The Paris climate targets

I’m currently in Oxford for a meeting and, having spent most of the train ride working on a book chapter I’m writing, I thought I would now spend some time writing a quick post about the recent Schurer et al. paper Interpretations of the Paris climate target. It’s essentially a response to the Millar et al. paper, that I’ve discussed in a number of recent posts.

Credit: Schurer et al. 2018

The key result is probably illustrated by the figure on the right. It shows how different ways of treating the observations influence how close we might appear to be to a target (in this case 1.5oC). For example, the top panel shows HadCRUT4 and how close it is to 1.5oC based on the standard dataset (blue), a version that corrects for HadCRUT4 being a combination of surface air temperatures (SATs) and sea surface temperatures (SSTs) (green), changing the pre-industrial baseline for the standard HadCRUT4 dataset (yellow), and doing the same but for a case where it is all SATs (purple). The middle panel does the same as the top panel, but corrects for HadCRUT4’s coverage bias (i.e., makes it global). The bottom panel is the same, but for the Cowtan and Way dataset.

Essentially, if we use observational datasets to infer how close we are to a target, it will depend on how that dataset is constructed (SST + SATs, coverage) and on the assumed baseline. There are a number of reasons why it’s important to understand this effect. For example, Millar et al. claimed that we’d warmed less than expected, given how much we’ve emitted. Therefore, we have a larger remaining carbon budget that had been realised. However, this difference was (as I understand it) mostly because the observations were blended (SSTs + SATs) and suffered from coverage bias, while the model used to estimate how much we should have warmed was based on global coverage and SATs. If the comparison had been like-for-like, then the difference would have been much smaller.

Additionally, if you’re going to estimate some warming at which impacts become sufficiently severe that we should aim to keep below that, then the observations used to determine how close we are to that target should be consistent with what was used to determine the target. If the latter was determined using global coverage and SATs, then either an equivalent observational dataset should be used, or the target should be corrected to account for form of the observations (blended and masked, for example).

Ultimately, however, I’m not entirely sure I quite get the fuss. I think this is an interesting scientific puzzle. I think it’s useful to understand why there are these differences. However, whether the target is 1.5oC or 2oC, achieving the target is going to be difficult even if we do have a few tenths of a degree more to go than we had realised. It’s essentially still start reducing emissions as soon possible, and reduce them as fast as we can.

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46 Responses to The Paris climate targets

  1. Everett F Sargent says:

    So if atmospheric CO2eq today is ~544ppmv and total CO2 emissions to date are ~633 GtC (fossil fuel and industry + land-use change emissions) and we follow the baseline from the UNEP Emissions Gap Report 2017 through into 2030 (I had to reverse engineer their graphics since their numbers are not fully tabulated), oh say add another ~212 GtC (or an additional 777 GtCO2eq), so maybe ~845 GtC in 2030 we will still be A-OK for a 1.5C target?

    Personally, I think the UNEP numbers are perhaps a bit high. But then again, I think the only way to see if we can stay below 1.5C in 2100 is to bust right through 1.5C in ~2040! 😦

  2. KiwiGriff says:

    I was hoping ATTP would have a relevant post for this to be discussed.
    We may be past 2C already.

    Unrealized Global Temperature Increase: Implications of Current Uncertainties
    Stephen E. Schwartz


    Unrealized increase in global mean surface air temperature (GMST) may result from the climate system not being in steady state with forcings and/or from cessation of negative aerosol forcing that would result from decreases in emissions. An observation‐constrained method is applied to infer the dependence of Earth’s climate sensitivity on forcing by anthropogenic aerosols within the uncertainty on that forcing given by the Fifth (2013) Assessment Report of the Intergovernmental Panel on Climate Change. Within these uncertainty ranges the increase in GMST due to temperature lag for future forcings held constant is slight (0.09–0.19 K over 20 years; 0.12–0.26 K over 100 years). However the incremental increase in GMST that would result from a hypothetical abrupt cessation of sources of aerosols could be quite large, but is highly uncertain, 0.1–1.3 K over 20 years. Decrease in CO2 abundance and forcing following abrupt cessation of emissions would offset these increases in GMST over 100 years by as little as 0.09 K to as much as 0.8 K. The uncertainties quantified here greatly limit confidence in projections of change in GMST that would result from any strategy for future reduction of emissions.
    Plain Language Summary

    Earth’s surface temperature has increased by more than 1°C in response to anthropogenic greenhouse gases, principally CO2. This warming influence is offset in part by a highly uncertain cooling influence by anthropogenic aerosols. As CO2 is long‐lived in the atmosphere (centuries), whereas aerosols are short lived (weeks), the reduction of sources of aerosols that would result from reducing fossil fuel combustion would lead to temporary warming before cooling from decreased CO2 became dominant. Within current uncertainties a hypothetical abrupt cessation of anthropogenic sources of CO2 and aerosols could result in minimal increase in global temperature to as much as 1.3°C additional increase over a few decades. These findings have important implications on the consequences of prior and prospective future emissions.

