this just means greater resistance from Antarctica, maybe some regional stuff, I don’t know much about ocean circulation to say…

  9. 9

    May I reprint your first paragraph somewhere in RealClimate is greatly needed there.

    [Response: Be my guest. Thanks for the interest. –raypierre]

  10. 10

    A good post, but your cavalier dismissal of the gradient problem surprises me.

    For those who are unfamiliar with the issue, paleoclimate indicators during hothouse times indicate a little warming in the tropics and a lot of warming at the poles. Hence the temperature difference from poles to equator (i.e. the gradient) was much reduced. However GCMs do a poor (some might say dismal) job of replicating that change. For GCMs to get upwards of 20 C of polar warming you generally also need very substantial warming in tropics (way more than the ~5 C observed).

    Or at least that is the gist of the problem discussed multiple times at AGU last month and mentioned (very briefly) in the last IPCC report. If you know something different Ray, please share.

    From my perspective it seems that the paleoproxy data for low gradient has gotten pretty compelling, but the only GCMs that can “fit” that data that I’ve heard of need to be substantially fudged by building in large poleward heat transports. The only model I’ve seen that started to reproduce that without a simple fudge factor, created somewhat enhanced heat transport through abundant super-hurricanes.

    [Response: It’s not a dismissal of the gradient problem. I only said that the new tropical data helps — it’s a far cry when people thought the Eocene tropics was even colder than today. There are two lines of thinking now going on now: (1) High climate sensitivity combined with partly effective thermostatting of the tropics by increase in hurricanes (notably Sriver and Huber, Nature), (2) The upward revisions of tropical T aren’t done yet, and in fact it’s even hotter than currently thought (though one has to be more speculative here and reconcile such high temperatures with the likely die-back of tropical land plants. And yes, the upward revision of Eocene Arctic temperatures puts part of the gradient problem back. –raypierre]

  11. 11
    Philip Dolan says:

    One group say CO2 increases first then temperature increases, another says the opposite, whilst others say that this is a normal cycle just like summer, autumn, winter, spring, and that the temperature was on the increase before man started burning fossil fuels and we can’t reverse it.
    Who do we believe and why?
    Any change in mankind’s behaviour that improves the quality of our air, water and soil has to be good.
    Thanks for the interesting read.

    [Response: I think you have misinterpreted the lesson of the CO2 theory of the hothouse. Sure, CO2 has been increased by natural processes at times in the past. However, it is known with 100% certainty that the present rise in CO2 is not part of a natural cycle. Multiple lines of evidence, including isotopic composition of atmospheric CO2, prove that the rise is due to fossil fuel burning and deforestation. It’s not a matter of believing the group of scientists subscribing to the CO2 theory of the Cretaceous vs. the bunch subscribing to the CO2 theory of global warming. Almost without exception, the groups are one and the same. There’s no conflict there. –raypierre]

  12. 12
    Nick Gotts says:

    An excellent popular science essay – up with the best of Gould or Dawkins. It would be nice to think this approach – describing what climate scientists are actually working on and arguing about – could engage the interest and thence loosen the dogmatism of denialists resistant to a frontal assault using evidence and logical argument.

    [Response: Thanks so much. Believe me, this sort of thing was much more enjoyable to write than Chevaliers–Part II, even if the latter created more of a stir in the French press. Much nicer to be writing about good science again. The good science is thankfully much more typical of the scientific enterprise, and gives a much better picture of what life is like for scientists. –raypierre]

  13. 13
    Johnno says:

    I searched the article for ‘decade’ as in warming per decade to see if 0.2C was exceeded. Maybe those gators had plenty of time to adjust. Also I doubt there were 6.6 billion of them with high metabolisms requiring permanent nest sites plugged into feeding tubes.

    Instead of conjecturing about the distant past I think researchers should concentrate on stark problems like what if there are no viable fossil fuels left after 2040.

  14. 14
    Ben Hocking says:

    Is it possible that the reduced O-16 in the oceans is due to increased O-16 in places other than ice sheets? One thought that comes to mind is that hotter temperatures lead to increased moisture in the air, and that moisture is going to be naturally more concentrated in O-16 (being a lighter isotope). Of course, that doesn’t explain it lasting over only a “brief” interval of 200 thousand years, but it seems that there might be other explanations that don’t involve ice, and that might be either more likely during hotter temperatures, or due to a specific geography/aquagraphy that was present only during that “short” time.

