There are other theories around the PETM and the details remain uncertain. However it can be said with some confidence that the PETM started with a warming due to CO2 emissions that was then amplified by a low carbon13 source, methane seeming the least problematic option. In most areas of science one can afford to remain agnostic about issues in doubt, however Anthropogenic Global Warming (AGW) and the risks it poses, methane hydrate being just one, do not allow for a 'wait and see' approach. So my working assumption is that the PETM was indeed a warming amplified by methane, with marine hydrates being a likely candidate for a major player in the amplification.
Working on the assumption that methane emissions were the amplifying factor, the next issue is whether the geological record can tell us the speed of the emissions. McInerney & Wing's 2011 Review paper states that the onset of the negative excursion of carbon13 took less than 10,000 years, that being the time from the non-depleted samples to maximum depletion of carbon13. The onset in continental cores took an estimated 8 to 23,000 years. Whilst such timescales are geologically rapid, in human timescales this would be more of a chronic release of methane. Schmidt & Shindell investigate the PETM using an atmospheric chemistry model and climate model. They find that 0.3Gt (Gt=1000,000,000 tons) per year most closely matches the forcing that tallies with the geological record, and that a larger emission scenario of 3Gt per year over 500 years produces a higher forcing than is justified by geological observations. This does not mean that the PETM was not catastrophic, nor does it mean humanity is unlikely to invoke this feedback by emissions of CO2. Furthermore as I've argued in the previous post there is good reason to expect transient pulses of methane as part of the process of their dissolution. However overall the PETM was a catastrophe that unfolded at a slower pace than some public commentary on this issue might lead one to expect.
Figure 1c of Hansen & Sato 2011. Estimated deep ocean temperature changes for the last 500k years.
This figure is presented to illustrate the Holsteinian and Eemian interglacials, which were warmer than the Holocene, the interglacial from which the current epoch, the Anthropocene, has risen. It is noted in the text of Hansen & Sato that the temperature changes of the deep oceans were only around two thirds as large as global temperature changes. To that must then be added polar amplification, the physics for which would apply in the past as now, so there is good reason to suppose that the Arctic was warmer still.
Jakobsson et al examine beach records from the Holocene. They found that between 8,000 and 6,000 years before present Arctic sea-ice was substantially lower than at present. They show that July 65degN insolation was around 10% higher than at present. Funder et al examine beach ridges as indicative of sea-ice on the northern coast of Greenland, they find beach ridges that are indicative of the presence of open ocean where there are ice shelves at present. They also note that temperatures in the north of Greenland were around 2 to 4degC warmer than at present. Dyck et al find that the early holocene is dominated by a positive Arctic Oscillation (AO) mode, with enhanced first year sea-ice growth in the seas off Siberia due to an enhanced transport of sea-ice from the east to the west Arctic (enhanced transpolar drift).
The recent trend in Arctic warming. Provided for context. (If this is your image please let me know as I've lost the attribution.)
The Holocene Thermal Maxiumum (HTM) was a period of warming following the last ice age after the start of the holocene, the warmth was driven by higher summer insolation than at present. Kaufman et al found that the West Arctic (mainly Arctic North America) was only 2degC warmer than the 20th century average, which would make it only marginally warmer than present Arctic temperatures. However the pattern is complex with different areas of the region studied having different times of HTM, and different levels of warming. Renssen et al carried out a modelling study and found that Arctic temperatures were up to an average around of around 2degC, with the Laurentide ice-sheet reducing that by just under 0.5degC. However the Laurentide ice sheet was over North America and as Hansen & Lebedef have shown the climatic scale of global warming is robust to up to around 1200km, so it seems reasonable to suggest that the Laurentide ice sheet had little effect on Siberian temperatures. With regards the HTM in the Russian Arctic sector, I can't find much at all. Wikipedia states "The Holocene Climate Optimum warm event consisted of increases of up to 4 °C near the North Pole (in one study, winter warming of 3 to 9 °C and summer of 2 to 6 °C in northern central Siberia)". But the source of that claim is a Russian published paper that didn't come up in my searches and has a very brief abstract. So it is possible I've missed important Russian literature.
Even though temperatures show an equivocal picture, with temperatures being at least around current Arctic levels but perhaps some degrees warmer, sea-ice proxy studies show a significantly reduced Arctic sea-ice pack. It has already been observed that since 2007 end of summer sea surface temperatures in areas of open ocean are several degrees higher than previously in areas of open ocean, and the the ocean so exposed to sunlight is warmed (ice albedo effect), that warming being mixed down into the ocean column. The physics behind this are general, so would have applied during the HTM. Whereas the recent warming may only have come close to matching HTM temperatures in the last decade, the sea-ice state was lower, and crucially during the HTM the warmth (however warm it was) and reduced sea-ice lasted many centuries, enough time for warming of the Arctic Ocean and to penetrate sediments. Yet we don't see catastrophic releases of methane at during the HTM. This point also applies to the Holsteinian and Eemian interglacials. As they're much older periods data is sparse, however Hansen cites temperatures of the deep ocean as having been warmer than the present.
It is often asserted in public discussion that Arctic sea-ice has a tipping point that will drive a rapid transition to a seasonally sea-ice free state. Such assertions have been made in association with claims of an imminent destabilisation of methane hydrates on the ESAS. However these arguments seem to fall before the evidence, because evidence from the past shows less sea-ice and higher temperatures with substantially greater summer solar forcing produced neither a rapid transition to a stable seasonally sea-ice free state, nor a massive (enough to substantially raise atmospheric methane) venting of methane from the ESAS. One possible explanation for this apparent problem with regards methane and the ESAS is the history of methane hydrates on the ESAS.