  3. BBD says:

    Ultimately, however, I’m not entirely sure I quite get the fuss. I think this is an interesting scientific puzzle. I think it’s useful to understand why there are these differences. However, whether the target is 1.5oC or 2oC, achieving the target is going to be difficult even if we do have a few tenths of a degree more to go than we had realised. It’s essentially still start reducing emissions as soon possible, and reduce them as fast as we can.

    Nail on head.

  4. Mitch says:

    What 2°C from pre-industrial global temperature means is only important from a regulatory regime. So, expect fossil-fuelers to be pushing for the interpretation that gives the least change. From an earth systems point of view, what is important is the actual change that happens, locally, regionally and globally.

  5. Dave_Geologist says:

    Cynical me says:

    1) Angels-on-pinhead science. Arguing about 0.2°C when Paris has elasticity between 1.5°C and 3°C depending on how hard people try, and there is still a powerful lobby which would have us on a BAU 4°C or 5°C trajectory.

    2) But for that lobby, PR matters and when we officially crash through 1.5°C it will be bad PR. And the further away they can push that crash in the public’s minds, the longer they can keep the “greenies” off their back. Five years of inaction is not as “good” as ten but is “better” than none.

    3) And for the part of that lobby which doesn’t believe it’s happening, or does but doesn’t want to stop it for political, religious or economic motives, minimisation is the key to delay. They don’t care about the science, it’s 100% about the PR, and they’ll cynically choose satellite of thermometers, UAH over RSS, HadCRUT over BEST. Until the pecking order changes, then suddenly they’ll have a new favourite.

  6. Steven Mosher says:

    when you go through 1.5 and nothing bad happens…on the other hand after 1.5 every bad thing will be blamed on it.

  7. BBD says:

    when you go through 1.5 and nothing bad happens…

    Bad is happening already. Let’s not be unnecessarily rhetorical.

  8. jacksmith4tx says:

    If our technology created this problem it should be a foregone conclusion we will use technology to predict the outcome.
    “Deep Learning at 15 PFlops Enables Training for Extreme Weather Identification at Scale”
    Petaflop per second deep learning training performance on the NERSC (National Energy Research Scientific Computing Center) Cori supercomputer has given climate scientists the ability to use machine learning to identify extreme weather events in huge climate simulation datasets. Predictive accuracies ranging from 89.4% to as high as 99.1% show that trained deep learning neural networks (DNNs) can identify weather fronts, tropical cyclones, and long narrow air flows that transport water vapor from the tropics called atmospheric rivers. As with image recognition, Michael Wehner (senior staff scientist, LBNL) noted they found the machine learning output outperforms humans.”

    There is a good video of the Intel AI conference where the author describes how this technology was developed.
    In a few years I think they will couple a climate AI system to a biosphere AI system and we will be surprised to find out how few options we we have to change the outcome.

  9. Willard says:

    > when you go through 1.5 and nothing bad happens

    For Gremlins, perhaps:

    One problem which [Richie] didn’t note was the role of the changing minus signs in interpreting the estimates that were not garbled. In particular, his estimate of a big positive impact at 1 degree is a clear outlier in his analysis.

  10. David B. Benson says:

    Look more carefully to the climate of the mid-Pliocene to estimate just how bad things are going to be on a centennial scale.

  11. Dave_Geologist says:

    I’ll try to warm this thread up by putting in some geological perspective on what the various Paris targets might mean, and also look at: what if we don’t do Paris? I’ll look later at the mid-Pliocene. Let’s start with the really exciting stuff 😉

    The first one very much relegates arguments about which anomaly series to use and when to start into angels-on-pinhead territory. It might matter for some less severe analogues.

    I’ll look at the Paris targets not in the context not of 2100, but of hundreds of years (still the blink of a geological eye). IOW, assume the 21st Century projections are right, but that we’ll see more warming than projected in the following centuries, even if we go to zero net emissions, because ECS is at the high end of the range but TCS is in the middle.

    I’ll make some simplifying assumptions and use round numbers. For ML ECS I’ll use 3°C per doubling (middle of IPCC’s 1.5 to 4.5), and for the high end 6°C, the IPCC’s very-unlikely-to-be-higher-than. That also conveniently sits at the top end of the palaeo-based estimates, which of course as a geologist I like. 😉 . And rather neatly, the end-Permian event I’ll start with (5-6°C) provides one of those high-end constraints. The top end of instrumental estimates go much higher, but it’s in the nature of that record that long-term feedbacks won’t be captured because they haven’t happened yet. But nor can they be ruled out, because absence-of-evidence isn’t evidence-of-absence. Whereas we can say, for example, that we didn’t go into a Snowball Earth during the last Ice Age, nor did the sea boil at end-Permian (although it did warm to >35°C).

  12. Dave_Geologist says:

    So let’s start with the end-Permian. I presume we’d all agree that it’s not a good idea to replicate the Earth’s most severe known extinction event, with up to 96% of all marine species and 70% of terrestrial vertebrate species becoming extinct, the only known mass extinction of insects and some 57% of all families and 83% of all genera becoming extinct. And it took up to 10 million years for the biosphere to recover (although that may be because the early Triassic was also very warm, so it was not just a short hot spike). In fact, let’s say that it’s such a bad idea to replicate it that we should working towards having zero chance that it will happen again (at least, absent a massive Large Igneous Province breaking out).

