Home > Deconstruction > Taking the C out of CAGW

Taking the C out of CAGW

2012 May 18

The following is not intended to “prove” that AGW does not lead to CAGW (defined as extinction events and/or collapse of civilization), but rather outlines why I don’t currently believe it. I do not consider myself well-informed regarding palaeobiology or palaeoclimate, so I welcome corrections to what I have outlined.

Extinction level events

Argument 1A: A 2-6C rise in temps will trigger an extinction event.
Reply: There have been many step changes of similar magnitude (6-8C Antarctic, 2-3C global?) over the last million years, and again with the melt of Antartica 10 million years ago (~4C polar ocean), and again with the PETM 56 million years ago (6C global). Such abrupt changes are anomalous, but they are not unprecedented. None of the upward step changes are associated with named extinction events. Although the *cooling* in the midMiocene has been described as a disruption with notable extinctions.

Argument 1B: The rate of change over the next several hundred years is too fast for the biosphere to handle.
Reply: The rate of change during the PETM may be the best natural analogy. It might have occured over a period of 20,000 years or so, about 1.5 orders of magnitude faster than the rise due to the release of CO2 from fossil fuels. (about 500 years). Wiki says of the PETM: The PETM is accompanied by a mass extinction of 35-50% of benthic foraminifera (especially in deeper waters) over the course of ~1,000 years – the group suffering more than during the dinosaur-slaying K-T extinction. Contrarily, planktonic foraminifera diversified, and dinoflagellates bloomed. Success was also enjoyed by the mammals, who radiated profusely around this time. … There is no evidence of any increased extinction rate among the terrestrial biota. So while impactful, the PETM event does not support a theory that AGW induce a widespread biotic collapse.

Counterpoint: Bad analogy. 1.5 orders of magnitude is too great a difference.

Argument 1C: The equilibrium temperature post fossil fuels will be too high for the biosphere to handle.
Reply: The middle Miocene Climatic Optimum, ~17 to 15 Ma, is representative of climate under x2 preindustrial CO2 and about 3-6C warmer in mid-latitudes. While I haven’t seen actual NPP studies for the Miocene, most descriptions are ‘warmer and wetter’, suggesting higher NPP. I am intrigued by the apparent lack of arid deserts in the few Miocene papers I have read.

Counterpoint: http://www.pnas.org/content/107/21/9552.full.pdf Sherwood, Huber (2010)

2. Collapse of civilization

Argument 2A: Climate change will severely impact agricultural production leading to collapse.
Reply: Climate models show a strong increase in “Net Primary Production”, much of which is directly attributable to the increase in CO2. Models are much less certain if the changes in “climate” sans CO2 will have a net positive or negative effect, although they lean towards the negative. It does seem likely that there will be shift in C3/C4 plant ratios and distribution. In addition, barring collapse for other reasons, it seems likely that technological innovation in both modified crops and improved farming technologies will occur faster than climate stress will decrease production. Caveat: the net effect of climate change with other environmental degradations and economic stresses may outstrip our innovation speed. Which is not to say that some regions won’t be net losers in the shifting climate. I believe that the US South West and most of Mexico, North West China, the Mediterranian, and South African(?) regions will be counted among the losers.

See also:
Fig 5 in Terrestrial plant production and climate change Friend (2010)

Fig 3b from Climate–Carbon Cycle Feedback Analysis: Results from the C4MIP Model Intercomparison Friedlingstein et al (2006)
(models cover wide range of results for T-driven NPP; but all model large increase in net NPP)

Fig 3 in Evolution of carbon sinks in a changing climate Fung 2005
(NPP drops south of ~30N, increases north)

Modeling the Recent Evolution of Global Drought and Projections for the Twenty-First Century with the Hadley Centre Climate Model Bourke (2006)

Dustbowlifaction Romm (2011)

Argument 2B: Coastal flooding will lead to collapse.
Reply: While sea level rise is inevitable in AGW world, total melting of the Antarctic is unlikely. The Pliocene world at 2-3C warmer, sea levels were about 6 meters higher. Even if we had a rise of tens of meters, this is likley to occur over a period of centuries. Port infrastructure is likely to be replaced several times over that time scale, so it is hard to see how this would lead to collapse of civilization, although the loss of coastal lands is not insignificant.


