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The End of the Hothouse

Posted on 16 December 2011 by John Mason

A new study links major atmospheric CO2 drop to the onset of Antarctic glaciation, 33.7 million years ago

Forty million years ago, Antarctica had a pleasantly mild climate, its mountains and shores flanked by swathes of woodland in which a diverse mammalian fauna flourished. Today, it is one of the most inhospitable places on Earth. Throughout this time, the continent has remained in pretty much the same place, straddling the South Pole. It follows that a drastic climatic change must have occurred, but how?

That has been the subject of much research over the years and a good picture has gradually emerged. Now, a new paper in the journal Science has clarified the role of rapidly-declining atmospheric carbon dioxide levels in the temperature-plunge that saw the rapid onset of Antarctic glaciation, 33.7 million years ago. The fall in CO2 concentration was from 1000-1200 ppm down to 600-700ppm, at which point it was cool enough to allow glaciers to start to form. That our current emissions path takes us beyond the latter levels by 2100 means that we are heading straight towards a planet that may no longer sustain polar ice-caps, resulting in a steady melt and relentless sea-level rise that will duly threaten every coastal city in the world. We'll see what the research found out in a moment, but first let's take a quick look at the Cenozoic Era, the geological timespan during which the glaciation of Antarctica began.

The Cenozoic Era: 65.5 million years ago (MA) to the present day

The Cenozoic is subdivided into seven periods of geological time: from oldest to newest the Palaocene (65.5-55.8 MA), the Eocene (55.8-33.9MA), the Oligocene (33.9-23.03MA), the Miocene (23-5.3MA), the Pliocene (5.3-2.6MA), the Pleistocene (2.6MA-11,784 thousand years ago) and the Holocene, from then until the present day. It saw one of Earth's periodic transitions from the Hothouse climate (very warm; no polar ice-caps) to the Icehouse, the latter typified by the geologically recent glacial/interglacial cycles.

The early Cenozoic had a positively balmy climate, including the Paleocene-Eocene Thermal Maximum (PETM on the graphic below). The PETM featured such extreme warmth that subtropical flora and fauna occurred in some Polar regions, as evidenced in the fossil record. During the middle to late Eocene, global temperatures cooled, but around the Eocene-Oligocene boundary they literally fell off a cliff. This was the point at which glaciation rapidly initiated in Antarctica. Through the rest of the Oligocene and into the Miocene, temperate conditions saw Antarctic glaciation advancing and retreating, before the temperature-decline in the late Miocene and Pliocene led down into the Icehouse of the Pleistocene.

Climate trend of the past 65 million years

Above: The Cenozoic climatic trend (green line): figure prepared by Robert A. Rohde (citations via the link).

The cause of the precipitous end-Eocene fall in temperatures has been the subject of much research. The strata dating from this time have been mapped, analysed and their fossil fauna and flora described on an ongoing basis. The new paper in Science has added yet another piece to the jigsaw, refining the use of what was previously a problematic CO2 proxy - something whose properties can be calibrated with CO2 concentration at the time - and reaffirming the role of CO2 as Planet Earth's atmospheric thermostat.

However, the new study has also been doing the rounds in the skeptic sector of the Blogosphere. Typically, and unsurprisingly, the professional-skeptic spin on the paper has involved variations of the following theme: "so the Antarctic was a frozen hell at 600ppm and we are nowhere near that, so what's all the fuss about?"


The first Golden Rule when a new paper comes out is to read it, so let's take a look and see what it actually says. Here's the Abstract - the brief summary of the work that all scientific papers begin with:


Earth’s modern climate, characterized by polar ice sheets and large equator-to-pole temperature gradients, is rooted in environmental changes that promoted Antarctic glaciation ~33.7 million years ago. Onset of Antarctic glaciation reflects a critical tipping point for Earth’s climate and provides a framework for investigating the role of atmospheric carbon dioxide (CO2) during major climatic change. Previously published records of alkenone-based CO2 from high- and low-latitude ocean localities suggested that CO2 increased during glaciation, in contradiction to theory. Here, we further investigate alkenone records and demonstrate that Antarctic and subantarctic data overestimate atmospheric CO2 levels, biasing long-term trends. Our results show that CO2 declined before and during Antarctic glaciation and support a substantial CO2 decrease as the primary agent forcing Antarctic glaciation, consistent with model-derived CO2 thresholds.

What does that mean? We can now delve into the paper itself for the meat to put on the bones.

