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CO2 Currently Rising Faster Than The PETM Extinction Event

Posted on 17 June 2011 by Rob Painting

The Paleocene-Eocene Thermal Maximum (PETM) was a period of natural global warming that took place almost 56 million years ago.  It came at a time when the atmospheric concentration of CO2 was already higher than today, and global temperatures also much warmer. The PETM warming was a roughly 200,000-year long event where global temperatures rose by a further 6–8°C, and is thought to have been caused by a massive injection of CO2 into the atmosphere.

The PETM: A great carbon mystery 

The existence of the PETM was first noticed when analysis of marine sediment cores, retrieved near Antarctica, revealed an abrupt change in the ratios of carbon-12 & carbon-13 isotopes. Land and marine plants discriminate against the heavier carbon-13 isotope when they are photosynthesizing, and because the sediments are depleted in that isotope, the carbon isotope ratios indicate that the CO2 was organic in origin. This abrupt PETM change is shown in the right-hand dip (@55.5 million years ago) in Figure 1 below: 

Figure 1  PETM changes in the carbon isotopic composition of carbonate. The upper records are from Maud Rise in the southern Atlantic Ocean (Kennett and Stott 1991). The lower record is from the South Island, New Zealand (Nicolo 2007). BFEE = benthic foraminifera extinction events discussed in Nicolo (2007). Figure from (Dickens 2009)

A truly colossal amount of CO2 must have found its way into the atmosphere to create the observed fall in carbon-13 isotopes (called a negative carbon isotope excursion, or CIE, in the literature). Since the initial discovery, the CIE has been found in paleo records from around the world (the smaller CIE's in figure 1, show abrupt warming periods during the Eocene).  

The PETM pulse of CO2 has been linked with acidification of the deep ocean and the extinction of tiny marine life called forams (foraminifera), and proved to be a difficult time for coral reefs. It was also a time of rapid change in land plants and animals, with a quick turnover of species and large migrations, although extinctions were limited.

No smoking gun 

Hundreds of scientific papers have been published on the PETM, but because of the scarcity of paleo-data from this time, there has been no clear scientific agreement over what initiated this warming, or where all the CO2 came from.  

A number of researchers have converged on methane clathrates in deep sea sediments as a possible culprit. Methane clathrates are molecules of methane frozen in a cage of water molecules, which are buried in sediments at the bottom of the ocean. If heated, or de-pressurized, they can quickly break down (oxidize) to CO2, either in the ocean or in the air. An attraction of methane hydrates as the source of CO2, is that methane is highly depleted of carbon-13, and therefore much less of it (than CO2) need be released to match the carbon isotopes shifts observed.

A number of other sources have also been examined too, such as volcanic activity disrupting organic sediments, and methane from permafrost on Antarctica (when it was much warmer), but the case is very much still open.

Abrupt is how long?

Examination of the sedimentary records shows an abrupt increase in CO2, followed by a rapid drawdown (perhaps a rise in silicate weathering), then a long tail of recovery as the PETM warming interval comes to an end. The deep sea cores are affected by the ocean acidification event, which dissolved carbonates in the sediment cores, so it has been difficult to tie down exactly how long this release of CO2 lasted. Previous estimates have been between 10,000–20,000 years.

A recently published study, Cui 2011, indicates that this splurge of CO2 was at the longer end of estimates - around 19,000 years. Cui and co-authors loooked at a sediment core that had been drilled at Spitsbergen, Norway, in a shallow coastal marine environment. Unlike other cores which can be highly condensed, the section from Spitsbergen, covering the initial PETM warming, is some 70 metres long, which provided a more detailed analysis.

Cui 2011 fed the data, collected from the sediment core, into a bare-bones climate model (GENIE), and then ran the model under two scenarios: one with organic carbon as the source, and one with methane, to find a best fit with the core data. They found that the maximum rates of CO2 release that match the PETM data are much lower than present rates observed today from fossil fuel burning.

Figure 2  Model results of the PETM carbon release rate and cumulative amount of carbon added versus time from the onset of the CIE  (age model is from Charles 2011). a. carbon-13 atm that we used to force GENIE. b. model results of the PETM carbon release rate. c. Model results of the cumulative amount of carbon added. d. Model results of the PETM atmospheric pCO2. e. Model results of the PETM global average temperature (°C). The two best-?t simulations are shown in b–e: (1) Methane (CH4) simulation (black solid line); (2) Organic carbon simulation (red dotted line).