    Globally the latest Permian oceans were about 4°C warmer than the present-day ocean (but ranging from about 0°C at the equator to 10°C at the poles). Tropical SSTs warmed by 5-9°C during and after the extinction event, with a mode at 8°C, so let’s say 12°C warmer than at present with a range of 9-13°C (the poles would have been more like 20°C warmer than today). When we’re talking such big numbers, it doesn’t matter much which pre-industrial or even 20th Century starting point you use.

    Although the Paris target is <2°C, current commitments are consistent with 3°C warming. I’m assuming a high ECS so will double that to 6°C. Still only half of 12°C, so even without additional Paris commitments, we should be OK temperature-wise. But if we stick to BAU and follow RCP 8.5, the high AR5 projection for 2080-2100 is 4.8°C, and we double that to 9.6°C in subsequent centuries, were into the end-Permian range. So if the cards stack up against us (high ECS, BAU emissions and low end-Permian ΔT), there is a credible but unlikely case where we trigger an end-Permian scale extinction. Even if it is only a 1% chance, it’s not a chance I’d be prepared to take.

    The end-Permian saw about a 50% increase in pCO2, but from a higher base with most estimates in the low thousands of ppm. So to the extent that the marine extinction was caused by ocean acidification, even RCP8.5 would probably only take us to baseline Permian levels. OTOH it’s probably the change, rather than the absolute value which calcifying organisms can’t cope with. There were some pretty impressive reef-builders in the Permian, so over tens of millions of years they’d obviously got used to more acid seawater than today. The RCP8.5 ΔpCO2, approaching 1000ppm by end-century, is within the range of the end-Permian ΔpCO2 (two lowest estimates 700ppm and 1650ppm).

    And of course all the ice would melt. But we'd be too busy dying to worry about moving coastal cities. Probably just abandon them.

    So that’s actual scarier than I thought it would be. At least at this ball-park level, I can’t completely rule out an end-Permian temperature or acidification event if we follow RCP8.5. It makes nit-picking about Paris angels-on-a-pinhead territory, but to-Paris-or-not-to-Paris a big deal.

  13. Dave_Geologist says:

    In retrospect maybe I was double-dipping by using a high AR5 sensitivity and a high-end ECS which perhaps had that already baked in. So maybe I should have used 3.7°C and doubled it to 7.4°C. Not quite at the lowest end-Permian temperature estimate then. But presumably if we burn all the fossil fuel in the ground we’ll be adding CO2 after 2100 and won’t stop at 1250ppm CO2.

    And what about the early Triassic? It was even hotter than the end-Permian and is probably implicated in the slow recovery post-mass extinction, with two further extinction events of end-Permian survivors coinciding with extreme temperature spikes. SSTs were maybe 3-4°C hotter than at end-Permian, with a possible spike in equatorial SST to 40°C.

    If you like peer-reviewed science papers with apocalyptic titles, this is one: Lethally Hot Temperatures During the Early Triassic Greenhouse

    Here, we show that the end-Permian mass extinction coincided with a rapid temperature rise to exceptionally high values in the Early Triassic that were inimical to life in equatorial latitudes and suppressed ecosystem recovery. This was manifested in the loss of calcareous algae, the near-absence of fish in equatorial Tethys, and the dominance of small taxa of invertebrates during the thermal maxima. High temperatures drove most Early Triassic plants and animals out of equatorial terrestrial ecosystems and probably were a major cause of the end-Smithian crisis.

    Yep, no fish in the equatorial oceans and reptiles couldn’t hack it on the equatorial landmasses. Mammals? There were none back then but I don’t think so. From their Fig 1 the southern hemisphere, where most land was, looks worse (right down to 30°S), so if it happened today that would be the northern hemisphere’s fate.

    I’m not sure how much that period has been studied, but I find it interesting because it breaks the standard pattern of polar amplification and a degree of tropical buffering. You can sometimes learn a lot about a system by studying what breaks it. Life survived and there was a burst of evolution in temperate and polar refugia, from which the tropics were eventually re-colonised. It’s not relevant to policy because we’d have to go through an end-Permian stage to get there, by which time we’d be past caring. But if there is a cause and effect relationship between the two (very long-term positive feedbacks which don’t kick in during normal hothouses? Slow drawdown of CO2 because the normal weathering cycle is disrupted somehow?).

  14. BBD says:

    Very interesting, thanks.

    Noticed you referenced Cui & Kump (2015) which reminded me – apropos of your remarks about how much CO2 may have been associated with extreme warming – that it may be less than previously thought. Cui & Schubert (2017) looks at the Eocene hyperthermals rather than the end-Permian, but physics is physics.

  15. BBD says:

    Sigh. Link mess again: Cui & Schubert (2017). Sorry.

  16. Willard says:

    Speaking of geological perspectives, DaveG, you might like this one.

  17. Dave_Geologist says:

    Speaking of geological perspectives, DaveG, you might like this one.

    Just the usual failed logic. Although he plays the [Mod: I try to avoid this description] just sayin’ game of leading his readers by the nose without explicitly perpetrating the logic fail. It was warm in the past so today’s warming might not be caused by humans. It was much warmer in the past so things will be OK.