3. Ocean Acidification

Argument 3a. Ocean acidification will trigger an extinction event
See the following for both the point and the counterpoint …

The most salient paleo-analog to the current atmospheric CO2 increase is the strong ocean
acidification event at the Paleocene-Eocene thermal maximum (PETM) 55 Mya. The PETM is
marked by the sudden and massive carbon input to the ocean/atmosphere system, a shoaling of
the deep ocean’s calcite saturation horizon by at least 2 km in less than 2000 years that did not
recover for tens of thousands of years, global warming of at least 5?C in less than 10,000 years,
and major shifts in marine planktonic communities (Kennett & Stott 1991; Zachos et al. 1993,
2003, 2005). The only major extinctions occurred within the benthic foraminifera, though it is
unclear whether ocean acidification was the main factor or whether changes in ocean circulation
led to anoxia in bottom waters (Zachos et al. 2008).

However, the similarity of the PETM and several comparable, but smaller, Eocene events to
modern conditions is incomplete. First, whether the carbon excursion at the PETM was as rapid
as the present-day excursion remains unclear. Second, the PETM and smaller events occurred
within a background of already high CO2 and global temperature. Third, the Mg:Ca ratio, an
important factor that affects the carbonate mineralogy of many organisms, was also significantly
different from that of today (Stanley & Hardie 2001). Finally, the marine biota during the PETM
were also different. Corals and coral reefs had not yet re-established following the Cretaceous-
Tertiary extinction (Wood 2001); modern coccolithophores are very different from those of the
early Tertiary (Young 1994); and modern thecosomatous pteropod families appeared after the
PETM, in the Eocene and Miocene (Lalli & Gilmer 1989).

Ocean Acidification: The Other CO2 Problem Doney et al (2009)

There are many serious consequences aside from extinction events and collapse of civilizations.

Argument 4A: You are neglecting severe consequences by concentrating on extreme crisis.
Reply: I am not neglecting severe consequences. I am trying to define catastrophic consequences and (informally) assess their likeliehood. I am totally open to the possibility that my assessment could be off and welcome references that could be inform it.

Edits for typos. I am also likely to edit at will to include additional cites or details.

  1. 2012 May 19 at 12:23 am

    You say elsewhere “My mind is not closed on any of this” & say that present CO2 levels are not high on a 10 million year timescale.
    Present CO2 levels are high on a 10 million year timescale because we are breaking 15 million year records.
    20 million year records are well within reach if a “it’s not a big problem” attitude is taken & cuts are not made quickly.
    Add in a helping hand from nature in the shape of reduced ocean sinks & thawing permafrost & we could be knocking on the door of 40 million year records, 50 million year records.
    So where would that put us on your annotated “65 Million Years of Climate Change” graph?

  2. 2012 May 19 at 2:58 am

    In a yet-unpublished paper of mine, I define “F” as the fraction of Earth’s land surface in “severe drought.” This is in turn defined as a Palmer Drought Severity Index score of -3.0 or below. F was fairly steady from 1948 to 1970 at around 12%. Since then it peaked at 31% in 2003 before dropping down to 21% in 2005. A very volatile series.

    I have shown that F is a function of past values of F, and current values of dT (temperature anomaly), with 86% of variance accounted for. I did all the standard checks for residual autocorrelation, integration, multicolinearity, etc. Granger causality is unequivocally from temperature to drought.

    This logically implies that

    1. Some time in this century, most likely the 2050s, F will reach 70%.
    2. At or around this point, every country in the world will bring in a poor harvest.
    3. Since world food stores are a nice Biblical number, 40 days, most of the world will be dead the following year.
    4. Since F is a highly variable series, the year of agricultural collapse may well come as a surprise,

    Which part of “every harvest in the world will fail and almost everybody will be dead in a year” would you define as “not catastrophic?”

  3. 2012 May 19 at 7:58 am

    Obviously, we don’t all have access to your paper, so we must evaluate what you suggest based on your comment. It’s not clear to me that your increase in F would result in crop failures for a number of reasons.