Glaciation always leaves calling-cards

Firstly, due to their very nature, glacial episodes on Earth always leave behind their distinctive geological calling-cards. The onset of mid-Cenozoic glaciation in Antarctica, ~33.7 million years ago, is no exception in this respect. Initiation of this abrupt climatic change is marked by a sudden shift in deep-sea oxygen isotope values and also by two strong signals in the local sedimentary rock sequence: firstly, the sudden introduction of sediments with a glacio-marine origin and secondly a change in in the types of clay-minerals present that identifies a shift in the predominant type of weathering of the rocks of the continent, from chemical dissolution of minerals to a physical erosion regime. So we have known for a long time that Antarctica went from being ice-free to having a system of glaciers, we know that this happened around the Eocene-Oligocene boundary and we know that, geologically-speaking, this change happened rather quickly.

So how did it cool so quickly?

The abrupt cooling episode that led to the appearance of Antarctica's glaciers has been the subject of much research and there have been a number of proposed mechanisms, involving changing concentrations of greenhouse gases, orbital variations and changes to oceanic currents in various combinations.

Brief mention should also be made of the slightly fainter sun at the time: as a main sequence star, the sun has been brightening at a rate of ca. 10% per billion years of geological time, so that 33.7 million years ago it would have been about 0.3% less bright than at the present day. However, this variability clearly occurs over a very long timescale, so that it would not cause a sudden and drastic climate event.

CO2 values in the Hothouse climate of the Eocene (and indeed over the preceding millions of years) were significantly higher (1000-1500ppm) than any relatively recent levels (180-280ppm respectively in glacial-interglacial cycles and ~390ppm and rising in the Industrial Age of today). However, the exact timing, nature and causes of the Cenozoic CO2 decline have remained somewhat elusive. Reconstructions have been attempted by examining the chemistry of alkenones in marine sediments. Alkenones are organic compounds of the ketone family that are in most cases highly resistant in nature. They are produced by a particular class of phytoplanktonic algae and their stable carbon isotope ratios have been used in palaeoclimatology as a method of estimating atmospheric CO2 levels (Pagani, 2002).

Alkenone-based CO2 reconstructions for the period of cooling that led to the onset of Antarctic glaciation have to date been a little problematic. They do indicate that the large CO2 decline occurred in several steps during late Eocene-early Oligocene times from the Hothouse high of 1000-1500ppm.  However, a previous attempted reconstruction showed an increase in CO2 at the time of the cooling and a large decline occurring 2-3 million years after maximum ice-sheet expansion (Pagani et al, 2005). This was found to be at odds with a separate proxy based on boron isotopes (Pearson et al, 2009) which indicated a C02 decline consistent with the Eocene-Oligocene boundary cooling, a subsequent weak rebound then a continued decline. It was also quite at odds with modelled estimates for the CO2 threshold - the level below which a rapid onset of Antarctic glaciation was likely to occur. Why?

The existing, problematic alkenone-based CO2 record is based on samples taken from a variety of environmental settings. In cases where they were obtained from poorly-stratified, nutrient-rich waters, the new paper suggests, it was possible that the samples did not accurately reflect the atmospheric CO2 concentration. In this new study, the authors examined regional differences in CO2-alkenone estimates from six well-separated localities around the globe, representing a range of environmental conditions, in order to investigate the problem further.

The paper goes on to describe in considerable detail how the issues with alkenones as proxies may be addressed, based on biochemistry work with cultures of the alkenone-producing algae in which physical and nutritional conditions could be varied and the results noted. The research led to the understanding that it was in fact very difficult to glean useful reconstructions from high southern latitudes (i.e. the seas around Antarctica) because of uncertainty with respect to these critical physical and nutritional conditions around the Eocene-Oligocene boundary. This was because the rather different geography at the time would have affected ocean circulations - and hence those physical and nutritional constraints - in that particular area, in an unpredictable manner. However, it was found that the uncertainties were greatly reduced at low-latitude sites - of which, in this study, there were two - in the Atlantic Ocean a little south of the Equator. It is upon these two sites, therefore, that the new attempt to reconstruct atmospheric CO2 was focused.

The sites yielded a reconstruction that revealed a persistent and substantial CO2 decline that began approximately two million years before the start of the rapid cooling and continued through and just beyond the event. A slight rise occurred in the mid-Oligocene before a long-term decline then set in towards Miocene times. The decline over the period 35.5-32.5 million years ago was from an initial high of 1000-1200ppm down to 600-700ppm in just three million years. This is consistent with the boron-based study and, importantly, is also consistent with the modelled estimates for the threshold CO2 level required for rapid glaciation in Antarctica (e.g. DeConto et al, 2008). In other words, it is consistent with what the physics would expect.

Other recent research has suggested that the development of the Circum-Antarctic Oceanic Current through the remainder of the Oligocene contributed to the further cooling of the continent due to thermal isolation (Anderson et al, 2011), so that by the Miocene the only vegetation left on Antarctica was localised areas of wooded tundra: some time after 12.8 million years ago this, too, had vanished as the great ice-sheets expanded. In addition, there is the positive feedback of albedo-rise once extensive permanent ice and snow are present, which would also favour further cooling. However, the take-home point of this study is that sharply-falling CO2 levels had the leading role in abruptly transitioning the global climate from the Hothouse to one in which it was cool enough for Antarctic glaciation to commence.