    On Panama, I’ve never been a great fan of the closure of the isthmus being relevant to ocean circulation, just to species migration. The Atlantic and Pacific oceans are pretty big and the channel would have been a significant barrier long before it closed. The Faroe-Shetland channel is 200 miles wide and 2000 feet deep, but it still does a pretty good job of restricting flow between the Arctic and Atlantic.

  18. Dave_Geologist says:

    Thanks for the timely link BBD. I’d missed that one.

    You’ll have perhaps gathered from my posts that I’m concerned about the long-term impacts and the fact that palaeo estimate give higher ECS than models. And that the models are known to omit some of the long-term feedbacks.

    Re Willard’s link, the other common bit of failed logic among lukewarmers is the idea that the more variable temperatures were in the past, the less we should be concerned about man-made warming. Which only makes sense if they’re actually deniers pretending to be lukewarmers. Otherwise they’d grasp the fact that more past variability means higher climate sensitivity and more to be concerned about, not less. So it only makes sense if they don’t actually believe we’re doing anything, that it’s all natural and we just have to take it.

  19. Dave_Geologist says:

    I try to avoid this description

    OK, apologies, point taken.

    And on my second post, an alternative interpretation is that for some reason anthropogenic forcings have drastically different impacts than natural forcings. Even though some of them involved CO2. But that just leads into “how do we know CO2 behaves the same in the wild as in the lab”. And then if they reject the “because physics” explanation, it’s just another form of denial. Science is just too joined-up to let you pick and choose.

  20. BBD says:


    You’ll have perhaps gathered from my posts that I’m concerned about the long-term impacts and the fact that palaeo estimate give higher ECS than models. And that the models are known to omit some of the long-term feedbacks.

    Have you come across the PALAEOSENSE Project? You’ll find it interesting reading if not.

  21. Dave_Geologist says:

    Yes, thanks, I’m familiar with it.

  22. DG:
    I’ve never been a great fan of the closure of the isthmus being relevant to ocean circulation, just to species migration. The Atlantic and Pacific oceans are pretty big and the channel would have been a significant barrier long before it closed. The Faroe-Shetland channel is 200 miles wide and 2000 feet deep, but it still does a pretty good job of restricting flow between the Arctic and Atlantic.

    Before the Caribbean Plate came along, the gap between the Americas was neigh1000 milea wide and abyssally deep, and there was nothing there to make the Gulf Stream do a U-turn.

  23. David B. Benson says:

    The end-Permian mass extinction was the result of the nickel spread over the face of the earth by Siberian Trapps formation explosions lofting nickel sulfide in the basalt lofted into the stratosphere. For that, see Svensen et al. and also Ogden & Sleep, PNAS 2012. For the result of spreading the nickel around, read
    Methanogenic burst in the end-Permian carbon cycle:

    This caused anoxic shallow ocean and the ecosystem took a very severe hit, at least until well into the Triassic Period.

    But this excursion in nickel, sulfer cycle and carbon cycle following, seems irrelevant to the result of the anthropogenic flux of carbon dioxide in the atmosphere. For that, consider the mid-Pliocene.

  24. Dave_Geologist says:

    Thanks Russell, I was aware of that and was thinking more about when in the Neogene it closed enough to matter for climate.

    If you look, for example, at Scotese’s maps there were quite a lot of shallow obstructions in the Caribbean by the mid-Tertiary. But people get hung up on when exactly the isthmus closed and when you could walk from South to North America. Which may be relevant to species mixing and separation, but ocean currents would have been impeded before that. And there is some evidence that the isthmus itself was there in the mid-Miocene. Middle Miocene closure of the Central American Seaway (although that interpretation has been challenged). A couple of 2007 papers, one using a Pliocene GCM, showed that it was far from clear-cut that (a) an open or closed isthmus made much difference (yes, larger ice sheets, but only 5cm of SLR worth) and (b) that the increased glaciation ~3Myr ago needs a special explanation, as opposed to just being a tipping point on the back of long-term cooling. Closure of the Panama Seaway during the Pliocene:
    implications for climate and Northern Hemisphere glaciation
    and Closing of the Central American Seaway and the Ice Age: A critical review Molnar-2008-Paleoceanography.

    The relevance to Paris is that when the Pliocene is used as an analogue for the long-term impact of 2-3°C of warming, a counter-argument is often made that it doesn’t count because Panama was open and ocean circulation was different. Probably because that’s what Wiki says

    The isthmus formed around 2.8 million years ago.[1] This major geological event separated the Atlantic and Pacific Oceans and caused the creation of the Gulf Stream.</blockquote
    But Wiki isn't the last word.

  25. BBD says:

    A couple of 2007 papers, one using a Pliocene GCM, showed that it was far from clear-cut that (a) an open or closed isthmus made much difference

    It is, I think, arguable that while such an ocean gateway closure might have (geologically) short-term climate impacts, global climate would return to something close to its prior state assuming no major forcing change in the interim.

  26. Mitch says:

    Dave_Geologist–the middle Miocene closure of CAS has very poor evidence. It is based on a correlation of zircon ages and U/Pb in formations in Northern Colombia vs Panama and arguing for a river to do the transport (Montes et al, Science, 2015). Never mind that the river would have to travel along the spine of the isthmus for around 500 km to get from Panama to Colombia. It is much easier to explain as an uplifted Caribbean Miocene deep sea fan scraped onto Colombia.