    [Response: The atmosphere won’t do it, because the total reservoir of water in the atmosphere is equivalent to something like a 2cm deep ocean. It’s not a big enough repository to affect the ocean isotopic composition. You could do it with massive amounts of cold (but unfrozen) freshwater in lakes or aquifers, but where would you put that much water? With that, you are also fighting the fact that when you rain out liquid you don’t get as much fractionation as when you snow out moisture onto a glacier, because the proportion of moisture lost from an air parcel start to finish is lower when you make rain. It’s less distilled. The only thing I can think of that would conceivably (and I hasten to add only conceivably, since I haven’t worked through the arithmetic) come close is to make the entire Arctic ocean into a freshwater lake, and while that may have happened episodically in the Eocene there’s no support for that in the Turonian. (BTW, I took the liberty of correcting your typo of 200 million for 200 thousand.) –raypierre]

  15. 15
    Nick Gotts says:

    Re #13 “Instead of conjecturing about the distant past I think researchers should concentrate on stark problems like what if there are no viable fossil fuels left after 2040.”

    That misunderstands the way applied and fundamental science interact, particularly when you are studying complex structures with many unique features, such as the Earth’s climate and geobiochemical systems. In fact, knowledge of the distant past is crucial to the study of fossil fuel reserves – ask a petroleum geologist.

    [Response: Indeed, one of the ironically disconcerting things about the Brinkuis et al Arctic core work is that it revealed massive amounts of organic carbon, in part from blooms of freshwater duckweed when the Arctic ocean went fresh. There was at least one poster at fall AGU modeling petroleum formation in these conditions. These cores just don’t look like normal cores — lots of organics, no forams, not much carbonate. At the same time that they are telling us to be cautious about increasing CO2, they are providing information about possible additional petroleum deposits — in a region that will become more accessible to oil exploration as global warming causes the ice to melt. Some people will make a ton of money out of that, but it’s a pretty vicious cycle. –raypierre]

  16. 16
    Steve Reynolds says:

    Chris Colose>

    it wasn’t a scientific fact, it was a scenario…if the globe warms a lot, and antarctica doesn’t change much, that means from basic math that somewhere else must get much warmer. Call it “hot” “toast” “fried chicken” or what have you, but it probably won’t be good

    There were two reasons for my question:

    1. I am sincerely interested in a knowledgeable answer, especially if these conditions might be a worst case result of unmitigated AGW.

    2. If this speculation is just an idle remark, it seems rather unprofessional and irresponsible to come from a climate scientist representing objective climate science in a politically controversial subject.

    [Response: Steve, you’re really going over the top here. “unprofessional and irresponsible,” come on now. For the record, since Steve seems overly literal minded, let me emphasize that my statement was not meant to imply that 35C tropical temperatures would turn the planet and its inhabitants into a common heat-treated wheat-based product. Nor do I think the moon is made out of green cheese. Now please why don’t you find something more substantive to argue about. I’m sure a lot of readers can give you an idea of what a 35C tropics would do to marine and terrestrial ecosystems — keeping in mind that if it’s 35C in the ocean, it will get a lot warmer on land. –raypierre]

  17. 17

    Could the resistance of Antarctica to warming,at that time, be due to the fact that CO2 concentrations in the atmosphere weren’t as “well mixed”, as they are at present, because of temperature, pressure, convection or other anomolies?

  18. 18
    Pascal says:

    thanks Ray for this article.

    When you write this:

    “However, if Antarctica glaciates, the deep ocean should be filled with cold Antarctic bottom water, which should produce an additional positive isotopic shift in the uncorrected bottom dwelling forams”

    I suppose that cold water is richer in 18O than warm water or is there another explanation?

    [Response: Not so much that. What I was alluding to was the fact that forams fractionate oxygen relative to the composition of sea water, and they fractionate more strongly in colder conditions. According to the paleotemperature equation, you could get a 1 permil shift in benthic foram carbonate delta O18 from a 4C cooling of benthic waters. 4C cooling is hard to do for bottom water if you’re starting from a cold Pleistocene type climate, but quite easy if you’re starting from a warm climate. If it weren’t for the planktic data in the paper, I’d say that their isotope shift was just due to a flush of moderately cold deep water, not from ice. We badly need independent benthic temperature proxies for the Cretaceous and Eocene. Mg/Ca is good for the Pliocene and Pleistocene, but I’m not sure how reliable it is when pushed back to the Eocene. –raypierre]

  19. 19
    Dodo says:

    Thanks for an excellent post. It would have been even better if you had resisted the urge to take the obligatory RC cheap shots against “dogmatic denialists”.