    Do you mean this:http://www.drought.noaa.gov/palmer.html

    The advantage of the Palmer Index is that it is standardized to local climate,…

    Or do you mean an index that is not standardized to local climate?

    If you mean the index standardized to local climates, it seems to me that if F is very large, then starvation could be largly avoided by adaptation.

    First, there is the possibility that people would abandon farming in drier regions that became drier relative to the current climate and shift to farming in currently wetter different regions that might be a bit too wet for many crops. For these other reasons “semi drought” defined based on the local climate might improve yields relative to what is possible now. Around here we sometimes see spots where crops failed because that portion of the field remained too wet in a particular year. (Other years, we see problems because the growing season was too dry and generally, I suspect too dry is a bigger problem that too wet. But that’s because farmers generally simply avoid places that remain soggy– or drain them and so on.)

    Also, since that index is relative to local conditions, some degree of adaptation merely involves planning. Some regions that currently don’t irrigate– like Central Illinois– might pump available water long distances– like Lake Michigan. People could also store water– as my husband’s grandparents did in the 20s-40s. (His mother was a laundress and they had a cistern to store rain water which was softer and plentiful, but less predictable than well water.)

    It’s costly to store water, but if the alternative is risking starvation in 2050, by 2040, farmers and farming communities could likely build reservoirs. With planning, lower rainfall might not result in crop failure around here. (FWIW: The village of Lisle tries to promote installation of water barrels — partly to relieve the water needs in the summer, and partly to reduce the sudden inundation of low lying areas during sudden thunderstorms. If we had several very dry summers in a row, I might install a cistern or barrel. )

    Second: But assuming extrapolation based on your model predicting F works on out of sample data, why would the massive crop failures happen all of a sudden? It seems more likely that if F rises sufficiently high, average crop yields would decline over time. If this impacted yields sufficiently, population would decline hand in hand with that. The result after that would be a reduction in demand for food as population falls. But under the slow scenario, the total decline in population would likely be smaller than under one where we assume no impact on population until, suddenly, one year, F hits 70% and every country in the world brings in a poor harvest.

    Also, if yields began to be highly variable in 2030, it’s quite likely more people and communities would begin to store more than 40 days worth of food. My pantry isn’t stocked like a Mormon’s– but if we began to experience frequent food shortages, that would change. I’d stock bags of grain, soy, canned goods, tuna etc. I imagine others would do so as well. I wouldn’t be surprised if the amount of stored food didn’t rise well beyond 120 days worth. I might also develop a taste for squirrel meat which would further stretch the “reserves”. But the cute critters are edible and are fairly easy to trap.

    So while your model might be correct, and major crop failures are certainly a bad thing–many things could prevent the consequence you forsee. It’s just not clear that if the thing you define as F=0.70%, most the human population would die in one year.

  4. 2012 May 19 at 8:53 am

    Al, I doubt that any of the climate conditions post-Cretacious are hazardous to the modern biome with the glaring exception of polar ecosystems. That includes temps up to 6C warmer globally (higher in the poles) and CO2 up to 1000ppm.

    That might even render parts of the globe uninhabitable to human populations exposed to the elements without cooling technologies.

    But, to be blunt, those parts are not the heart of the modern technological industrial civilization.

    I set the bar high for ‘catastrophe’. Collapse of civilization or an extinction event notable on geologic timescales. (Notable also implies a magnitude or greater than the extinction events already occurring due to human development, overfishing, overhunting, overgrazing, desertification, and deforestation. Might as well add in riparian pollution and nitrogen blooms).

  5. 2012 May 19 at 9:03 am

    BPL: unfortunately I don’t have access to your model, so I am at a disadvantage in discussing it. But, I would like to see how agricultural production responds to F before I conclude that widespread starvation follows high F.

    Considering your problem space, my line of attack (ie … in understanding the potential impacts to agriculture due to future changes in locations/intensity/persistance of drought) would be to1) calculate PDSI from models/ensemble and 2) model agricultural production as a function of PDSI. But 2 is also a function of technology – growing methods, harvesting methods, new breeds/strains/genetics, new fertilizers, new water management techniques.

    But an additional confounding factor is what the positive impacts on agriculture might be, especially in relation to longer seasons, but also perhaps in more favorable precipitation.