    While it is convenient to point to CAS closure to cause the descent into ice ages, what is lacking is how the mechanism might work. Most constructions have only low volume shallow flow through the isthmus area after about 6 million years ago.

  27. Dave_Geologist says:

    A final let’s-not-go-there-please palaeo example – The Palaeocene-Eocene Thermal Maximum. I’ll split it into bits as this is getting to guest post length 😉

    A climate slightly warmer then today (3°C or thereabouts) warmed quickly (but still ten times slower than today) by another 5°C or so. So now we’re in Paris territory if ECS is in the middle of the IPCC range. 3°C is not-the-PETM territory, whereas 7.4°C or 9.6°C (my back-of-envelope high-end-palaeoclimate estimate for post-2100 temperatures assuming BAU emissions) is very much PETM territory.

    Why do I regard that as a don’t-go-there, must-be-avoided-at-all-costs case? It might be tempting to think that it was OK because there was no general mass extinction. But the geological evidence across the world shows a marked change in the rock record. Any event big enough to change depositional and erosional conditions sufficiently to lay down completely different rocks is huge by comparison to the sort of climatic shifts we’ve seen so far. On a par with a glaciation/deglaciation, not just a local change in rainfall or seasonality. At a minimum it would require wholesale movement of people, probably of whole countries, to adapt.

    From Wiki

    Fossil records for many organisms show major turnovers. For example, in the marine realm, a mass extinction of benthic foraminifera, a global expansion of subtropical dinoflagellates, and an appearance of excursion (sic), planktic foraminifera and calcareous nanofossils all occurred during the beginning stages of PETM. On land, modern mammal orders (including primates) suddenly appear in Europe and in North America. Sediment deposition changed significantly at many outcrops and in many drill cores spanning this time interval

    Anyone who thinks “oh good, it triggered the rise of the primates, we’re primates, we’ll be fine” should consider the possibility that what’s good for a six-inch quadruped looking like a cross between a squirrel and a lemur, might not be good for 7 billion six-foot bipeds with a civilisation to maintain. Plus ask yourself: why was there an outburst of evolution after the PETM? Maybe there were a bunch of empty ecological niches to fill. Remember that land fossils have very poor preservation potential, especially the sort of small animal early primates would have competed with.

  28. Dave_Geologist says:

    What was the geology doing?

    In the Ager basin in Spain, there was a massive increase in rate and depth of erosion, and in transport of coarse material. It lasted about 20,000 years and coincided with a switch from an arid climate to a hot, humid climate subject to major floods. The rain might be welcome to farmers, but not when it’s so torrential that it cuts deep canyons, reminiscent of those cut in continental shelves during glacial see-level falls. They have a lovely turn of phrase:

    The coarser-grained clastic deposits associated with the earliest stages of the PETM and the occurrence of debris flows could represent the catastrophic impact of a twisted climatic regime over a landscape in equilibrium with arid conditions.

    There was a massive influx of kaolinite into the sea, too sudden to be explained just by the onset of humid tropical conditions.

    Instead the kaolinite influx may reflect deeper physical erosion on the continents that mobilized buried Mesozoic kaolinite-rich sediments formed in an earlier tropical paleogeographic setting

    The unusually high erosion rates … are consistent with generally dry conditions. If humid conditions, as in the present tropics, had evolved, then a dense vegetation cover would have protected hinterland soils from being eroded. … Summers may have become hotter and drier, preventing the development of a vegetation cover. During rainy seasons, more intense rain fronts contributed the necessary water to erode the hinterlands.

    Abrupt increase in seasonal extreme precipitation at the Paleocene-Eocene boundary generated a megafan like that of the modern Kosi River (which today needs a Himalayan-scale mountain range to source it).

    An example is the Kosi River megafan in northeastern India, where the river has migrated westward on average 0.5 km/yr the past 228 yr…

    The development of vast braid plains or a megafan at the P-E boundary in the Pyrenees is consistent with model predictions of increased intra-annual humidity gradients and associated seasonal flash floods in the subtropics in a strengthened greenhouse situation (Houghton et al., 2001). The … soils … indicate a generally semiarid climate … The CC disappeared as abruptly as it appeared … The hydrological cycle apparently shifted again, leading to rapid soil formation, but still in a generally semiarid setting … Drying immediately after the PETM is indicated by the appearance of thick gypsum beds in some areas

    IOW a greatly enhanced version of the standard dry-seasons-get-dryer, wet-seasons-get-wetter response to a hotter climate. You too can have a city or farm like those on the Kosi megafan, where three million people were flooded out and 2000 killed in 2005. No need for Himalaya-scale mountains, a small mountain range will do, maybe just some hills.

  29. Dave_Geologist says:

    What about places that were already forested like southern England?

    In the late Paleocene there is a close association of high abundances of Cicatricosisporites (Schizaeaceae) fern spores with microscopic and mesoscopic charcoal representing a low diversity fire prone fern and woody angiosperm community. By contrast, the PETM vegetation is characterised by the loss of ferns and cessation of fires, an increase in wetland plants … a more varied flowering plant community with palms and eudicots.