Paleoglobe from the late Eocene

Above: The late-Eocene world. Image courtesy of Ron Blakey of CP Geosystems.

Where did the CO2 go?

What processes would have been responsible for the strong CO2 drawdown event that led to this cooling? Burial of organic matter is one way of removing a lot of carbon from the system. This goes on all of the time. Chemical weathering of silicate minerals is another significant and ever-present carbon-sink. But here, we are dealing with a specific event in which a large amount of CO2 was taken out of the system - a carbon cycle "hiccup", if you like.

The Hothouse of the early Cenozoic was accompanied by very widespread tropical to sub-tropical weathering of silicate minerals but at the same time there was also the Alpine-Himalayan Orogeny (mountain-building episode), which commenced in Palaeocene-Eocene times and continued into the Miocene. The massive increase in mechanical erosion as mountain ranges rise leads to two enhanced carbon sinks: firstly a massive increase in surface area available to weathering agents, amongst which dissolved CO2 in rainwater is a lead player (think of the surface area of a lump of rock weighing a kilogram compared to the surface area of a kilo of sand scattered over the ground), and secondly a massive increase in particulate flux i.e. sediment getting swept down rivers - which in turn leads to rapid burial in sedimentary basins, including burial of organic carbon. So as a candidate bulk sink of carbon, it is certainly worthy of further consideration. Large phytoplanktonic algal blooms are another possibility - the mid-Eocene also saw the "Azolla event", a massive bloom of a freshwater fern over the then isolated and highly stratified Arctic Ocean, conditions in which dead organic matter was preserved and buried (e.g. Brinkhuis & Schouten, 2006). Did similar events occur elsewhere in the final years of the last Hothouse Earth?

The implications of this research

The graphic below depicts atmospheric CO2 concentrations, as observed at Mauna Loa from 1958-2008 (black dashed line) and projected from 2008-2100 under the six IPCC scenarios. Given that current emissions are in line with the A1F1 scenario, there seems a high risk of crossing the 600ppm threshold by later this century if the situation remains unmitigated. What does this mean in practice, with respect to the recent paper?

IPCC emissions scenarios


The Pagani et al (2011) reconstruction suggests that a significant and rapid episode of CO2 drawdown occurred just before and during the cooling that led to the onset of Antarctic glaciation, and the drawdown took CO2 levels to 600-700ppm - below the modelled threshold value for the initiation of Antarctic glaciation. The converse of this is that, in an ice-free world, atmospheric CO2 levels much above 600-700ppm would not favour temperatures low enough for the development of glaciers in that continent.

Heading into a future with CO2 levels in the high hundreds of PPM therefore seems unfavourable for the long-term survival of the ice-sheets of Antarctica. In this context, it is important to remember that the current study deals with the onset of glaciation in Antarctica - as set out above, the extensive ice-sheets came later in the Oligocene and Miocene when CO2 levels were lower still, the Circum-Antarctic Current had fully developed and albedo had increased massively. Skeptical Science has already covered land-ice loss from Antarctica here: the current rate of loss is 100-300Gt/year. Arguments, generated by professional climate change skeptics, that Antarctica is gaining ice are thus off-target: the continent is losing land-ice, whose melting leads to sea-level rise. Fluctuations in sea-ice (which is what these skeptics have seized upon) do not affect sea-levels: indeed, they are to be expected as the input of massive amounts of fresh water from melting land-ice dilutes surface sea-water salinity, thereby raising its freezing-point and promoting sea-ice growth - for the time being.

In conclusion, we are already into a world where the long-term survival of parts of the ice-sheets is not favoured: the further towards the high hundreds of ppm CO2 we head the further we head into a world that does not favour any Antarctic land-ice, although of course to melt all that ice would likely take many centuries. That is no comfort when considering the ecological, humanitarian and economic effects of a steady sea-level rise of several tens of metres over that time, submerging all of our coastal cities one after another. If that's not worth making a fuss about then what is?