    And last, in 100 years, we may well be farming the seas in a way that we have only just begun.

    I’m not a big advocate of converting the ‘wild seas’ to ‘domesticated aquaculture’, but hungry future generations aren’t terribly interested in my aesthetic or ethical preferences.

  6. 2012 May 19 at 9:09 am

    To be clear, I am not discounting the possibility of severe and costly consequences to climate change – costs that (potentially) could be reduced through appropriate carbon policies today.

    But I have not been persuaded that this is an ‘existential crisis’ for either the planet or our civilization.

  7. 2012 May 19 at 11:27 am

    Ron Broberg
    This is mouth-dropping stuff I’m reading here.
    You say “But I have not been persuaded that this is an ‘existential crisis’ for either the planet or our civilization,” which is the position you stated prior to my being directed here. Yet I find it difficult to accept that we are talking about the same “civilisation.” If we affluent folk who dwell in the temperate regions of this planet cause pollution that renders the tropics uninhabitable, I would consider it entirely uncivilised to explain it away by saying “But, to be blunt, those parts are not the heart of the modern technological industrial civilization.”
    I will be blunt. This is a statement of facism.

  8. 2012 May 19 at 12:10 pm

    Al, mouth dropping, perhaps. But am I wrong?

    Would modern technological industrial civilization collapse if parts of the tropics became uninhabitable to humans for part of the year without tech assistance?

    And this point we are discussing is a very unlikely, worst case scenario of a rise in global T of 12C which is well outside the range of current projections. It is precisely because this is an extreme worst case scenario that I mention it. And if a case as extreme as this won’t directly lead to a crash of civilization, then it is very unlikely that any AGW scenario based simply on the end result (argument 1c) will lead to it either.

  9. 2012 May 19 at 10:53 pm

    Ron Broberg
    You ask my views – “Would modern technological industrial civilization collapse if parts of the tropics became uninhabitable to humans for part of the year without tech assistance?”
    I don’t think it would survive. But this leaps ahead. To address your follow-on comment which you use to frame your question.

    An uninhabitable tropics you discuss here is not, as you assert, “very unlikely.” Neither you or I need look far to see this. Support for what I say is embarrassingly close to hand as you reference above Sherwood & Huber 2010 –
    “it would begin to occur with global-mean warming of about 7°C, calling the habitability of some regions into question. With 11–12°C warming, such regions would spread to encompass the majority of the human population as currently distributed. Eventual warmings of 12°C are possible from fossil fuel burning.”

    And I remain extremely uncomfortable with the idea of an ‘expendable tropics’, even if I ‘civilisation’ were able to survive such an occurrence! (You will have noticed my earlier accusation of fascism.)

  10. PeteB
    2012 May 20 at 12:44 am


    I thought that the “mainstream” position that AGW is unlikely to lead to the collapse of civilisation / extinction events , I’ve not seen anything in the IPCC reports that suggest otherwise, although it could result in pretty severe regional pressures and costs. I’m pretty sure I’ve seen similar statements from James Annan, William Connolley, Gavin Schmidt and others.

    But I think we have to be careful to not buy into the narrative “AGW is not catastrophic (whatever that means) so we are better off adapting than mitigating”

    We have to balance the damage done by AGW (including environmental and ecological damage) with the economic damage of limiting fossil fuels.

    If we can estimate the damage done by different AGW scenarios and their likelihood (and I recognise there will be some difference in estimates for things like environmental and ecological damage) we can build this into the price of fossil fuels as a carbon tax. This will ensure fossil fuels are competing against their competitors on a level playing field as the price includes the cost of the damage they are doing. For some applications I think the alternatives will quickly become more competitive, other areas will probably take longer until an alternative becomes more cost effective.

    This seems the best way of balancing mitigation with adaption.

  11. PeteB
    2012 May 20 at 12:55 am

    from realclimate

    But I think in general the consensus gut feeling among small-minded working scientists like me is that the odds of such a catastrophe are low.