    Sounds like it got a bit more hospitable – perhaps somewhere for the refugees to go.

    How about somewhere a bit warmer like the US Gulf Coast… Oh Dear. Paratropical floral extinction in the Late Palaeocene–Early Eocene

    taxonomic diversity increases over c. 1 Ma in the Late Palaeocene but this trend is replaced, with the first occurrences of taxa that mark the Early Eocene, by a pronounced extinction into the Early Eocene (c. 20% of the palynoflora). Taxonomic diversity also decreases by up to 38% in the Early Eocene.

    Further north, in the Bighorn Basin: Plant response to a global greenhouse event 56 million years ago

    rates of local extirpation were exceptionally high. Eighty-eight percent (46 of 52) of plant taxa present in the last 100 kyr of the Paleocene are not recorded in the PETM, and four of those that are present in the PETM are known only from the recovery phase of the CIE, as CO2 concentration was declining and climate became cooler and wetter again.

    Note that none of these species went extinct. However they did move to refugia outside the basin. And any farmers living at that time would have had to move hundreds, perhaps thousands of kilometres to keep farming the same crops. Or to farm something complete different, probably something subtropical.

    In what is now Egypt, again there was an increase in rainfall (northward migration of rain-belts?) but presumably somewhere else dried out. And it takes more than rain to turn the desert green, at least with crops like wheat or rice as opposed to natural desert succulents which don’t need topsoil.

    the topmost Palaeocene experienced a period of aridity, … followed by humid climatic conditions, then a return to semi-arid conditions.

    In northern France

    an abrupt shift from a closed, quiescent marsh pond to an open eutrophic swamp subjected to algal blooms, concomitant with the onset of the CIE … and a moister climate associated with a stronger seasonality during the early PETM.

    In the Adriatic

    The strongly 13C-depleted δ13C record of our shallow-marine carbonates compared to open-marine records could result from organic matter oxidation, suggesting intensified weathering, runoff, and organic matter flux

    I could go on, but basically wherever you look, the climate was turned upside down. It’s impossible to imagine that happening to the modern world without unprecedented levels of starvation, migration (hundreds of millions or billions, not millions) and war. And some of the countries that would become uninhabitable have nuclear weapons. As has at least one of the countries refugees would be heading for (Russia, the USA would likely be a source of refugees).

    And the PETM jumped so quickly geologically, we can’t say where the tipping point lay. Somewhere between 3°C and 8°C, but it could be 4°C or 5°C, with 8°C representing overkill. The smaller ETMs might help there.

  30. Dave_Geologist says:


    the middle Miocene closure of CAS has very poor evidence

    Yes, I saw the Comment and Reply and agree that it is weak.

    OTOH the mechanism for the isthmus to cause Ice Age onset is also weak. So correlation without causation. Dating back to pre-numerical-model days. One could play devil’s advocate and say the same applied to Milankovitch cycles, timing fitted but the forcing looked too weak once people actually started to plug in numbers. But then with the proper feedbacks added, suddenly the forcing isn’t too weak after all. So maybe there’s something missing from the models and the isthmus can do it.

    But as you say flow was restricted sooner, and now the dates don’t even match. So I’d put it in the looked-like-a-good-idea-at-the-time-but-not-any-more category.

    My Faroe-Shetland Channel analogue was tied to the idea of flow being restricted well before complete closure. Of course, although mixing between the Arctic and Atlantic is restricted, there is flow. Very vigorous flow due to its confinement, which cuts a deep erosional channel and deposits contourites. So one way to test for large interchange of Atlantic and Pacific waters across a shallow sill would be to look for evidence of rapid bottom-water flow and scouring. In the FSC its impact can be seen as far back as the Oligocene. By coincidence, I see Panama has licensed some offshore oil exploration acreage, so maybe commercial seismic surveys will answer that in the near future.

  31. John Hartz says:

    David Wallace-Wells pulls no punches in this chilling article…

    The Paris Climate Accords Are Looking More and More Like Fantasy by David Wallace-Wells, The Intelligencer, New York Magazine, Mar 25, 2018

  32. John Hartz says:

    Here’s how Carbon Brief summarized the David Wallace-Wells article that I cited above in today’s edition of its Daily Briefing alert:

    “Remember Paris?,” asks David Wallace-Wells in the New York Magazine “It was not even two years ago that the celebrated climate accords were signed – defining two degrees of global warming as a must-meet target and rallying all the world’s nations to meet it – and the returns are already dispiritingly grim.” Wallace-Wells goes on to discuss recent findings from the International Energy Agency announced that carbon emissions grew 1.7% in 2017, as well as the recent “raft of distressing papers about what beyond compliance is required to stay below 2C”. In particular, he points to one article in the new issue of Nature Climate Change which tried to quantify the suffering that would be avoided if the planet were kept below 1.5C of warming, rather than 2C. “Their answer: 150 million more people would die from air pollution alone in a two-degree-warmer world than in a 1.5-degree-warmer one. … Numbers that large can be hard to grasp, but 150 million is the equivalent of 25 Holocausts.”

