Sorry, I don\’t understand this

If the world is to stay below 2C of warming, which scientists regard as the limit of safety, then emissions must be held to no more than 450 parts per million (ppm) of carbon dioxide in the atmosphere; the level is currently around 390ppm.

We see this point being made all the time. But I\’m not actually convinced that it\’s the scientists saying it.

The point is that how much the temperature rises depends upon the value of climate sensitivity.

That is, how much does the temperature go up with a doubling of CO2? Pre-industrial levels were, can\’t remember, 260 ppm? 280 ppm?

So, 520 ppm or 560 ppm is a doubling.

And climate sensitivity is estimated to be somewhere from 1.5-4.5 oC I think? With 3 oC being the most likely value (William, help out here).

And here\’s the bit I don\’t understand. That 2 0C \”limit of safety\”: that\’s usually taken to mean, is often used by campaigners to mean that, passing that means we get \”runaway climate change\”. The feedbacks all swing into action and thus once we\’ve passed the point it all just gets catastrophically worse.

But the whole point about this climate sensitivity number is that it\’s already including all of these feedbacks. That\’s why it\’s so imprecise, because we don\’t actually know how all the feedbacks interact. Whatever the number is, say it\’s 3 oC, then that\’s how far the temperature goes then stops: because that\’s how all of the various feedbacks interact and play out from a 520 ppm CO2 level. Over quite some period of time too: not a few years, not even a few decades.

So I don\’t understand the 2 oC point.

OK, rather, I do understand it if it is being used to mean that that\’s the sort of limit before changes that we really won\’t like very much. But I don\’t understand it as it\’s often used, as some trigger point after which we get a cascade of ever more horrible things. Because our climate sensitivity number is already including the totality of the feedbacks.

Anyone care to straighten me out on this?

24 comments on “Sorry, I don\’t understand this

  1. It’s “Spock’s Brain” reasoning, which is quite commonplace. You start off with an estimate, and then that becomes a precise limit in future discourse.

    At the start of possibly Star Trek’s greatest episode, Spock’s brain is stolen. Kirk asks McCoy how long he can survive with no brain, and the surprising answer is, “no more than 24 hours”; that is, about a day. Subsequently, this becomes a hard deadline with a countdown, fine at 23:59, no hope at 24:01.

    It’s quite a common trope in both fiction and real life.

  2. Tim, if you have a read through the ‘Dangerous Climate Change’ thread at Bishop Hill you might find some answers. The 2 degrees limit seems to have been based on not much at all. An interesting thread.

  3. The theory of catastrophic global warming rests entirely on strong positive feedbacks, but we don’t know what they are. We don’t know if clouds are negative or positive for example, and a one per cent change in cloud cover can explain all the warming in the past two hundred years.

    The science is at a rudimentary level – and most people don’t know this.

    In theory, a doubling of CO2 should add 1-1.5C degrees of heat energy to the earth’s atmosphere, all other things being equal. But in a dynamic system it’s not that simple. More CO2 will certainly be removed through increased vegetation in the short term, and increased calcification in the long term (over 1,000 years).

    Lindzen puts climate sensitivity at 0.7C, and thinks we’ve seen most of it already.

    Even 1.5C over 50 years is entirely manageable through adaptation – and there are many positive to a warmer climate. But it doesn’t provide a rational basis for restructuring industrial society, imposing taxes and regulations, re-ordering lifestyles, which the Greenies want.

  4. As I understand it, climate sensitivity already includes feedback but I’m not sure why it is focused upon so heavily.

    Of more interest is the transient climate sensitivity. The earth is already very good at balancing the carbon cycle and human emissions of CO2 are only a small percentage of that yearly cycle but over a bigger time-frame that looks like a step input and therefore measures of transient climate sensitivity would seem more appropriate.

    My analogy would be that if you were to punch someone in the face they would be more concerned with the transient impact and how that might effect the steady state nature of their face than a thousand slow tender caresses over a lifetime.

  5. Ref. Spock’s Brain. Apart from Ritchie, any nominations for public figures who have outlasted Spock?

  6. “Even 1.5C over 50 years is entirely manageable through adaptation …”

    Ah, and there’s the rub. These people don’t want things to change.

  7. “Apart from Ritchie, any nominations for public figures who have outlasted Spock?”

    Well, there was the famous Mike, but being a chicken, he unaccountably is considered of less value to society than the Murphster…

  8. Kevin,
    Feedback is critical because the effect because the heat trapping effects of CO2 decline in a logarithmic fashion as it rises.

    So if we get 3 degrees from the first doubling, then we get the logarithm of 3 degrees for the second doubling which is an incredibly small number (note: this is not the actual formula, just an example to show the nature of the decline). So CO2 wont do it on it’s own.

  9. You invoked me by name, so…

    Regarding about 3 oC as the value for climate sensitivity makes sense. We don’t know the exact answer, and probably won’t in time. I tned to take my views from the IPCC modified by James Annan: for example http://julesandjames.blogspot.com/2011/01/better-late.html.

    Regarding 2 oC as “dangerous” is not easy to justify. People have linked to Richard Betts comments, which are reasonable. I’ve also written similar: http://scienceblogs.com/stoat/2011/05/tim_lenton_is_silly.php. 2 oC looks to be a sort-of meme that people have agreed to unite around, because attempting to discuss the full complexity of the uncertainties (emissions X sensitivity X damage X …) is just too big a search space.

    Note, though, that believing the likely T change is very uncertain is a poor argument for inaction.

    However, your most important point is “That 2 0C “limit of safety”: that’s usually taken to mean, is often used by campaigners to mean that, passing that means we get “runaway climate change”. The feedbacks all swing into action and thus once we’ve passed the point it all just gets catastrophically worse.”