Paper under discussion in this post:

Pagani, M., Huber, M., Liu, Z., Bohaty, S.M., Henderiks, J., Sijp, W., Krishnan, S. & DeConto, R.M. (2011): The Role of Carbon Dioxide During the Onset of Antarctic Glaciation. Science, 334, 1261-1264

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Comments 1 to 37:

  1. "The fall in CO2 concentration was from 1000-1200 ppm down to 600-700ppm, at which point it was cool enough to allow glaciers to start to form." If this result holds up to scrutiny, this could easily be a landmark paper, calling for a re-evaluation of the 'CO2 lags temperature' myth. This comment from lead author Pagani is especially relevant: "The onset of Antarctic ice is the mother of all climate 'tipping points,'" he said. "Recognizing the primary role carbon dioxide change played in altering global climate is a fundamentally important observation." An important prior work by the second author is Huber et al 2004: Considering that (1) Antarctica was not kept warm by subtropical currents, (2) evidence of above-freezing Eocene continental interior winter temperatures occurs in high latitudes of both hemispheres, and (3) substantial carbon cycle changes coincide with the Eocene-Oligocene boundary, the most parsimonious explanation of the enigmatic global warmth of Eocene climate was that it was caused by atmospheric mechanisms, such as greenhouse gas radiative forcing ... and potential feedbacks ... . The implication is that changes in greenhouse gas concentrations and subsequent feedbacks were primarily responsible for climatic deterioration into the Oligocene.
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  2. If a drop in CO2 down to 600-700 ppm caused the onset of Antarctic glaciation, that is additional evidence we can use to rebut the skeptic’s argument that the CO2 effect is saturated.
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  3. Another important point to take away (and one that leapt out at me): "1000-1200ppm down to 600-700ppm in just three million years" What that says to me is that if we crank up atmospheric concentrations to ~1000ppm, it's going to take millions of years to get back to anything resembling 'normal' levels for all of human history. I.e. if we screw up the climate, it's darn well gonna stay screwed up, probably for longer than humans will exist on the planet...
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  4. Doc Snow @ 1 Yes, a difference between the threshold for glaciation from the threshold for deglaciation would fit the definition of historesis. Differing history and/or internal state can vary the threshold values. I too would be keen to learn more about this.
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  5. The research led to the understanding that it was in fact very difficult to glean useful reconstructions from high southern latitudes (i.e. the seas around Antarctica) How robust is that statement? I can imagine the cries of the deniers saying that ignoring the data from the high latitudes is cherry-picking. Am I right in assuming that the high-latitude data gives indeterminate results, rather than being emphatically in the 'wrong' direction?
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  6. Also of interest, and free access, the DeConto & Pollard 2002 paper for a model-based perspective linking Antarctica glaciation to CO2 change.
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  7. In the conclusion of the OT "In conclusion, we are already into a world where the long-term survival of parts of the ice-sheets is not favoured: the further towards the high hundreds of ppm CO2 we head the further we head into a world that does not favour any Antarctic land-ice, although of course to melt all that ice would likely take many centuries. That is no comfort when considering the ecological, humanitarian and economic effects of a steady sea-level rise of several tens of metres over that time, submerging all of our coastal cities one after another. If that's not worth making a fuss about then what is?" From this web page. "Ed Ring says: April 30, 2008 at 2:43 pm A net loss of 150 gigatons against a mass of 20.5 million gigatons is nothing." Antartica has a measured loss of ice at the rate of 150 gigatons year. This rate would increase with warming but at this current rate it would take around 137,000 years to melt the 20.5 million gigatons of ice frozen on that contintent. If mankind cannot find a way to move their cities or figure out ways to stop sea water leakage, then we truly are a dumb race and time for our extinction. Seems like we would be a really stupid people to watch this very slow rising water and just wait until the city streets are ten meters under water before taking some intelligent steps to avert disaster.
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    Moderator Response: Your misconception is addressed by the post Greenland has only lost a tiny fraction of its ice mass.
  8. You presume SLR due to ice sheet losses will be linear. Much evidence exists to the contrary.
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  9. Norman#8 "This rate would increase with warming but at this current rate it would take... " If you expect ice loss rate to increase, why do you even mention this extrapolation of the current rate? And if ice loss increases, why do you expect SLR to have the same rate? Move cities? Stop sea water leakage? Are you aware that salt water inundation is already doing damage? And who is Ed Ring?
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  10. Norman: Since the pre-industrial concentration of CO2 in the atmosphere during the Holocene interglacial was approx. 280 ppm, suggesting we are "saving" the ecosystems by burning fossil fuels and preventing CO2 concentrations from falling below 150 ppm is IMO complete nonsense. All: You might find this press conference from this year's AGU fall meeting of interest (hat tip to the comment threads at Deltoid where someone posted it). It's topical to this thread.
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  11. Norman, who said anything about reducing CO2 levels that low? Are you also aware of the amount that ice loss is accelerating? See the relevant articles at SkS. Entire, very large, ice sheets melted within ~<10,000 years at the end of the last glacial period. Within 2-3000 years between 11,000 and 8,000 years ago sea level rose ~60m. Meltwater Pulse 1A saw a rise of 20m in 500 years. What this shows is that large sea level rises in short periods of time are possible, given the right forcing and conditions, and that given appropriate forcing, deglaciation does not take over 100,000 years. Note that prior to Meltwater Pulse 1A, sea levels rose 20m in 6000 years, or ~3.3mm/yr. Coincidentally that's close to the rate sea level is rising today. It looks like abruptly around 15ka, sea levels began to rise rapidly. I'm not suggesting this will happen to Greenland or the WAIS, but rapid deglaciation and sea level rises have precedents, quite apart from the accelerations we observe today. source: globalwarmingart
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  12. Doc Snow, indeed there may be a hysteresis factor especially WRT the East Antarctica ice-sheet; however there are already signs that the West Antarctica and Antarctic Peninsula ice is in trouble. A reasonable analogy is switching off and opening a deep-freeze: for a while nothing much happens, then a slow melt starts, it speeds up and all of a sudden everything is defrosted! Doug H: "How robust is that statement? I can imagine the cries of the deniers saying that ignoring the data from the high latitudes is cherry-picking. Am I right in assuming that the high-latitude data gives indeterminate results, rather than being emphatically in the 'wrong' direction?" That has certainly been a response in some quarters, but one which ignores how proxies are developed: quite crude to begin with when discovered, they are refined over time as weaknesses are identified and addressed. The high-latitude samples have problems WRT environmental conditions, especially nutrient levels: thus they give results inconsistent with other proxies and modelling. In time, these problems themselves may be ironed-out. However, if a problem is identified in a sampling area that introduces a skew, then until the problem is rectified that area is best left out, because of the "garbage in, garbage out" issue. Better to drive a vehicle with a four-cylinder engine in which all is well than to drive a V8 with worn-out piston-rings in three of the cylinders! Cheers - John
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  13. Skywatcher, indeed: Meltwater pulse 1A shows us what is possible: 3.3m/century or ~ ten times the current rate, already projected to increase through the current century under a BAU scenario. Cheers - John
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  14. 1. Doc Snow: hysteresis sounds pretty likely. We expect heating or cooling and subsequent ice sheet changes to respond to TOTAL heating. Cooling down, we have a lower albedo because there's no ice. This should mean more heating from the Sun, so you need to reduce CO2 further to get enough cooling to trigger ice sheet formation. When you have an ice sheet, then higher albedo = more cooling = you need more CO2 to trigger the end of the ice sheet. But when it comes, the heating is faster. Of course, that's just local albedo feedback. I suspect the answer is lots more complicated and might even be completely different!
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  15. Norman, on the issue of taking actions to deal with sea level rise... consider the possibility, I know it may seem remote, that some people might argue that no action need be taken because sea levels aren't really rising, or will rise so slowly that there will be plenty of time to act. Sure, if we made every effort to deal with the problems caused by sea level rise we could probably do so. But, in my hypothetical, there are these people who deny it is happening and oppose any action to address it. If you want to call such people "really stupid" and deserving of extinction, well I think that's a bit harsh, but I won't try to dissuade you from your opinion.
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  16. C B Dunkerson writes at 23:44 PM on the 16th of December, 2011: "But, in my hypothetical, there are these people who deny it is happening and oppose any action to address it." Not so hypothetical: To paraphrase Haldane: People are not only more stupid than we imagine, they are more stupid than we can imagine. sidd
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  17. So what caused the CO2 to drop? Could it be the formation of the Southern Ocean that was taking place as the Antarctic Circumpolar Circulation started at the same time. Interesting that a 10 C drop in the 100m of the Southern Ocean would be enough to absorb large amounts of CO2 and cause the levels to drop.
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    [DB] Link to non-science blog snipped.