    Low odds of catastrophe does not imply negligible. Nordhaus [2001] considered the possibility of catastrophe in his analysis of the economics of climate change. He defined catastrophe as comparable to the Great Depression, a 25% decrease in global economic activity that lasts for a long time. The probability of such an event he estimated by polling the gut instincts of a group of climate scientists; for what it’s worth, they came up with probabilities of a few percent. Economically, Nordhaus found that this possibility imposed the largest cost of adapting to climate change, greater than the costs of sea level rise, potential change in storminess, and so on. My own belief is that economics is a flawed tool for managing global climate, because it neglects issues of fairness, and reduces the value of the natural world to units of money. The point is that, within this framework, a small possibility of a large catastrophe looms large as a practical issue.

  12. 2012 May 20 at 5:21 am

    “defined catastrophe as comparable to the Great Depression, a 25% decrease in global economic activity that lasts for a long time”
    This is not an the same as an existential threat.

  13. 2012 May 20 at 9:06 am

    PeteB, I agree with all that you say. I don’t think catastrophe (defined as existential threat) is a mainstream view. However, I think Dr Hansen takes a step or two into that territory in The Storms of my Grandchildren when he discusses the ‘Venus syndrome’. To be fair, I haven’t read the book, just this snippet

    Catastrophe defined as “severe” economic stress is quite likely. I don’t like the cumulative economic stress of climate change, oil depletion and Western economic debt – especially when the counterplay is “not yet economically realized technological innovations”

    It is interesting that of the three common fossil fuels, two of them have seen price hikes that I think are much higher than commonly proposed CO2 taxes/surcharges. Oil has tripled in price over the last decade; coal has doubled (with a x5 peak in 2008).

    So if pricing carbon higher is the key policy, resource depletion and global markets are likely to do it for us anyway and far more comprehensively than any government policy. But if it were the key policy, we wouldn’t have Hansen picketing coal plants and McKibben trying to shut down Keystone.

    The real policy is the creation of a non-fossil-fuel economy. But given today’s tech, that economy is either nuclear or has a lower energy intensity. And without solid improvements in energy efficiency, lower energy intensity is tantamount to saying materially poorer. And if the fossil fuel economy collapses, the resulting damage that 9-12 billion people trying to survive on a planet that will only sustain a fraction of them without the energy and fertilizers that fossil fuels produce will be, IMO, far more devastating than that of climate change itself.

  14. 2012 May 20 at 9:32 am

    re Al:

    We conclude that a global-mean warming of roughly 7 °C would create small zones where metabolic heat dissipation would for the first time become impossible, calling into question their suitability for human habitation. A warming of 11–12 °C would expand these zones to encompass most of today’s human population. This likely overestimates what could practically be tolerated:

    if Ts = 3C for CO2 doubling, it would take roughly two doublings to get to the edge of the danger zone and 3 doublings to be smack in the middle. Given preindustrial CO2 ~300ppm, that puts 2 doublings around 1200ppm (and I think it also pushes us out of the post Cretaceous climate).

    Given that the paper describes a warming greater than 6C and a CO2 regime likely higher than 1000ppm, I will concede that it actually describes a component of catastrophic climate change – not due just to the introduction inhabitable zones, but in conjunction with other effects that such a step size, over the course of a single millennium, and an effective T greater than the biosphere has experienced in (very roughly) 65 million years. Recall that my basic (admittedly ill-informed at this point!) intuition is that the Paleogene/Neogene climates are not too extreme for the modern biome. Sherwood and Huber are describing a world which is leaving the Paleogene/Neogene climate behind.

  15. 2012 May 21 at 10:29 am

    Ron Broberg

    Surely no sane man would chose to practise his cartwheels & handstands on the edge of a high cliff. Thus to “get to the edge of the danger zone” would be likewise a step too far.

    Sherwood & Hubber 2010 talk of ‘A 4°C increase in TW (that) would then subject over half the world’s population annually to unprecedented values and cut the “safety buffer” that now exists between the highest TW(Max) and 35°C to roughly a quarter.
    With a conversion factor of 0.75:1, that ‘4°C increase in TW‘ would equate to an average global temperature rise of 5.3°C.

    Let us examine this 5.3°C as a speculative limit beyond which severe global devastation lurks. (It would be a policy limit set far too high by my own reckoning.)

