The debate is just beginning — on the Cretaceous!

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!