    Or did I miss the part where the significance of mentioning Avery, Crichton and Singer in this context is explained? If not, please delete the first three paragraphs, and your points are better taken in the dogmatic circles we all love to hate so much.

    [Response: Try reading the post, and don’t quote things that aren’t there. Where did you get “dogmatic denialist?” Crichton, Avery and Singer are brought in as real-life examples we have been asked to debate. They are also brought in as examples of people whose ideas are in an area that scientists don’t consider worth debating. Sure it’s a cheap shot, but it’s not worth an expensive shot. These are people with cheap (even worthless) ideas. –raypierre]

  20. 20
    John L. McCormick says:

    RE # 15

    Good comeback Nick; [ask a petroleum geologist]

    But, I feel Jonno was expressing the frustration many feel about the ticking clock giving us less time to tackle massive challenges facing us: food/water availability, liquid fuels, etc. in a warming world of more people.

  21. 21
    Hank Roberts says:

    Gavin, could you open a “Friday” thread (top of page) to intercept driveby/naive/elementary questions, so this can stay focused?

    The (very brief) interview, 1/4 of the _Science_Podcast_ (found via links in first post by Ray) ends thus:

    Interviewee – Richard Kerr:
    “…. call for more data, but more data from different parts of the world. There’s concern that maybe there’s something peculiar about this one particular spot in the western tropical Atlantic where they’re reading this global signal.”

    — good point. I recall Gavin’s comment after his China trip that paleo folks don’t cross-compare cores much yet. Needed for sure.

  22. 22
    Ray Ladbury says:

    Ray Pierrehumbert,
    Excellent post as usual. The seeming glee that denialists sieze upon any result that could be interpreted as calling model results into question has always amused me. The empirical data are sufficient to establish that warming is occurring. The fact that nobody can construct even a semblance of a scientific model that explains these data without anthropogenic CO2 being the driver establishes convincingly the cause. And the paleoclimate is sufficient to establish that the consequences of rapid, significant warming can be severe indeed. The models are the only tools we have that could LIMIT how much we should be concerned. Right now it is the models that are suggesting scenarios by which we could limit the consequences of climate change without significantly harming our global economy. If the models are wrong, the upside risk of climate change cannot be limited, and arguments for draconiam measures are strengthened rather than weakened. That is why I keep telling responsible skeptics that the models are the best friends they have.

  23. 23

    Antarctica without any ice is harder to imagine, for instance the South Pole is at a much higher elevation than the North Pole, there is always cooler air at higher elevations (say 600 mb) anywhere in the world , combine this with a low sun along with a weaker equator to Poles gradient (less advection) and there is room for a giant glacier. How then could some ice not survive in higher terrain? I think people forget about 24 hour a day darkness lasting for 6 months, and how cold it gets when heat radiation escapes to space constantly. Antarctica under permanent clouds would be much warmer, only then I can imagine a temperate climate at the South Pole.

  24. 24
    JB says:

    If things are that “settled”, then what is the percentage, plus or minus 5%, of human-caused warming? Because it matters a great deal whether that is 90% or 10% when people are talking about spending trillions of dollars. There is at least some evidence out there to suggest that number is closer to 10%. I’m not saying that evidence is convincing, but it does exist, and ignoring it is not science.

    For what it’s worth, many solar scientists are willing to admit that solar science is quite unsettled. That giant super-furnace not too far way might just have a bit of impact on Earth’s climate.

    Please don’t hem and haw; if things are that “settled”, a number should be easy to come up with.

    [Response: This is just the kind of rhetorical device that comes up in debates all the time. Do you want to play Perry Mason, or do you want to actually try to reflect on and understand the issues? If I said that the proportion of global warming to date due to anthropogenic influences is known to within 20% would that satisfy you? Would you then demand 1% accuracy? .001%? In any event you are asking the wrong question, because the forecast is not based on an extrapolation of the warming to date. Models employing a variety of different physical assumptions, yielding a variety of different equilibrium sensitivities, are equally compatible with the warming observed to date — and give greater or lesser forecasts for the future warming, as expressed by the range in the IPCC report. There will always be continuing developments in the scientific literature, attempting to narrow the sensitivity range by using constraints based on instrumental or paleoclimate data.