    I often hear a business-as-usual CO2 level of 780ppm by 2100. Let that be an end on it – the folly of burning fossil fuels should be impossible to ignore well before then, shouldn’t it?.
    Add 100ppm CO2 extra in the atmosphere released by melting permafrost (not necessarily by 2100, although there is estimated to be 1.5TtC up there if temperatures rise high enough) and double it for an estimated 2% of the C emitting as methane. We are pretty much at 1000ppm CO2e, or 1.8 ‘doublings‘.
    Now you say above “if Ts = 3C for CO2 doubling.” A Ts of 3 is the value which would yield that global temperature increase of 5.3°C for 1.8 doublings. If however Ts were higher, so also would be the resulting global temperatures. Add in safety tolerances for other assumed values used & pushing CO2 up to 780ppm starts to look like sheer lunacy.

    This is all very much back-of-a-fag-packet analysis but I would assert more valid that say setting prior Cenozoic climates as some allowable precedent.
    (& I hope the HTML works in this domain.)

  16. Rob Honeycutt
    2012 May 21 at 10:37 am

    Ron… I believe your figures for 3C are based on fast feedbacks and for equilibrium temperature. If we got to 3C that would likely mean 4C+ was baked in (so to speak). And even those figures have a large degree of uncertainty. The CS response might be higher or it might be lower.

    If we were watching this happen on another nearby planet that was not our own it would be extremely fascinating to watch and learn what happens. When we’re in the test tube it changes the experiment a little bit. It’s certain that there are tipping points out there. We don’t know where they are but we’re certain they are there. We’re likely watching one right now with Arctic sea ice.

    It’s also not just one experiment going on inside our test tube. We have atmospheric CO2 levels rising and climate change. We have human population reaching near 10B in the same time frame. We have large numbers of human’s attaining first world status. And we also have a wide range of potential technical solutions to any of these given problems.

    We can’t know the exact results of the complex experiment ahead of time. That means we have to apply some risk analysis for what the potential range of outcomes might be. What are we willing to risk on the chance that this might NOT be catastrophic?

  17. 2012 May 21 at 11:14 pm

    Al, I took a closer look at Sherwood and Huber, and note two quick things: 1) they also use the upper limits of the Paleogene as a kind of potential natural mammalian limit (see discussion section) and 2) they misread Meinshausen’s discussion of 90% range of sensitivity and overestimate the possibility of ECS > 7C.

    Rob, I took a closer look at a range of ECS and likely fossil fuel forcings in my latest post.

    I moved towards a more pessimistic position, not because I believe the arguments that I outlined above are undermined, but because it seems to be easier to move (a higher probability to move) past the Paleogene climate than I realized.

  18. 2012 May 22 at 1:25 am

    Ron Broberg
    You say you “note two quick things.” Perhaps you are a little too quick.

    Point 1). Here you are wrong. Sherwood & Huber 2010 do not use palaeo-climates as some gauge of severity for what is or isn’t acceptable as an outcome from AGW. They use it to test their thesis asking “Mammals have survived past warm climates; does this contradict our conclusions?

    Point 2) Here you agrue no more than a technicality. The reference to Meinshausen et al 2009 is not essential to Sherwood & Huber 2010. There is a ‘fat tail’ on the estimates of climate sensitivity. If Meinshausen et al 2009 do not provide such a reference (& I go no further than their abstract), there are many papers that do.
    Whether a sensitivity of 7.1 is 0.5% or 5% makes no impact on Sherwood & Huber 2010. Indeed they continue this passage thus:- “Because combustion of all available fossil fuels could produce 2.75 doublings of CO2 by 2300 (5), even a 4.5 °C sensitivity could eventually produce 12 °C of warming.

  19. Ned
    2012 May 22 at 7:12 am

    Al Rodger writes:

    An uninhabitable tropics you discuss here is not, as you assert, “very unlikely.”

    A tropics where some areas are periodically subject to dangerously severe heat waves is not the same as “an uninhabitable tropics.” You radically underestimate people’s abilities to adapt to climate via behavior and technology.

    Much of the globe already has climates where a person without access to technology (clothing, shelter, wells, etc.) would die of cold, heat, or thirst if exposed to the climate long enough at the wrong time of the year. Yet people do live in all those places.