    That is almost totally wrong (it may be what is passed into the general campaigning view, but don’t mistake that for the scientific view). 2 oC isn’t any kind of “tipping point” beyond which everything qualitatively changes. http://en.wikipedia.org/wiki/Runaway_climate_change might help, though it isn’t a great page, but The scientific consensus in the IPCC Fourth Assessment Report[19] is that “Anthropogenic warming could lead to some effects that are abrupt or irreversible, depending upon the rate and magnitude of the climate change.” Note however that this statement is about situations weaker than “runaway change”. is reasonable.

    > So I don’t understand the 2 oC point. OK, rather, I do understand it if it is being used to mean that that’s the sort of limit before changes that we really won’t like very much.

    Yes, that is the correct interpretation (remembering that 2 oC is global, warming is larger towards the poles and larger again over land, so that a not-very-big 2 oC might be 4 oC locally or more). Again, most of the really serious proposed problems are ecosystem feedback type thingies, which I know little about.

    > Even 1.5C over 50 years is entirely manageable through adaptation

    Clearly people, or at least those living around here, can adapt to +1.5 over 50 years, because people living further south already have. It is the ecosystems you want to worry about, and of course the other effects (changes in rainfall most obviously).

    > effects of CO2 decline in a logarithmic fashion as it rises…

    Yes, this is correct.

    > So if we get 3 degrees from the first doubling, then we get the logarithm of 3 degrees for the second doubling

    No, this is wrong. It is 3 oC for every doubling. To first order, the warming is log(concentration). Do the maths :-).

  10. jorb,

    I don’t think that there’s any argument regarding the logarithmic nature.

    For example based upon a climate sensitivity of 3C…

    ?T = 0 (280 ppm)
    ?T = 3 (560 ppm)
    ?T = 6 (1120 ppm)
    ?T = 9 (2240 ppm)

    where ?T is change from pre-industrial levels

    I don’t think anybody is debating that. I think they’re just debating what that climate sensitivity might be (and in Tim’s case whether that already includes feedback – which I believe it does). Current estimates are 1.5 to 4.5C.

    The point I’m trying to get at is that from my limited knowledge, the 1.5 to 4.5C is pretty meaningless because it represents the steady state – i.e. if the climate sensitivity is 3C then 560ppm for a million years will result in a 3C change. But this is the steady state value. What is of greater interest is how the earths temperature will behave in the short term (the next 1000 years) as a result of a step change in CO2 levels. The best guess 3C is measured from looking at the steady state (glacial samples etc) where the levels changed relatively slowly.

    We are in unchartered territory here and we may find that the transient climate sensitivity looks very different due to lagging feedback or overshooting and THAT is going to be very hard to predict.

  11. My understanding was that the 1.5 to 4.5 range was derived from two guestimates by alarmists , one being 2 and the other 4. This was combined in to the range 1.5 to 4.5

    Empirical evidence suggests that it is below the bottom of the range. For example the energy been measured leaving the Earth is higher than it would of the 1.5 to 4.5 range was valid.

    To some extent BEST confirms this as it clearly shows the warming trend was in progress for at least a century before CO2 started to rise.

  12. The 2C safety level is derived in exactly the same way as the governments safe limits for drinking.

    In other words, it’s plucked out of thin air with a view to (a) scaring the populace a la HL Mencken and (b) keeping the grants and the politicial power flowing in the right direction.

  13. > My understanding was that the 1.5 to 4.5 range was derived from two guestimates by alarmists , one being 2 and the other 4. This was combined in to the range 1.5 to 4.5

    Then you’re an idiot. You’re entirely welcome to disagree with it if you like, but don’t be so lazy that you spout nonsense without even bothering to check.

    Most obviously, try: http://en.wikipedia.org/wiki/Climate_sensitivity

    That will point you towards http://www.ipcc.ch/publications_and_data/ar4/syr/en/mains2-3.html as the first reference.

    Which will in turn point you at http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch8s8-6.html

    Which provides lots of reading and further sources.

    But I suspect you’re far too lazy to read that, so I recommend http://julesandjames.blogspot.com/2006/03/climate-sensitivity-is-3c.html instead for a nice simple intro and even the paper itself isn’t too hard http://julesandjames.blogspot.com/2006/03/climate-sensitivity-is-3c.html

  14. > The point I’m trying to get at is that from my limited knowledge, the 1.5 to 4.5C is pretty meaningless because it represents the steady state

    Only very partially true. You’ll find discussion of the equilibrium vs steady state in the IPCC reports, but the difference isn’t large when thinking of warming to 2100. At the level we’re discussing here, the distinction can be ignored.

  15. Have you been un-banned from Wikipedia yet, William?

    Here we find you quoting your own misinformation as a reliable and impartial source.

    As a Green Party candidate, you’re anything but impartial.

  16. “Then you’re an idiot”

    @ sigh @

    You can take the boy out of Wikipedia – but you can’t take Wikipedia out of the boy.

  17. @Andrew: yes, thanks for asking. Not that I ever was, but I know what you mean.

    > Here we find you quoting your own misinformation…

    *No*: you’re too stupid to even be an idiot. That wiki page *points you to other sources of information*. You don’t have to believe the contents of that page (I think it is largely accurate, but you don’t have to agree), all you have to do is *follow the links*.

    Which will direct you to an number of places that estimate climate sensitivity. Again, you’re under no obligation to believe them – feel free to substitute your own totally uninformed judgement – but what you aren’t allowed to do is pretend that those sources don’t exist.

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