    The notion of what is “settled” is not tied to some arbitrary precision level of some individual statistic one is trying to reproduce or forecast. The point is that right now the science says that there is a very low chance of low climate sensitivity, and a very real risk of high climate sensitivity. The big damages come from the high end, and politics makes decisions in the face of such uncertainty all the time — even decisions involving trillions of dollars of spending. If you think about it, you’ll surely be able to come up with plenty of examples, most of which have involved considerably greater uncertainties than those involved in the climate forecasts. –raypierre]

  25. 25

    Were there any massive volcanic eruptions or large meteorite impacts around the time of this possible glaciation? What we might be seeing in this Cretaceous event is a temporary response to in injection of reflective aerosols into the upper atmosphere. A sudden cooling would be first manifested in Antarctica if the present is the key to this event in the past.

    Whatever, happened, it’s an excellent problem for the modelers to address.

  26. 26
    Chris Colose says:

    #10 and raypierre

    I’m not that familiar with the “gradient problem” but I just ran the GISS model with 8x CO2 (, and even under 4x CO2 there is substnatially more warming over the poles than the rest of the globe (Actually Africa warms up as much as well). Just reading from #10, where is the problem here?

    Also, if someone can answer this, but just of curiosity why is it so hard to get that one blotch in southern greenland to warm up?

    [Response: The question is whether you can get a 20C Arctic and above-freezing winters without making the tropical ocean warmer than 35C. The extent of the problem is somewhat model-dependent, and somewhat dependent on just which part of the hothouse period you’re looking at, but it is a problem that hasn’t completely gone away. –raypierre]

  27. 27
    Jim Eager says:

    Re Philip Dolan @ 11: “One group say CO2 increases first then temperature increases, another says the opposite”

    And those who are paying attention know that CO2 can both lead temperature as a forcing, or follow temperature as a feedback, depending on the circumstances, and either way increasing CO2 makes it warmer.

  28. 28

    Re: Wayne #23

    It’s not a question of how cold is winter. There is little difference between -5 C and -25 C when it comes to precipitating snow and ice. Rather, the important question is how much of that snow and ice deposited in winter can survive through the 24 hour sunlight of summer in order to allow multi-year accumulation into ice sheets.

    If you really believe the 20 C numbers for sea surface temperatures in the Arctic, then you’d need elevations of 3000-4000 m in Antarctica to counter an equivalent amount of warmth. A deglaciated Antarctica does have some terrain at that level, but like most continents the lion’s share of the terrain is below 1000 m. Similarly, keep in mind that those 20 C numbers are being reported in spite of an Arctic that would also suffer from 6 months of darkness.

    In other words, the data really has been pointing to a ridiculous amount of warmth at the poles that would be extremely hard to overcome and form ice sheets. As Ray shows in a figure, models give you very little ice at >3 times CO2, and that’s in spite of the fact that models seem to underestimate high latitude warming when compared to proxy data.

  29. 29
    pat n says:

    The demise of the dinosaurs about 65 million years ago was influenced by the impact of the Chicxulub asteroid. Perhaps the Turonian (93.5 to 89.3 million years ago) ice sheets, if they existed, came about due to asteroid impacts temporarily (200 K-yrs) clouding up the stratosphere, cooling the lands with dampened influence on oceans.

  30. 30

    Re: Chris #25

    You are starting with an annualized, sea level air temperature gradient of +25C at the equator to -20C at the poles today.

    In order to match the data for the most severe hothouses you need a equator temperature of order 35C with a polar temperature of perhaps as much as +20 C. In other words, a warming of 10 C at the equator coupled to a polar warming of 30-40 C. The GISS model you point to gives values of like 7 C at the equator with only 12 C at the poles. That effect is in the right direction, but not nearly large enough. The polar amplification still needs to be far larger than in that model in order to reduce the gradient to a level consistent with the current interpretations of the paleoproxy data.

  31. 31
    Nick Gotts says:

    Re 16# (raypierre’s response) “For the record, since Steve seems overly literal minded, let me emphasize that my statement was not meant to imply that 35C tropical temperatures would turn the planet and its inhabitants into a common heat-treated wheat-based product.”