    I think Ron is right that the scenarios at the upper end of Sherwood and Huber’s range are unlikely. I think he’s also right that they would not likely lead to “the collapse of civilization”. That does not mean that Ron, or I, or anyone else is blithely willing to raise global temperatures by 8C.

    There are many things I would prefer to not have happen that still do not fall under the heading of “collapse of civilization”!

  20. 2012 May 22 at 11:14 am

    Do read the reference here being discussed Sherwood & Huber 2010 which explains how the tropics, and indeed much more, will become uninhabtable for humans (& also for other large fauna) without access to air con. This is why some species shrank in size 55 million years ago when there was a similar warming to the AGW we are creating today, only back then it happened much much slower.

  21. Ned
    2012 May 22 at 3:41 pm

    Al Rodger, I’ve read S&H 2010. Unless I had read it, I would not have written “I think Ron is right that the scenarios at the upper end of Sherwood and Huber’s range are unlikely.”

    I don’t think “…explains how the tropics, and indeed much more, will become uninhabtable for humans” is a reasonable summary of S&H2010. The language used in S&H2010 is alarming enough without exaggerating it further.

    In addition to not exaggerating the findings of S&H, you need to be very careful when reading it to pay attention to what degree of warming is associated with what impact. The more dramatic impacts they cite are associated with warming of ca. 10C in the tropics, which corresponds to a global mean increase of well above 10C.

    So, in summary:

    1. The more extreme scenarios discussed in S&H 2010 are not especially likely to occur.
    2. The more extreme scenarios discussed in S&H 2010 are not as extreme as you suggest.
    3. The more extreme scenarios discussed in S&H 2010 would not necessarily lead to a “collapse of civilization”, which is what I believe Ron is focusing on here.

    Again, there is plenty of room for something to be bad, awful, or even monstrous, and yet not lead to the collapse of civilization.

    I like the current distribution of climates around the globe. I would prefer not to undergo a large-scale experiment that might perturb global temperatures by 3C, let alone 6C. But exaggeration doesn’t lead to improved understanding.

  22. 2012 May 22 at 9:56 pm

    Al, I agree with Ned. I’m not welcoming the changes that might come with 3-4C of warming, but I don’t see them as an existential threat – to us or to the biosphere. And Sherwood and Hubert agree with me.

    Quoting: A
    global-mean warming of only 3–4 °Cwould in some locations halve the margin of safety (difference between TW max and 35 °C) that now leaves room for additional burdens or limitations to cooling. Considering the impacts of heat stress that occur already, this would certainly be unpleasant and costly if not debilitating.

    warming 3-4C cuts the margin of safety in half and is unpleasant and costly
    warming 7C begins to create uninhabitable regions
    warming 10-12C displaces a majority of humans

    Looking at fossil fuel emission scenarios and the range of likely climate sensitivities (some published yesterday, more tomorrow or next), shows that is is very unlikely that we will reach 7C warming directly from fossil fuel consumption (although I haven’t looked at methyl hydrates/methan feedbacks yet)

    In addition, my ‘rule-of-thumb’ that 6C as an upper safe limit is below the SH2010 level for the beginning of uninhabitable zones.

    If you have another ‘dire consequences’ paper to propose we examine, I would be happy to continue this, otherwise, we seem to be repeating ourselves.

  23. 2012 May 23 at 2:43 am

    Our interpretations of S&H2010 appears wildly at variance which is why I took the view you had yet to read it fully.
    If we are after “a reasonable summary of S&H2010“, perhaps the following would be authoritative, it being from their abstract.
    …it would begin to occur with global-mean warming of about 7°C, calling the habitability of some regions into question. With 11–12°C warming, such regions would spread to encompass the majority of the human population as currently distributed. Eventual warmings of 12°C are possible from fossil fuel burning.

    This 12°C warming is among the “more extreme scenarios discussed by S&H2010” & I interpret your comment as saying that this is the “unlikely” outcome that ‘would not necessarily lead to a “collapse of civilization”.’
    (I am not sure if our host, Ron Broberg, agrees with you on this point but he has said it is outwith the present limits of previous Cenozoic climates, if not by its equilibrium state then by the speed of its onset.)