  33. jacksmith4tx says:

    All we have to do calculate (invent) a premium that will completely absolve us of any responsibility. Don’t laugh. Just look at the size of the global debt and derivatives markets.
    “The Mesoamerican Reef, the second-longest barrier reef system in the world. Funds for the trust and insurance premium will be collected as a portion of the tourism taxes and also come from other government sources.”
    “Data shows that since 1980, 80 percent of live coral cover in the Mexican Caribbean has been lost or degraded due to disease, bleaching events, diminishing herbivores, and algae overgrowth.”

    Great stall tactic. Should be able to extend the life span of the reefs for a couple of years I’m sure.

  34. Dave_Geologist says:

    David B

    But this excursion in nickel, sulfer cycle and carbon cycle following, seems irrelevant to the result of the anthropogenic flux of carbon dioxide in the atmosphere..

    An interesting hypothesis. Some random thoughts:

    AFAICS they’re not suggesting poisoning as a kill mechanism, other than perhaps H2S as a result of ocean anoxia. Warming stratifies the oceans which on its own is enough to explain anoxia. And other warm spikes in the geological record coincided with an increase in anoxia. And the excess carbon argument can be explained by coal as well as by methanogens. Other mass extinctions coincide with LIP volcanic activity. So Occam’s Razor would say: why the need for a special cause? Which also applies of course to the K-T (K-Pg). The meteorite was an extraordinary claim which required extraordinary evidence before the consensus was convinced. Same will apply to this hypothesis.

    The DNA divergence estimate is +/-40Myr, so hardly an exact date match. The paper strengthens the LIP data match by confirming a Ni spike in the right rocks.

    Meishan is very close to the Siberian traps. 2-7x increase in Ni doesn’t sound like much – what is it like on the other side of the world?

    So I think it is worth considering a P-T extinction as potentially a global warming kill. Especially as I was taking a precautionary-principle approach: does Paris do enough, even without the promised additional commitments, to essentially rule out going into such deadly territory.

    I do however agree that a mid-Pliocene world is a likely outcome, even with Paris.

  35. Dave_Geologist says:

    David Wallace-Wells pulls no punches in this chilling article

    I do think we’re storing up problems for the future by the way Paris has been spun/presented.

    As I understand it we have agreed a set of commitments which should deliver 3°C by 2100, with non-binding “promises” which, if implemented should deliver 2°C by 2100. And there’s some hand-waving ambition to get to 1.5°C.

    But most of the press has been about 1.5° and 2°C, not 3°C. So most of the public probably thinks that commitments their governments have already made will deliver 1.5°-2°C. Whereas they’ll have to do a lot more to get there.

    Which leads me to fear that when those additional measures are announced, people will say “no fair, you told us what we needed to do back in 2016, we agreed, now 5 or 10 years later you’ve moved the goalposts and want more”. And of course there will be actors who say that even if the scientists are honest, they can’t be trusted because they change the numbers every 5-10 years. Whereas the numbers haven’t changed, they’ve just been poorly communicated, perhaps out of a desire to spin Paris as more of a success than it really was.

  36. David B. Benson says:

    Dave Geologist — You couldn’t have bothered to read the linked paper, I fear. From Ogden & Sleep we have 3 events of a trillion tonnes of carbon each. That is not enough to explain the carbon and the sulfur isotope excursions. More happened and the linked paper explains how the approximately 27 billion tonnes of nickel catalyzed the methanogens. Then there were numerous undesirable results, including the sulfer excursion.

    Ogden & Sleep point out that fly ash from the coal combustion is found in Canada. The nickel sulfide is found at many locations in Eurasia, without indicating whether anybody has looked on other continents. As all these materials were lifted into the stratosphere one suspects worldwide depostion.

    In contrast, the less severe mass extinction at end-Triassic was the result of the largest igneous province formation. Also, the Deccan Trapps formation, about the same size as the Siberian Trapps, has no significant extinctions correlated, via recent fossil finds.

    So I continue to assert that it was the nickel what done it.

  37. Dave_Geologist says:

    We’ll have to agree to disagree David.

    I’d put it in the same place as the the recent paper suggesting it was an aerosol-induced glaciation what done it. Which doesn’t mean I rule it out, as I had the K-T impact in the same place in the 1980s. But one swallow doesn’t make a summer, and I like to wait for consilience. I was wrong (actually just a true skeptic) about the impact, but I was right about cold fusion and faster-than-light neutrinos, even though I had expertise in the first of those but not in the last two.

    Time will tell.

  38. angech says:

    “Ogden & Sleep point out that fly ash from the coal combustion is found in Canada. The nickel sulfide is found at many locations in Eurasia, without indicating whether anybody has looked on other continents. As all these materials were lifted into the stratosphere one suspects worldwide depostion.”

    If, nickel sulphide (was looked for and found) is found in many locations in Eurasia (quote) it logically stands to reason that it would have been looked for and found and documented on other continents.

  39. David B. Benson says:

    angech — Were that geologic exploration were so systematic and thorough. And even so, there would have to be a 252 million year old outcropping which withstood the ravages of time.

  40. @Dave_Geologist,

    Excellent stuff!