    Dang, I’d already bought into butter futures on a huge scale!

  32. 32
    SecularAnimist says:

    Dodo wrote: “It would have been even better if you had resisted the urge to take the obligatory RC cheap shots against ‘dogmatic denialists’ … mentioning Avery, Crichton and Singer in this context …”

    The phrase “dogmatic denialists” does not appear in Raypierre’s article and is Dodo’s invention. Raypierre refers to Avery, Crichton, Singer and Senator Inhofe as “skeptics.”

    In my opinion, those folks are not “skeptics”. “Dogmatic denialists” is a more accurate characterization. Both Avery and Singer are closely associated with policy advocacy organizations that have received many thousands of dollars from Exxon-Mobil, Senator Inhofe is a well-known ally of the fossil fuel industry, and Crichton is looking to sell his books of fiction.

    None of them exhibit actual “skepticism” about anthropogenic global warming; they are simply determined to deny it for reasons of their own. Since they have no interest in the truth, only in disseminating denialist propaganda (usually long-discredited propaganda), Raypierre is correct that it is worse than useless to “debate” with them.

  33. 33
    Will Baird says:

    If I may, iirc, Drs Thomas Rich and Patricia Vickers-Rich found evidence of permafrost at least one of their digs in Australia. One of them was their extended Dinosaur Cove site. I think it was about 114 my, but my copy of their book is at home.

  34. 34

    I’m a great fan of David Mamet, whose eloquence you invoke:
    ” scientists are justifiably wary of being drawn into staged debates on such diffuse, ill-defined and largely meaningless topics as whether global warming counts as a “crisis.” In the war of the sound bites, the people who feel free to lie and distort can always win.”

    But Mamet’s plays are melodramas ,a medium of catharsis unreal as the waves breaking over The Statue Of Liberty in the most famous extant scene of what climate models are presumed to mean by most people, because most people have never pick up a copy of Nature, and can no more read JGR than cuneiform.

    Yet intelligent laymen , witness the fact that Mamet plays make it to the screen, are capable of “recognizing cant when they hear it and cartoons when they see them”, which is why lawyers may strive to dumb down juries by selection in tort cases, lest courtroom rhetoric set off baloney alarms. “Science is not generally thought of in terms of semiotics, which deals with the creation and manipulation of symbols ” but some cases , legal and scientific , are of such immense cultural and political import as to attract the full arsenal of cultural amplification, from TV to the Op-ed pages.

    One of the loudest of baloney alarms sounds when those who know that they are right declare their opponents too contemptible to debate, while endorsing the cultural icons that embody their own creations ;if anything is more hazardous than coveting our own theories , it is presuming that those with political agendas will not use them for purposes of their own.

    The quotes, by the way, RayPierre , come from the op-ed page of The Wall Street Journal. I wrote them in regard to the political abuse of a 1-D optical depth model that having acquired a cartoon life of its own courtesy of a K-Street PR firm , was incorporated into an apocalyptic film called “The Day After.”

    I believe you have seen the sequel. I know Al is familiar with both , as he chaired a 1987 debate on nuclear winter between yours truly and the ‘A’in TTAPS. Though Al found for the my side – the title of the event was ” Is Nuclear Winter Real And Relevant ?” as usual,he did not stick around to take questions. Plus ca change, mon vieux- toujours la semiotique!

    As to Dennis Avery , I’m willing if you’re not-

    [Response: I think Nuclear Winter is a good example of science dealing with its own problems on its own terms. The first estimates were made using over-idealized models, and made a big splash. Other scientists jumped on the phenomenon, and it didn’t take long before the estimates of the magnitude were reduced. That hasn’t happened with global warming, despite a much greater amount of research lavished on the subject than ever was expended on Nuclear Winter. It is true that there has been a small revival recently in thinking about Nuclear Winter in the context of regional exchanges. This is largely based on new estimates of soot production in urban fires, and vertical transport processes of soot. These, too, will be examined and cross-checked by others, and time will tell whether the phenomenon will turn out to be something worth additional worry. I don’t think a public debate is a good way to settle issues regarding self-lofting of soot particles by solar absorption. With regard to the original global-war Nuclear Winter scenario, the sort of things that indeed are appropriate for debate in the sense understood by the better sort of politicians are things like whether the additional damages from climate effects of Nuclear Winter are of a sort to significantly change the calculus of global thermonuclear war, in the face of the impacts from hazards like direct blast casualties and radioactive fallout. –raypierre]

  35. 35
    Walt Bennett says:

    It really is time to rise above both basic “who do I trust?” type posts as well as outright denialist provocations.

