Recovering from the LIA?

I can’t remember if I’ve written about this before, but since I haven’t written a post for a few days and would prefer to address something physicsy (rather than something controversial), I thought I might address the issue of us simply recovering from the Little Ice Age (LIA). There are some people who suggest that some (or all) of the warming we’ve experienced is simply a recovery from the LIA. This has a tendency to then irritate a lot of other people. The reason for this is that it doesn’t really make physical sense. It’s not that you can’t use the word recovery, but you do have to be careful about what you mean.

So, why is it irritating? Well, our climate isn’t some kind of bouncing ball that has an equilibrium that’s defined by it’s own properties. The mean temperature of the planet is essentially determined by 3 factors, the amount of energy we get from the Sun, the amount that we reflect directly back into space (albedo), and the composition of our atmosphere (the greenhouse effect). If one of these things changes (what we’d typically call a change in forcing) we will warm if the changes causes us to get more energy than we’re losing, and it will cause us to cool if we lose more energy then we’re gaining. There are other effects (internal variability) that can cause the surface temperature to change, but if these other effects do not change the solar flux (impossible), our albedo (unlikely), or the composition of our atmosphere (possible in some cases), we will return to equilibrium quickly.

So, the period referred to as the Little Ice Age, was a period where the solar flux was lower than today (we were getting less energy from the Sun) and – I think – was a period of enhanced volcanic activity. Volcanoes release aerosols which act to reflect incoming sunlight, producing a negative forcing and hence producing cooling. So, some of the warming since the LIA has been because the volcanic activity has reduced, and the aerosols have precipitated out (which happens quite quickly). Could this still be occurring today? No, because the “recovery” time after enhanced volcanic activity is quite short. As I point out in this post, even in the extreme case of cooling resulting in us falling a full degree below the equilibrium temperature, it would still only take a few decades to “recover”.

What about responding to the increasing solar forcing? Sure, that’s indeed happening and is incorporated in all the models. What to bear in mind though, is that the solar forcing has dropped in the second half of the 20th century and we’re still warming. Also, we’ve warmed by maybe 1 K since the LIA and solar forcing has increased by no more than 1 Wm-2 (probably less). This gives a sensitivity of at least 1 K/Wm-2 which – multiplying by 3.7 Wm-2 – would imply a climate sensitivity of 3.7 K per doubling of CO2. So, if you want to attribute our warming mainly to the slowly increasing solar forcing since the LIA (which has started dropping since the mid-1900s) you have a bit of a contradiction. How can we be so sensitive to changes in the solar forcing while being completely insensitive to changes in greenhouse gas forcings?

There’s probably more that could be said, but I hope this helps to explain why you may find yourself irritating others if you happen to suggest that we’re simply recovering from the LIA.

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67 Responses to Recovering from the LIA?

  1. John Hartz says:

    ATTP: Thanks for posting this. You might want to append links to some of the learned articles about the LIA that have been posted on Skeptical Science and Real Climate.

  2. John H.,
    I was thinking about adding some links, but I did this rather quickly as I had some other things to get done. May do so if I get a free moment.

  3. andrew adams says:

    To an expand on a point I made on this topic in the previous thread, it is worth looking at this in the context of what we know about long term (in human terms) historic temperature trends. Over the last ~800k years the climate has indeed been a bit like a bouncing ball as it has swung between glacials and interglacials, and for most of the last 5k years we have seen a steady downward trend as we made the slow descent towards the next ice age. The Marcott reconstruction illustrates this latter point nicely.

    http://www.realclimate.org/index.php/archives/2013/09/paleoclimate-the-end-of-the-holocene/

    Therefore in the absence of any unusual and long lasting peturbation in the earth’s energy balance we would expect that temperatures 1,000 years ago would be relatively high compared to today and that they would have dropped in the centuries following. Now it does appear that there were additional factors that caused warming in medieval times and accentuated the cooling in the latter centries of the last millennium, so we can see a MWP “blip” and LIA “trough” but in neither case was there a departure from the existing temperature trend of anything like the magnitude we have seen over the last 100 years or so.