  18. There is a more likely candidate for the faster melting than GHG alone and that would be the dumping of the heat which will be transported to the poles. Not only dumping but also the release of methane and its reduction in the air to carbondioxide + water will release quite an amount of heat. If permafrost is escaping lots of ch4 that might be quite a good reason why the north cap is melting faster than the south pole.
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  19. TIS @18, a more likely cause of the drop in CO2 was the collision between the Indian Sub-continent and Asia which was occurring at that time. The consequent raising of the Himalayas resulted in massive erosion, a process which absorbs CO2. The cooling of the Ocean due to the opening of the Magellan straight (the gap between Antarctica and Australia) would also have resulted in increased absorption of CO2 into the ocean, but based on the correlation between temperature and CO2 levels observed in the Vostock Ice Core, that would have reduced atmospheric CO2 levels by around 150 to 250 ppmv, and could not account for all, or even most of the 700 - 1200 ppmv reduction observed.
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  20. Ger - the height of South pole and the circumpolar current would be the major factors in the difference.
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  21. 12, Ger, I think you have cause and effect reversed. The methane is being released because temperatures are rising and things are thawing, not vice versa.
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  22. Tom #20, The Strait of Magellan is inland of Argentina. It is the Drake Passage that is the land between South America and Antarctica. That is what opened up at the same time that Antarctica froze over and the CO2 dropped. Hypothesis #1: The geography of the Earth changes the ocean currents and this causes: Southern Ocean to Cool and Antarctica to start freezing. This in turn lowers the level of CO2 in the atmosphere because cold water absorbs more CO2 than warm water. The snippet that was politely cut showed the science of solubility of CO2 in water, it was just inconvenient science. Hypothesis #2: The level of CO2 drops for no particular reason, but this causes everything BUT the geographical change and the ocean current change..... The science on this subject is very clear. There is every scientific reason to dismiss claims that CO2 drop triggered the temperature drop of the Southern Ocean and Antarctica.
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    [DB] "The snippet that was politely cut showed the science of solubility of CO2 in water, it was just inconvenient science."