    There are many examples in these threads of legitimate skeptical or educational questions, which the access we have to climate scientists doing science permits.

    That access should not be abused. Newcomers need to be pointed to the pages where basic information is posted. All others are free to do their own legwork. Most denialist rhetoric has been well examined and well publicized. If a denialist refuses to believe what they learn from such analysis, or if they refuse to do the analysis, that is not RC’s problem.

    We need more frequent posts; regular unthreaded posts or a forum; more editing of comments to keep them semi-focused and to whittle out ad-hom or rhetorical arguments.

    RC needs a sort of rededication to its original mission: to provide the lay public access to climate scientists doing climate science.

    Where is Gavin’s post about the GISS/Hadley discrepancies? I anxiously await his detailed analysis of two “reasonable” approaches to the data which yield different signs.

    [Response: What we need is an extra 48 hours in the day. –raypierre]

  36. 36

    JB writes:

    [[For what it’s worth, many solar scientists are willing to admit that solar science is quite unsettled. That giant super-furnace not too far way might just have a bit of impact on Earth’s climate.]]

    It does, but it has very little impact on recent climate change. The Solar constant hasn’t varied significantly in 50 years, so it’s unlikely to have caused the sharp upturn in warming of the last 30:

  37. 37

    Re #21 on intercepting naive, elementary questions, I remember being told in my early school days that there were no foolish questions, just foolish answers. An exaggeration for sure, but it was meant to encourage questioning things that we didn’t fully understand.
    My question above about the homogeneity of the distribution of CO2 in the atmosphere during the Paleocene period was thrown out as a possible area to explore to account for the cold Antarctic during an otherwise warm earth. Not all greenhouse gases are evenly distributed throughout the atmosphere. There are physical- chemistry explanations for this,I’m sure,which are not all fully understood.

    [Response: Your question might have been naive, but I hope nobody was implying that it was foolish. It is, however, easily answerable as the long lifetime of CO2 in the atmosphere gives it time to become quite well mixed. What I had in mind for the sort of things that could (speculatively) make Antarctica less sensitive to CO2 increase were things like the surface inversion or changes in the lapse rate; a weak lapse rate inhibits the greenhouse effect. I suppose ozone could also be involved, along the lines that have been invoked by Susan Solomon and others as a factor in Antarctic climate change today. –raypierre]

  38. 38
    pat n says:

    Polar warming is amplified by increasing clouds in the winter season in mid-high latitudes and decreasing clouds in the summer seasons in lower latitudes. That seems to have been happening for many years already. What is NOAA NWS saying about that?

    [Response: Pat, would you care to say what studies you are referring to that support the cloud feedback you are invoking? –raypierre ]

  39. 39
    David B. Benson says:

    I suggest volcanic activity at this time depth in Antacrtica may have produced enough aerosols to sufficiently lower the temprature that some ice formed. See the first abstract in

    to note the sentence

    Hypabyssal basaltic and andesitic dykes and plugs of the Admiralty Bay Group (Upper Cretaceous and Palaeogene) cut the two stratiform complexes, sometimes also the Wegger Peak Group plutons.

  40. 40
    raypierre says:

    Here I am going off-topic on my own article, but while I have your attention, I thought this was as good a place as any to note that Andy Revkin’s piece today over at Dot Earth discusses the recently released AGU statement on global warming. Marc Morano (of Inhofe 400 fame) has tried to swift-boat this by claiming the AGU council doesn’t represent the membership as a whole. Andy has responded by asking AGU members to comment on the statement, to see if there is any groundswell of dissent. So, if you are an AGU member, by all means go over there and make your voice heard.

  41. 41
    pat n says:

    The “studies” are from my observations (what I saw happening) while I was Snow Hydrologist for the North Central River Forecast Center (NCRFC NWS NOAA) – responsible for issuing annual Spring Snowmelt Flood outlooks for the Upper Midwest from the late 1970s until I was forced by NWS to retire because I chose to study and speak out on climate and hydrologic change which I observed happening in the Upper Midwest.