    OTOH it’s probably the change, rather than the absolute value which calcifying organisms can’t cope with.

    it’s not what you meant, but I’ve often asked the question

    What might the rate of increase in forcing mean for the climate? I mean, if one goes up +4C in 100,000 years that’s one thing, but doing it in 100 years is, geologically speaking, a shock, an impulse.

    I get silent nods and evades. Either there’s no coupling between the shock and the climate system at that rate, or no one knows, or no one knows how to figure that out, so they won’t. I’m particularly interested in ice sheet destablizations, but there could be other parts of the system. I agree it won’t affect deep oceans much.

  41. @David B Benson

    So I continue to assert that it was the nickel [sulfide] what done it.

    From paleontology, the trace of the extinction was pretty complicated, with indications that there was intermediate recovery. Evolutionary time constants are reasonably long, 100 Ky to 1 My, so it seems whatever happened to kill was itself complicated. Even the K-T looks like bolide+Deccan Traps.

  42. David B. Benson says:

    hypergeometric — The initial extinction of end-Permian took place in less than 20,000 years. Yes, it then went on and on…

    The nickel enabled methanogens so the available carbon was made all that much worse. Then lots of other things happened, made possible by the anoxic shallow oceans. But 30 gigatonnes of lithium, in three equal sized pulses is not to be ignored.

    For example, after the precisely end-Permian carbon isotope excursion, which soon disappeared, there was another, longer carbon isotope excursion in the early Triassic.

    One estimate is that the biosphere took fully 30 million years to recover.

    By the way, I disagree that the Deccan Trapps formation had anything significant to do with the extirpation of the non-avian dinosaurs but that is independent and a topic for another time. The end-Permian discussion came about by asking whether it offers any insight into the next several centuries; it doesn’t.

  43. Dave_Geologist says:

    Late addition to the thread 😉

    The end-Permian discussion came about by asking whether it offers any insight into the next several centuries; it doesn’t.

    As I said, we’ll have to agree to disagree on that.

    However, if you’re right the early Triassic does bear on what would happen if we get some combination of burn-all-the-fuel, high ECS and a CH4 kick from tundra, permafrost or clathrates. I only said it was irrelevant because I assumed we’d already have been done-in by our own P/T event. We really, really can’t afford to go there either.

  44. @Dave_Geologist, and all,

    Yeah, for sure, repeating the end-Permian is a tough act. But that’s hardly any reason to be sanguine regarding our course. There are many other events in geologic history which well could mean the demise of humanity, even if the rest of the biosphere simply adjusted.

    But, as @Dave_Geologist has noted, going there to see what happens is far from wise. As Prof Ray Pierrehumbert (PNAS, 2013) and, separately, Caballero and Huber (also PNAS, 2013) broadly remark, climate sensitivity is a non-linear thing and is likely to be state dependent. Caballero and Huber suggest that “hothouse climates” are likely to see higher sensitivity, and Pierrehumbert explains all this. and summarizes:

    One sure solution to the problem posed by uncertainty of climate sensitivity in hot climates is simply not to go there. Unfortunately, it looks increasingly like Nature will step in to answer some of our questions for us, and I doubt we’ll like the answer. The highest emission scenario currently being considered by the Intergovernmental Panel on Climate Change is Representative Concentration Pathway 8.5 (8), which would bring CO2 concentrations up to 2,000 ppm, which is in the upper reaches of the range considered in ref. 2. Even this scenario can be considered somewhat optimistic, in that it assumes that the annual growth in CO2 emissions rate (which has been hovering around 3% for decades) will tail off by 2060 and that the emissions rate will cease growing altogether by 2100, whereafter emissions will trend to zero; unrestrained growth could easily dump twice as much carbon into the atmosphere. It is not known if there are actually
    enough recoverable fossil fuels to emit that much CO2. Hoping that we run out of fossil fuels before bringing on a climate catastrophe does not seem like sound climate policy, but at present it seems to be the only one we have.

  45. Dave_Geologist says:

    In reality, I don’t think we’ll experience a P/T or an Early Triassic without a natural feedback such as permafrost or hydrate destabilisation kicking in.

    A PETM world will be more than enough to do in the current economy, probably with billions dying through war and starvation. That will force a reduction in emission rates.

    The hyperactive hydrology which existed then (megadroughts, megastorms and megafloods, inferred indirectly for ocean sediments where there is no land exposure) would make most modern agricultural practices impossible. There does seem to be a consistent pattern of decadal to multi-decadal cycles which are more extreme than the seasonal cycles, which is why I see it as being the other side of some sort of climatic tipping point.

    There is probably some research into why, but I would speculate the interaction of a warmer world with El Niño, QBO etc., such that what is currently a drier phase becomes a total drought, and a wetter phase becomes a megaflood time. Perhaps also the end point of a trend towards more intense but less frequent cyclonic storms. The chance of a particular place being hit by one goes down (so more years between hits) but when it hits, man it hits.

  46. John Hartz says:

    A very intereting analysis indeed…

    Shell has just released a new scenario, Sky, which meets the goals of the Paris Agreement. How does it compare with other low carbon pathways?

    Shell in a low carbon world by Glen Peters, Cicero, Mar 28, 2018

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