    Actually it was just giving publicity to a non-science website, as was politely delineated before.

    "The level of CO2 drops for no particular reason"

    Now you posit magic.

  23. TIS, The opening of the Drake Passage was a progressive process: it is by no means certain that it was sufficiently deep (i.e. oceanic) at the time this piece is discussing: however as I note in the piece, it is believed to have been of importance later in the Oligocene and Miocene, which is when the ice-sheets themselves developed. By that point you have three things at work, namely thermal isolation due to ocean currents, steadily-increased albedo and decreasing CO2 - but not at the same rate as in the massive drawdown in the latest Eocene. Interestingly, orogenesis has been linked to other sudden coolings - e.g. the Hirnantian glaciation and the Taconic Orogeny that came a little before. The isotopic signature for weathering then is one of the biggest in the entire Phanerozoic. Similar studies would be useful in this case and I have suggested this to the paper's authors. Cheers - John
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  24. Here's a fairly recent summary of research into the development of the ACC following the opening and then deepening of the Drake Passage: I'll just quote the final paragraph of the discussion therein: "While an open marine gateway between South America and Antarctica is required for the development of the ACC, evidence of a zenith of Eocene tectonic activity in the Scotia arc alone is insufficient to constrain the ACC onset that may have caused or contributed to Oi-1 glaciation because of the significant sill depths required to allow such circulation (Livermore et al., 2007). However, the ~5 Myr lag between the timing of the provenance shift recorded herein (ca. 39 Ma) and the Oi-1 glaciation (ca. 34 Ma) may have allowed time for the development of sufficient sill depths. The sediment provenance data and kinematic interpretations presented herein are temporally consistent with independent evidence of a middle Eocene onset of Drake Passage opening (Scher and Martin, 2006). Whether sufficient opening and associated subsidence occurred to allow ACC development in time for the Oi-1 glaciation remains to be seen. Considering the ACC's dominant influence upon modern global ocean circulation and temperature distribution (Barker, 2001), constraining the timing and nature of its onset remains vitally important. It appears that at least one part of the gateway's development involved the Eocene kinematics of the Fuegian Andes." Note the caution expressed in this. As ever, more research is needed! Yes, it is also possible that increased oceanic CO2 drawdown was a factor - but until we have a more definitive timing for the onset of the full ACC then it remains a "possible". However, what we DO know is that at full-on Eocene CO2 levels, i.e. 1000-1500ppm, the planet is ice-free, so we ought not to be heading anywhere in that direction! Cheers - John
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  25. I'll also quote from the end of Anderson et al 2011 (link in text of post above): "Whereas changing atmospheric CO2 concentrations undoubt- edly contributed to the abrupt cooling that occurred across the Eocene/Oligocene transition, continued cooling and gla- cial expansion in the AP region is best explained by gradual de- velopment of ocean passages, extended isolation of the continent, and development and expansion of the Circum Antarctic Cur- rent. The formation of a complete circum-Antarctic passage has been a continuous process spanning the past 50 million years. That's as up-to-date as I can find.... Cheers - John
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  26. TIS#23: "The science on this subject is very clear. " So a hand wave is sufficient to dismiss the content of the paper that is the subject of this post. Perhaps that is because it is necessary to be dismissive of this particular point, rather than be truly skeptical. As noted here, this paper and prior work by the authors suggest that if CO2 drove temperature drop, it becomes very difficult to avoid a re-examination of the CO2-lags-temperature meme. A meme that is vital to pseudo-skeptic posturing over 'its not CO2.' BTW, if you look into that, the uncertainty in dating CO2 in ice cores is quite large (an oft-quoted lag of 800+/-600 years ought to make any true skeptic curious). Or is 75% uncertainty good enough for what passes as 'very clear science' in the world of pseudo-skepticism?
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  27. TIS @23, thankyou for the correction on the Drake Passage (and serves me right for relying on my faulty memory). With regard to your two hypotheses, the first Antarctic glaciation occurred 34 million years ago, at which time CO2 levels had fallen from less than 1000 ppmv. At that time, the Drake Passage looked like this: Although there is some evidence of Pacific water flowing through to the Atlantic at that time, there is no evidence it was anything but surface water passing through narrow channels. In the words of Lyle et al, 2007:
    "The Antarctic Circumpolar Current (ACC) is a key feature of the Southern Ocean. Its development may have helped cool Antarctica and initiate Southern Hemisphere glaciation. The deep circulation of the ACC must have been established after both the Tasman gateway (between Antarctica and Australia) and the Drake Passage (between South America and Antarctica) opened. However, estimates for ACC initiation range over 20 m.y., from the middle Eocene to early Miocene. A new piston core of upper Oligocene to Holocene sediments from the South Pacific has allowed us to delimit the formation of the ACC to the late Oligocene (ca. 25–23 Ma). Upper Oligocene, current-worked sediments and a hiatus to the upper Miocene result from the beginning of the modern ACC flow; i.e., when strong currents and mixing throughout the water column were established. Previously published Nd isotope data date the first intrusion of Pacific water into the Atlantic much earlier. The discrepancy with our results can be reconciled by the different methods measuring different flow regimes. Tracer methods such as Nd are sensitive to relatively small and shallow incursions of water, whereas pelagic erosional regimes require vigorous deep flow."