  42. 42

    Re 39

    I am and I will.

    as to your response to 34, may I remind you that while Carl Sagan began his Foreign Affairs article on “Nuclear War And Climatic Catastrophe” with the words ;

    “Apocalyptic predictions require ,if they are to be taken seriously, higher standards of evidence”
    he subsequently refused to debate the matter with any of “the better sort of politicians” to whom you refer. It then,as you point out , took several years for climate modelers like Steve Schneider and Starley Thompson to rescue modelsof the KT impact from being hijacked as a vehicle for the rhetoric of extinction. They were of course accused of being in denial , but we won the cold war anyway.

    Which is a Good Thing since it leaves us at liberty to read Bornemann, Sluijs and the rest of this fascinating stuff- thanks for reviving the Snowball Isotope Wars !

    [Response: Right, Carl Sagan exaggerated, maybe overplayed the TTAPS study. But this wasn’t settled or sorted out by debates with Carl Sagan. Other scientists got on his case and published stuff. That’s why we know the real dimensions of the risk today. It all got sorted out rather quickly. It will be interesting to see how Toon et al’s attempted revival plays out in the literature. But my point? Sure, it can happen that scientists go public too soon, overplay their hands. But it was regular climate scientists who got in their and cleaned up the mess, even climate scientists whose political disposition would make them want to believe Sagan’s picture.

    I wish to emphasize that I still admire Carl Sagan. Scientists are prey to all the same imperfections as anybody else, and Carl Sagan’s provocations served to shake loose some very important ideas. As I said before, a lot of good thinking about the effect of the KT bolide impact came out of the Nuclear Winter work. Sagan was also wrong in detail about the resolution of the Faint Young Sun (he thought it was ammonia that did it), but he still was the one who pointed out the problem and launched the field on the pathway to the solution. It’s good that science is self-correcting, since it allows us to enjoy mavericks like Carl Sagan without any long-term harm.

    By the way, your facile claim that the massive over-investment in the US nuclear capability won the cold war is not very well supported. Let’s not get into that here, but since I let your one-liner through on this, you owe me a one-liner as well. –raypierre]

  43. 43
    Mare team says:

    Wow, modeling things that happened about 90 million years ago within a timespan of 200k/years seems to be hugely ambitious. Even our solar system was not spinning at the same speed as today (fractionally faster) which would imply slightly different Milankovitch cycles as well, then count in a nasty fly-by or two … good luck guys !

    And yes, there are some people who try to think ahead and offer solutions for the next 40 years as well. We are :-)

    Cheers, Mare team

  44. 44
    AK says:

    Re: #26

    [Response: The question is whether you can get a 20C Arctic and above-freezing winters without making the tropical ocean warmer than 35C. The extent of the problem is somewhat model-dependent, and somewhat dependent on just which part of the hothouse period you’re looking at, but it is a problem that hasn’t completely gone away. –raypierre]

    I’m assuming the problem is that the warmer the arctic is, the less gradient there is to drive advection, while the more heat is needed at the pole to counter increased radiative cooling.

    Assuming this is the problem, I’ve speculated that perhaps the current system of Hadley, Ferrel, and Polar cells doesn’t apply to these scenarios, being replaced, at least during the summer, with something more longitudinal. Would the current models be able to show such a reorganization, or is the current system built-in to them?

  45. 45
    JB says:

    Barton Paul,

    “It does, but it has very little impact on recent climate change.”

    That is a huge assumption. You are assuming that solar irradiance is the only variable we should be looking at with regards to the sun. Assumptions like this are exactly the problem. There are a large number of solar variables and they should all be taken into account; not dismissed and ignored.

  46. 46
    Mark A. York says:

    Nice job Ray as ever. Great stuff. In this vein, I got this response from a critic who does not believe in AGW. 0.4% percent of the atmosphere can’t have that kind of influence. When asked to provide an alternat theory this is what he said:

    “One of my pet theories is that there is much more coupling between the hot core and the oceans that cover 71% of the earth’s surface (particularly the deep parts of the ocean where the crust is thinnest) than is accounted for in the GCMs. If someone offered me grant money, this is what I’d look into. I read that volcanic activity in Greenland (in areas where, again, the crust is thin) might be partially responsible for its glaciers receding…”

    What would your answer be to this Captain Nemo Theory of global warming?