    Myself, I certainly cannot agree with you. A technology-using society of some form would likely not be snuffed out but with a socio-political discontinuity far far greater than that of the mid-first millennium, civilisation cannot be anything other than “collapsed”.

    I will develop this to consider less extreme AGW when your position here has been clarified.

  24. 2012 May 26 at 7:54 am

    Looking over Hansen and Sato Climate Sensitivity Estimated From Earth’s Climate History (2012), my earlier guess that Cenozoic temps max out around 6C higher than today may be a significant underestimation. This paper (referencing Zachos 2008) caps Cenozoic temps around 12C above today with early Cenozoic in the 8-12C range and not falling to 6C above present until about 40Ma.

    You can see the same information in Beerling and Royer Convergent Cenozoic CO2 history” (2011). However, Beerling and Royer does somewhat support my guess of a Cenozoic upper limit of about 1000 ppm CO2 re ocean acidification.

  25. Kevin
    2012 June 18 at 8:59 am

    It seems odd to equate the word “catastrophe” with “existential threat.” To grab the dictionary dot com definition off the top of the google pile, catastrophe is defined as “An event causing great and often sudden damage or suffering.” This condition seems likely to be satisfied many times over by various heatwaves and floods, and indeed this has already happened in recent years. To say you are looking at whether the consequences of global warming will be “catastrophic” by analyzing the likelihood of the destruction of human civilization is really torturing the English language. In the normal use of “catastrophe,” the “C” in “CAGW” is a done deal. The premise of your post here strikes me as an enormous re-positioning of goal-posts.

  26. 2012 June 19 at 7:06 pm

    I accept the idea that common language uses ‘catastrophe’ at a lesser level than ‘existential threat.’ But I think it is worthwhile to be able to distinguish between those who believe that AGW will be the dominant cause in future massive human die-offs, the collapse of civilization, or the mass extinctions.

    In the end, I am just exploring the high end risks/probabilities. I’m willing to consider alternate terminology if you have something to suggest.

    OTOH, I don’t think I have ever heard the phrase ‘catastrophic heatwave.’ (Although google has some hits. Drought and heat tend to be slow events. I think ‘catastrophe’ tends to be used in common language for more sudden events like earthquakes, tornadoes, and tsunami.

  27. mk
    2012 August 30 at 2:09 am

    Ron Broberg :
    warming 3-4C cuts the margin of safety in half and is unpleasant and costly

    You seem to have a reading comprehension problem. S&H say that it is at least unpleasant and costly; it can be debilitating. You radically underestimate the threat to societal coherence of such conditions, and loss of coherence makes adaptation more difficult. Creating uninhabitable regions and displacing a majority of humans are far beyond the point of societal breakdown, so blathering about how likely or unlikely they are is a diversion.

    I accept the idea that common language uses ‘catastrophe’ at a lesser level than ‘existential threat.’ But I think it is worthwhile to be able to distinguish between those who believe that AGW will be the dominant cause in future massive human die-offs, the collapse of civilization, or the mass extinctions.
    In the end, I am just exploring the high end risks/probabilities. I’m willing to consider alternate terminology if you have something to suggest.

    In the end, your goal is to downplay. Your title is “Taking the C out of CAGW” — that’s dishonest when every normal human being would consider the consequences to be catastrophic even if it falls short of mass exticntions … which is a very long way off from collapse of civilization. You absurdly talk about “named” extinction events, but they only get names because the number of extinctions are on the order of half or more of extant species; collapse of human civilization does not entail the extinction of even one species … although we are already seeing species extinction as a result of GW, and can reasonably predict many more.

    That your title is a lie and that you are spouting BS about what you are distinguishing is illustrated by your own illustration, which pictures a catastrophe but no extinctions, massive die-offs, or collapse of civilization.

    Finally, it is the deniers and faux-skeptics who put the “C” in “CAGW” … it’s just one prong of many of their disinformation campaign, that allows them, when they need to, to acknowledge AGW as opposed to the “hoax” and “alarmism” of the “profiteering warmistas”. To even use the term is to play their game.

  1. 2012 May 21 at 11:06 pm
  2. 2012 May 27 at 10:42 am
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