    (My emphasis). By 25-23 million years ago, the first time for which we have firm evidence of a circumpolar current, the Drake Passage looked like this: And here is the full progression of its development: So, on the evidence, Hypothesis 1 is that the geography of the Earth changes so as to cause a change in ocean currents, which causes the CO2 levels in the atmosphere to decline 10 million years before hand, by amounts which exceed the capacity of oceanic temperature changes to account for. Personally, I prefer the real hypothesis two, not your strawman version, ie, that mountain building in the Rockies and Himalayas results in increased erosion, which results in increased sequestration of CO2, thus drawing down CO2 levels over a period of millions of years.
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  28. "The science on this subject is very clear. There is every scientific reason to dismiss claims that CO2 drop triggered the temperature drop of the Southern Ocean and Antarctica. Well I find the "science" somewhat lacking. Faced with Hypothesis 1 - development of ACC caused sufficient ocean cooling to draw-down CO2; and Hypothesis 2 - increased weathering from orogenesis caused CO2 drawdown, the scientific approach would be look for data that could separate them. Off the top of my head, I can think of a couple of things to check. Firstly, as others have done, there is the question of timing. Which event preceded the CO2 drop? Second, if ocean cooling was cause of CO2 drop, then do ocean temperature proxies support sufficient cooling to match the CO2 draw down? Since the same core would yield both the CO2 proxy and temperature proxy, then dating shouldnt be a major issue. Finally, if orogenesis is the cause, then a proxy for weathering would be sediment flux into ocean basins (if you are eroding then the sediment has to go somewhere). My own databases on the basins do not have sufficient time resolution to answer this (oh for some more funding), but I would imagine that the basins are sufficiently well known to at least do a back-of-the-envelope calculation. Sadly full-blown sedflux studies are very expensive but its possible that data is there. GeoCarb III might have done this - bound to have tried it at first order anyway. Both approaches require arithmetic rather than handwaving. I would find this more convincing.
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  29. A quick look at GeoCarb revealed another tool that I had forgotten - Sr isotopes as a proxy for weathering rate. eg this paper. Quite definitely a himalayan signal here.
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  30. #23 The moderator response to "The level of CO2 drops for no particular reason" was that this is positing magic. It is unclear how magic and lack of a particular reason are equal. Isn't this how we get statistical anomalies entrenched as the truth? For example, cancer clusters that aren't real? A whole lot of quantum physics seems pretty magical, so would that not be science? Please clarify how magic and no particular reason are the same.
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  31. What the moderator is saying is that there will always be a reason for drop in CO2 - the idea that something happens without a cause is in fact magical thinking. Moreover, particularly large changes of the sort referred to can only be caused by a limited number of mechanisms, which we understand very well. In many cases we know enough to determine if those mechanisms could or couldn't be acting. I don't see how your cancer cluster example is relevant. That is simply a case of being properly rigorous in statistical analyses of data so that we don't try to explain a phenomenon that does not actually exist. Such false phenomena can actually happen for no particular reason, because they are a figment of our imagination. While quantum mechanics seems magical to the unitiated, it has proven to be very predictive - so it classifies as a particular reason for lot of stuff that happens. It would be magical thinking if we concluded instead that the phenomena explained by quantum mechanics happened for no particular reason.
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  32. @ doubtingallofit
    "The moderator response to "The level of CO2 drops for no particular reason" was that this is positing magic. It is unclear how magic and lack of a particular reason are equal."
    CO2 does not rise nor fall without causative physical reasons/mechanisms. To posit that it simply "drops for no particular reason" is to betray either a lack of knowledge about those physical, causal mechanisms (unfortunately for TIS, he is very aware of those mechanisms but conveniently omits them) or to just make a comment that is intended to waste the time of others. I.e., trolling. For more info on CO2, I recommend watching this video on why CO2 is the biggest climate control knob in Earth's history.
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  33. Causation is a fundamental tenet in science. True randomness is very hard to find. Even what is called "unforced variation" still happen because of causes. These are of fundamental importance to say weather-forecasters or ocean dynamic modellers but not so much for climate since they are energy-bound. For CO2 drop, you cannot simply expect CO2 to disappear. A process must be operating that takes it from atmosphere and changes one or more the carbon-cycle fluxes. These are all governed by physical and chemical laws. You can believe in magic but I find science more useful. Obligatory XKCD