  47. 47
    Gareth says:


    Thanks for a most interesting discussion. It illuminates a significant part of a problem that I blogged about last week – namely, that until we can refine the models so that they can cope with the observed rates of Arctic warming, they can offer little relevant information for policy makers on the climate states that could emerge over coming decades. (Very interesting discussion of the modelling challenge – ”An Ice-Free Arctic? Opportunities for Computational Science” here: PDF – includes Gavin as an author (hat-tip: Steve Bloom in the comments to the post linked above)).

    You describe the northwards heat transfer as a “fudge factor” in getting models to generate realistic warm Arctic conditions. That transfer is happening now, and it would be interesting to know what progress is being made in measuring it.

    Equally interesting, from a “what’s going to happen” perspective, is what do northern hemisphere climates look like when the Arctic’s ice free year round?

  48. 48
    David B. Benson says:

    Mark A. York (46) — Suggest he read The Discovery of Global Warming, especially the section on carbon dioxide as a greenhouse gas. The web pages are linked on the sidebar, first in the Science Links section.

  49. 49
    Carrick says:

    If we are to make this a scientific discourse (it seems to me the word “debate” is the wrong word to use), then it seems to be necessary to go away from language like “believers”, “deniers” or “contrarians”.

    If somebody is practicing skepticism on a scientific basis, that is a necessary component of the scientific mechanism, and it doesn’t make them a contrararian simply because they’re poking holes in the established lore on the subject (whatever that subject is).

    It is well established that Gore’s presentation of GW contains significant scientific errors, so it seems imminently reasonable for him to address these problems with his presentation. To many people, it appears that he is refusing public discourse on this subject because he doesn’t want to answer to his mistakes.

    That doesn’t fill the public with much sense of confidence that he has used this approach rather than update and correct his work. And in the end this type of error makes the sell of the real science more difficult, not easier. I also think that overselling the science doesn’t help. For example, we do need better monitoring of our climate than we have, especially in the Arctic and Antarctic regions. But it makes new funding more difficult if we are as a group trying to claim “it’s a done deal”.

    What’s to be researched if we already know the answers?

    When I am asked by lay people (I should mention I’m a physicist not a climate scientist) I do come down rather critically on the limits of the theory and experiment. This is how scientists function (if the science is to be healthy). And, after all, if the point is correct, tweaking it to address criticisms only makes it stronger.

    I usually have to explain to people that regardless of what is settled, there are some basic policy decisions that don’t depend on the details of the science. That we should moderate our production of CO2, CFCs and other greenhouse gases is just common sense. And, if as I advocate, there is more uncertainty that some people are claiming, doesn’t this uncertainty make it more necessary (not less) that we act in a responsible fashion to stop what could be an unprecedented human disaster?

    The fact is a greater uncertainty increases the risk factor, it doesn’t lessen it. And I think that’s a fact too often missed by some close-minded critics.

    [Response: Oh come now. A New York Times article by the sloppy William Broad (who also botched reporting of the Phanerozoic CO2 business) hardly constitutes a serious indictment of Gore’s science. Even the article you link, careless as it is with the data and people it quotes, shows a substantial support for the idea that while A.I.T. had some minor flaws, in the big picture it is spot-on. Broad’s article misquotes the National Academy report regarding climate of the past millennium, and came before the substantially stronger backing of the Hockey Stick result that was made in the current IPCC report. I can’t speak for Gore, but while I think there should be some kind of forum for clearing the air about the scientific issues raised by his critics, I’m not sure that subjecting him to the kind of sound-bite sniping you see quoted in Broad’s article is the way to go about it.

    I do very much agree with your statement about uncertainty increasing the risk. There are a few economists that have begun to understand this, and are starting to incorporate low probability catastrophic events in their models. If scientists sometimes get backed into a corner and feel obliged to try to minimize uncertainties, it’s because policy makers seem to forget your point about uncertainty when it comes to thinking about global warming. –raypierre]

  50. 50
    Peter D. Tillman says:

    Very thought-provoking post, and more catch-up reading. Is this part of your upcoming Climate book?

    Now, for your next essay, I hope you take on the Paleocene–Eocene Thermal Maximum ;-)

    Cheers — Pete Tillman
    Professional geologist – Amateur paleoclimatologist