The authors find that the maximum PETM rate of emission for organic carbon as the source is equivalent to 6.2 billion tonnes of CO2 per year, and for methane as the source, 1.1 billion tonnes of CO2 per year. For comparison: 2010 human-carbon emissions were 30.6 billion tonnes. So if organic carbon was the source, current emissions are almost 5 times faster than the PETM, and if methane, current emissions are rising 27 times faster.  

With the available data, it wasn't possible for the authors to determine which scenario was the more likely; however, many lines of research indicate that the massive pulse of CO2 was likely to have come from multiple sources. Therefore the above rates can be thought of as the most likely range of values. 

Lessons from a previous global warming event

The PETM took place at a point in Earth's development when the climate was very different than today. It's important to stress that none of the preceding discussion implies that direct and complete comparisons can be made between the Earth climate of today, and the Earth climate of 56 million years ago. Much has changed since then, such as the layout of the continents, and the development of major mountain chains such at the Himalayas and Andes, the growth of major ice sheets, major cooling of the deep ocean and the poles, slight warming of the Sun, and changing Earth-Sun orbital characteristics, all of which greatly alter global circulations and therefore climate. 

But now that we humans have embarked on a global warming experiment, there are some useful lessons from the past:

  • Ocean acidification (of the deep sea at least) can occur even under conditions of CO2 release much slower than today.

Whether the plants and animals upon which humans depend can survive the present rapidly changing environment remains to be seen.

Recommended reading: Sks post - Wakening The Kraken 

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Comments 51 to 65 out of 65:

  1. Rob, Laughing ... one of my favorite movies. The coal argument is an interesting one, but I think very much a "red herring" considering mass balance. The total amount of coal in strata of all ages is <20,000 Gt (Höök et al., Fuel, 89, 3546-3558, 2010). So, I find it difficult to suggest (without a long path through the chocolate factory) that the little bit of coal preserved in late Paleocene rocks, while greater than during some time intervals, is somehow meaningful. Received the email from John Cook and will pass along the paper and figure as it was originally posted on the EGU "open access" journal web site. I don't think there are any copyright issues this way (the final paper is slightly different, because of some corrections). Will be fun to see what people think. Jerry
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  2. 48, jerryd,
    With Lee, a classic exchange: "Lee, this is really hard to imagine; Jerry, you need to imagine harder."
    This is a fabulous quote, and maybe should be put on a poster for all scientists, as well as anyone interested in climate science -- since both sentiments are keys to solving scientific problems and riddling our way towards the truths, and at the same time doing so in a congenial spirit of adversarial-cooperation. Interestingly, the climate debate is replete with this exact scenario and pair of opposing behaviors, with each component present in due proportion (understand the boundaries, and find the occasional, unexpected ways around them).
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  3. jerryd - "It's from a paper that should come out in the next month or so in Climates of the Past. Maybe there is a way I can send the figure and paper to this site?" Anything you can point us to, such as a link to the paper, would be great, Jerry. Speaking just for myself, I've found this discussion extremely illuminating! It's always great to hear from someone working with primary data. I had been thinking about the "Antarctica peat" idea, but I suspect you're quite right about the mass balance not supporting that as a major contributor.
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  4. KR - To be honest, this has been very illuminating for me also. I had never heard of "Skeptical Science" before a few days ago when I accidentally clicked Rob's article when searching for something else. I discuss and argue these things with my friends and colleagues all the time, but didn't really think anyone outside our small circle was terribly interested. It's not like the topic will show up on Letterman, although Lee has an article coming out in Scientific American shortly, so who knows ... I think you can access my initial submission (and all the comments, including the imagine harder quote from Lee) as follows: 1. 2. click "most commented papers" 3. third one down I've also sent a copy to John (per previous post) and maybe we can set this into some sort of tutorial for everyone to comment on. Jerry
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  5. The paper which includes discussion on methane from sedimentary basins is Kroeger, di Primio and Horsfield 2011. The reservoir dwarfs other carbon sources.
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  6. The amount of papers on the PETM is actually “huge”, but the latest should not be overlooked. So I am surprised by the lack of work cited by scaddenp. There are interesting observations: “The mass of organic carbon in sedimentary basins amounts to a staggering 1016 t, dwarfing the mass contained in coal, oil, gas and all living systems by ten thousand-fold.” “... only a tiny change in the degree of leakage, particularly if focused through the hydrate cycle, can result in globally significant greenhouse gas emissions.” “Methane degassing from sedimentary basins may be a mechanism to explain increases of atmospheric CO2 to values as much as 20 times higher than pre-industrial values.” But these processes can take place quickly? “As geologic sources may have contributed over one third of global atmospheric methane in pre-industrial time, variability in methane flux from sedimentary basins may have driven global climate not only at time scales of millions of years, but also over geologically short periods of time.” How “short”? Most - perhaps - a "complete" - new - from this year - work on the PETM is a paper: Methane and environmental change during the Paleocene-Eocene thermal maximum (PETM): Modeling the PETM onset as a two-stage event, Carozza, Mysak, and Schmidt, 2011. - A corresponding well with the observations and conclusions of the work cited by scaddenp. So it describes the most likely “the course of events”: “To explain the observations, the carbon must have been released over at most 500 years. The first stage results cannot be associated with any known PETM hypothesis. However, the second stage results are consistent with a methane hydrate source. More than a single source of carbon is required to explain the PETM onset.” “... second stage (stage 2) ...” is therefore particularly interesting. “In particular, for DT = 3°C [Winguth et al., 2010], a mixed emission of 900 to 1400 Pg C consisting of 400 to 500 Pg C CO2 to the ocean and 400 to 900 Pg C CH4 to the atmosphere simulates stage 2. Durations of 50 and 250 years are data‐compatible (Figure 2c); however, only a duration of 50 years is compatible with 3°C of warming.” “Therefore, the emission of 400 to 900 Pg C CH4 to the atmosphere and 400 to 500 Pg C CO2 to the ocean, with a duration of 50 years and d13C ranging from −50 to −60‰, best simulates stage 2 ...” So we have 50 years - the same as for the period 1950 to 2000 AD. During this period the volume of our emissions are on average 5 - 6 Pg C - with the 2.5 - 3 Pg C „to the atmosphere”. For PETM - „stage 2” we have: 400 to 900 Pg C „to the atmosphere” during 50 years = 8 - 18 Pg C year - 1 ... Where else could take on carbon? Rob Painting writes about the permafrost - and His rightly - the organic carbon stocks in permafrost (poor in13C) (and the content changes over time) may be many times greater than we currently estimate. Example of arctic permafrost (not only Antarctic) - the Pleistocene - writes Zech et al., 2011. ( High carbon sequestration in Siberian permafrost loess-paleosols during glacials.): “Recent findings show that the amount of organic carbon stored in high-latitude permafrost regions has been greatly underestimated.
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  7. Phil Scadden - whoa.......15 million gigatonnes of organic carbon! Crikey! Are you able to access a copy of the paper? Jerryd - A tutorial would be cool. Maybe you can tee-up with John and we could get something underway?
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  8. „PETM changes in the carbon isotopic composition of carbonate”, and ”... carbon isotope excursion as recorded in marine organic matter ...” - may be saddled with significant error - incorrect ( Baczyński et al. 2011.). Dickens (2009) in his paper - quoted here - but interesting, writes: “A 2000 Gt input of carbon to the exogenic carbon cycle cannot explain the 6°C warming, unless earth surfaces temperatures increase by more than 5°C per doubling of pCO2 (Pagani et al., 2006a). Such climate sensitivity is more extreme than that in most climate models. Complicating matters, however, is the relative timing of environmental change and massive carbon addition at the start of the PETM. In several sediment sequences, changes in temperature and biota begin before the start of the CIE (Sluijs et al., 2007b). With available data, massive carbon addition during the PETM appears to have been a positive feedback to environmental change initiated by some process that remains highly speculative.” The most extensive - from this year - general study: The Paleocene-Eocene Thermal Maximum: A Perturbation of Carbon Cycle, Climate, and Biosphere with Implications for the Future, McInerney and Wing, 2011.; have interesting conclusions: 5. Although there was a major extinction of benthic foraminifera, most groups of organisms did not suffer mass extinction. 7. Rapid morphological change occurred in bothmarine and terrestrial lineages, suggesting that organisms adjusted to climate change through evolution as well as dispersal and local extirpation. Where best understood, these evolutionary changes appear to be responses to nutrient and/or food limitation.
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    [DB] And your point is...?