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  34. If everything happens for a reason, then there has to be a reason for the Big Bang, a reason for the reason for the Big Bang, etc. Causation is a tenet in science, but that does not mean everything has a cause. It means we search for the cause as a function of the discipline called science. Also, the statement says "no particular reason", not no reason. Maybe wording it differently would have helped, since all attention went to that statement rather than the entire post. As for calling a belief that things can be random or without causation believing in magic, I would reply that magic requires violation of physical laws and unless there's a physical law with mathematical verification that there cannot exist a phenomena with a cause, my belief is not magic.
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    Moderator Response: [muoncounter] Your 'beliefs' are of little general interest. Please confine your commentary to the topic of this post. Off topic comments are deleted.
    Please note that posting comments here at SkS is a privilege, not a right. This privilege can be rescinded if the posting individual treats adherence to the Comments Policy as optional, rather than the mandatory condition of participating in this online forum. Please take the time to review the policy and ensure future comments are in full compliance with it. Thanks for your understanding and compliance in this matter.
  35. doubtingallofit - Lets take this carefully. I assume that you are not doubting conservation of mass - if CO2 goes down in the atmosphere this it must be absorbed somewhere else. I would contend that this absorption somewhere else must be result of a process (eg absorption into seawater, take up by vegetation) which in turn is governed by law of thermodynamics. For this process to change with cause is magic to me (a breach of physical law). Whether there is anything truly random in nature is an open question though quantum processes appear so. What is absolutely not random is chaos. Predicting what face a dice a show when thrown is extremely difficult but the result is pre-determined by the throw. With AGW what we observe in the world matches what is predicted by the science. However, you appear to be preferring to hope in some other cause (or even no cause). It seems to me that your strange belief is likely routed in fact that you do not wish to deal with the consequences of AGW being true?? You would rather hope that we are going from suffer from natural causes than having the power to do something about it? It should be pointed out that EVEN if warming from natural causes we could reduce the impact be reducing our emissions.
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  36. "If everything happens for a reason, then there has to be a reason for .... (absolutely everything) ... and unless there's a physical law with mathematical verification that there cannot exist a phenomena with a cause ... " Just because there's a reason for everything, it doesn't necessarily follow that we have yet identified that reason or set of reasons. There are millions of issues in science that we don't even bother with - even though we know there are 'reasons' for them - because what we already know is good enough. Or often, in the case of earth sciences especially, we don't have equipment like enough satellites or deep ocean observatories or seismological sensors to gather and process the mountains of details we'd need to get things down to the umpteenth decimal place or the precise day and hour of expected earthquakes and the like. The 'reason' for warming of the ocean and atmosphere is, in fact, pretty straightforward. We do know how greenhouse gases work and we do know how much of them are emitted and absorbed. Physics plus other arithmetic tells us what kind of events to expect as a result of the net increase. The fact that we can't tell the precise place, date and time of floods or wildfires is irrelevant. (Just like we don't know which particular smokers will suffer which particular illnesses - or none at all for the lucky ones. What we do know is that a community with more smokers suffers more smoking related illnesses. And that's all we need to know to take action.)
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  37. My last comment was off topic, I guess. I am sorry I don't understand your rules, but my degree was in psychology/philosophy and I don't alway understand this stuff. Perhaps I should find a site that has more tolerance for lack of understanding.
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    [DB] "I don't understand your rules"

    Comments constructed to comply with the Comments Policy and also on-topic for the thread on which they are placed receive no moderation.

    "I don't alway understand this stuff"

    Understandable.  There was a time when I was just starting out in this field & knew little about it. 

    There are over 4,700 threads here at SkS on virtually every conceivable topic related to climate science.  If you have questions on things related to climate science, please use the Search function in the upper left corner of every page here to find a related post.  If you still have questions after reading it, place those questions there.

    "Perhaps I should find a site that has more tolerance for lack of understanding."

    The dialogue in the comments threads here at SkS rely upon science and peer-reviewed evidence to support positions.  Comments lacking substance or citations to said peer-reviewed evidence amount to opinion.  As such, little attention to them is typically given unless they also do not comply with the Comments Policy.

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