Recently Andy Revkin has interviewed Dr Shakhova and Dr Semiletov, in that interview he posts comments from them about modelling research into the effects of post glacial inundation of the ESAS, together with a response from the leader of the team who conducted the modelling study; Dr Dmitrenko. Firstly, elsewhere I've seen the Dmitrenko paper queried on the basis of citation numbers, that is not a sound method, science is about understanding, it is not a popularity contest. Dmitrenko et al model permafrost subject to the recent changes in the Arctic and find that the recent warming (last 30 years) has only lowered the hydrate stability zone by 1m, they conclude that the emissions of methane from the ESAS is due to a long process of warming initiated by the inundation of the ESAS by sea level rises following the last glacial. It is claimed by Semiletov & Shakhova that Dmitrenko et al assumed a thaw point of zero degrees (C), whereas observations show hydrate melting at below zero. This is important because the warming in that area is warming up from substantially below zero. Dmitrenko responds to this criticism stating unequivocally that the assessment of the model is wrong and that the model does simulate unfrozen sediments at below zero degrees, rounding off by saying that their paper had not been carefully read. This is a strong response that can be checked with reference to the paper (indeed it includes a quote from the paper). So I do not think Shakhova and Semiletov are correct in their criticism of Dmitrenko et al.
Dmitrenko et al raises a crucial issue. Shakhova, Semiletov and their colleagues working on the ESAS are involved in crucial cutting edge work on methane emissions from that region, and the possibility of increased emissions in future. However because their work is cutting edge, the observations are new and lack longer term context. What Dmitrenko et al's study suggests is that the current hydrate melt and consequent methane emissions are the result of a longer term process, so these emissions may have been going on for millenia, unobserved. I am reminded of the example of Bryden et al and observations of the Meridional Overturning Circulation (MOC) in the North Atlantic. Bryden's initial results appeared to show a large reduction in the MOC, leading some excitable commentators to declare that the MOC (otherwise known as the Thermohaline Circulation) was shutting down. At the time RealClimate expressed doubt about the slowing of the MOC, and were subsequently proven to be correct, see here for related posts at RC.
Likewise it is possible that the observations of methane emissions from the ESAS are sampling something that has been going on for some time, possibly millenia. With spatially limited observations over a short timescale it is no surprise that the observations don't yet answer the question of whether that is the case. However these observations are crucial because they at least present the start of a series of data that will reveal whether what is being seen is intensifying as the process of Arctic warming, driven by AGW, proceeds.
Semiletov et al 2012, states that:
Our measurements of CH4 taken in 1994–9 and 2003–10 over the ESAS demonstrate that the system is in a destabilisation period (Semiletov 1999a, Shakhova et al 2010a, 2010b, 2010c).Shakova et al 2010a is a paper that uses seven scenarios of methane emissions, different amounts over short and long timescales, and calculates the impacts of these on global average temperature and radiative forcing. Shakova et al 2010b & c contain more relevant detail to the question of whether the ESAS is destabilised, they can be taken to support the hypothesis that the ESAS is destabilised. Shakhova & Semiletov 2006 find that in 2003 the area weighted methane flux was 4.86 X 10^10 g/cmsq/hour and in 2004 it was 3.02 X 10^10 g/cmsq/hour, supporting the large interannual variability noted elsewhere. So whilst the research clearly shows that methane hydrates are destabilising this does not determine whether this is an exceptional condition unique to recent decades and due to AGW. Dmitrenko et al's findings that the destabilisation is a long term situation with its origins in the inundation and massive warming at the start of the holocene are not rejected by the above stated references.
All this said, the work of Shakhova, Semiletov, and colleagues is vital, and I've been impressed by what I've read of their work. Together with observations from ground stations and satellite, their work provides the first basis of a marine methane hydrate early warning system. When the currently destabilised methane deposits on the ESAS start to join their land counterparts in a substantially increasing trend of emission, the groundwork these scientists are laying will be crucial. Significant funding for their efforts can be viewed as a prudent insurance policy for the planet, especially as humanity seems hell-bent on Plan A (Business as Usual).
Dmitrenko et al, 2011, "Recent changes in shelf hydrography in the Siberian Arctic: Potential for subsea permafrost instability."
Dyck et al, 2010, "Arctic sea-ice cover from the early Holocene: the role of atmospheric circulation patterns."
Funder et al, 2011, "A 10,000-Year Record of Arctic Ocean Sea-Ice Variability—View from the Beach."
Hansen & Sato, 2011, "Paleoclimate Implications for Human-Made Climate Change."
Jakobsson et al, 2010, "New insights on Arctic Quaternary climate variability from palaeo-records and numerical modelling."
Kaufman et al, 2004, "Holocene thermal maximum in the western Arctic (0–180 W)"
Francis et al, 2006, "Interglacial and Holocene temperature reconstructions based on midge remains in sediments of two lakes from Baffin Island, Nunavut, Arctic Canada."
McInerney & Wing, 2011, "The Paleocene-Eocene Thermal Maximum: A Perturbation of Carbon Cycle, Climate, and Biosphere with Implications for the Future."
Renssen et al, 2004, "Simulating the Holocene climate evolution at northern high latitudes using a coupled atmosphere-sea ice-ocean-vegetation model."
Shakhova et al 2010a "Predicted methane emission on the East Siberian shelf."
Shakhova et al 2010b "Geochemical and geophysical evidence of methane release from the inner East Siberian Shelf."
Shakhova et al 2010c "Extensive methane venting to the atmosphere from sediments of the East Siberian Arctic shelf"