  9. 58, Arkadiusz, It's unclear if you have a particular point that you are trying to make, or if you are merely providing what you consider to be a variety of useful information on the subject. Without any dialogue to explain the segments that you choose to include, there's nothing coherent about the information. Could you state your point or purpose more directly?
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  10. Rob, Karsten Kroeger works in same section/team as me. So, yes. I am Phil Scadden at GNS Science. Just email me.
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  11. The conclusion is simple, works cited by R.P. give a very incomplete picture of the topic: what science says about PETM (despite citing the fundamental work). Among other things, Gavin Schmidt says that the changes were extremely rapid p.CO2. The second position (which I quote) is the conclusion that nature has "fared" surprisingly well during the PETM of ocean acidification and warming.
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  12. Arkadiusz - "conclusion that nature has "fared" surprisingly well during the PETM of ocean acidification and warming" Yes, but the current rate of ocean acidification is unprecedented in the last 65 million years. In fact, if the PETM carbon is from methane, acidification could be happening around 27 times faster today than during the PETM. Additionally, during the PETM ancient corals appeared to cop a bit of a hammering from bleaching events. Links for peer-reviewed papers, supporting these claims, are in the post. A global civilization did not exist during the PETM, but if it did, how do you think they would have fared given even that slow rate of change?
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  13. Regarding the above points, Abundant evidence suggests that a massive input of CO2 occurred during the onset of the PETM (this could include oxidation of CH4). There were no mass extinctions (except in many benthic foraminifera). HOWEVER, There were profound changes in the environment during the event. One only has to look at the sedimentary record. The PETM now has been clearly documented in about 150 locations from around the world (McInerney and Wing, Ann. Rev. Earth Sci., 2011, nicely show most of them on a map). At almost all these locations, the PETM is marked by an anomalous horizon. The lithological changes across the PETM relate to several factors. For example, in the deep-sea (>2000 m paleo-water depth), there is typically a drop in carbonate content caused by carbonate dissolution (aka ocean acidification). In several shallow marine sections, there is a black organic-rich shale, probably caused by enhanced runoff and water column stratification. There were also numerous prominent changes in biota. Indeed, this is why there exists an epoch boundary (i.e., the Paleocene/Eocene boundary) at the start of the event! Fro example, in Wyoming, the mammal fossils found before the event are completely different than the mammal fossils after the event. We don't really think of this as a mass extinction, though, because the number of orders does not drop precipitously. Instead, it is a wholesale migration (or perhaps even origination) of animals. So "fared well" depends on what you mean. Certainly, if you were a Plesiodapid living in Wyoming, the world was not a kind place; on the other hand, if you were a camel or a primate living somewhere unknown, Wyoming suddenly became the land of bounty. In any case, Rob's main point is entirely valid. In the last 65 million years, the PETM was by the most extreme short-term event associated with rapid warming and massive carbon input. It was marked by profound environmental changes. All indications are that carbon inputs are changing significantly faster at present-day than during the PETM. This has been spun in two different ways by many people: we're heading for catastrophe; things will be okay as Earth has been through this before. I'll just stick to the facts on this point, but note that, to suggest that changes will be minimal, is not consistent with the geological record. Jerry
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  14. Jerry #63 - excellent comment, well worth reading thoroughly. Seeing as modern civilisation could be described as 'well adapted' to current conditions, and more critically, can be described as 'tightly tied' to current conditions, in terms of the locations of global agriculture, coastal cities etc. We don't have the luxury of the camels or primates of Jerry's example, where we can simply migrate ourselves and all our infrastructure and agricultural zones to somewhere more pleasant. Regrettably maybe we're more like the plesiodapids, except that perhaps we can do something about the CO2 emissions?
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  15. The great Carbon influx into the atmosphere at the PTEM has been resolved.  The eruption of the Northern Atlantic and High Arctic flood basalts at the time caused the largest insertion of greenhouse gases in earth's atmosphere in the last 290 million years.

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    Moderator Response:

    [PS] An email address is not a substitute for a reference to a published paper. Such a paper would be interesting but at first glance, such an explanation fails to explain the observed carbon isotope signature.

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