    So there may well have been a “recovery” from the LIA as the sun recovered from the maunder minimum and volcanoes quietened down but there could be no expectation that temperatures would rise to the extent we have seen, and it would seem reasonable to think that in the absence of our GHG emissions the long term cooling trend would ultimately have resumed. Recent temperature changes simply can’t be explained in this context.

    Of course the corollary of this argument is that it does weaken the argument that the existence of the MWP is evidence of high climate sensitivity.

  4. BBD says:

    Of course the corollary of this argument is that it does weaken the argument that the existence of the MWP is evidence of high climate sensitivity.

    Why?

  5. BBD says:

    A few points:

    Over the last ~800k years the climate has indeed been a bit like a bouncing ball as it has swung between glacials and interglacials

    No, this is a forced response to changing orbital dynamics. As ATTP defined above:

    Well, our climate isn’t some kind of bouncing ball that has an equilibrium that’s defined by it’s own properties.

    I think he means “internal properties” but the point is that a ball bounces because its internal properties make it bouncy. Climate changes because it is pushed. It’s not bouncy.

  6. BBD,
    Indeed, I meant internal properties. A ball bounces when a force is applied to it (which can be regarded as similar to the climate in some sense) but how it bounces and the shape it returns to is determined – as you say – by its internal properties.

  7. andrew adams says:

    BBD,

    Sorry, I should have said it has the superficial appearance of being like a bouncing ball.

    http://en.wikipedia.org/wiki/File:EPICA_temperature_plot.svg

    I didn’t mean to imply there wasn’t an underlying forcing involved.

    My point about the MWP is that although it was relatively warm compared to the centuries that followed this was to an extent because it was warmer to start with, there wasn’t (as far as I can tell) the kind of sudden sharp warming “spike” comparable to what we have seen in recent decades – it was more of a “blip”. So if the warming wasn’t actually that dramatic then there is less of an argument that it demonstrates high climate sensitivity (although it doesn’t actually disprove it).

  8. BBD says:

    andrew

    Re MWP – yes, sorry, I should have been clearer about what I meant. First, let’s drop “MWP” which is confusing the issue. The evidence suggests that there was no “MWP” but rather a series of regional medieval climate anomalies (mainly NH) spaced over several centuries

    If there had been a single, global, synchronous warm event, then it would have been evidence for a moderately high sensitivity because the evidence suggests that contemporary forcing changes were relatively minor.

  9. John Mashey says:

    One more time: Bill Ruddiman’s http://www.whfreeman.com/Catalog/product/earthtransformed-ruddiman/tableofcontents“>Earth Transformed (2013) has a quite credible explanation of the Holocen, which started diverging from purely natural behavior with the start of agriculture. See his Tyndall Lecture from AGU 2013.
    Our interglacial is quite different from past interglacials, because of us.

    CO2 was higher during the Roman period than they would have been, higher in the MWP than they would have been, and then had the sharpest drop in thousands of years int o1600AD, *before* the Maunder Minimum. Here’s Law Dome CO2 and here are the corresponding “slope curves”,.
    I.e., compute 25,50,75-year regressions as a sort of first derivative. It’s really, really hard to drop CO2 that fast on the multi-decadal/century scale. Of course, I had to chop it had 1900, since including 20th century would have squished the scale.

  10. John Mashey says:

    One more note: I assume people understand that polar amplification works in both directions, and is especially visible in areas where it can and does snow, but does not have year-round snow cover.
    N. Europe is a good example. So, jiggles in CO2 are especially relevant there, added to any natural variability.

  11. izen says:

    “Recovering from the LIA” is a description masquerading as an explanation.

    The question to ask is – WHY are we “recovering”, and not continuing to cool as the climate has been doing since the Holocene maximum.

  12. BBD says:

    The “recovery” from the LIA ended by about 1850. With that qualification in mind, we could ask, why has it warmed since ~1850?

    I still think we should not talk about a recovery from the LIA after ~1850 because it is misleading and generates confusion.

  13. John Hartz says:

    ATTP: Kudos for once again posting an article that attracts high-quality comments from people who know the subject matter.in greater detail than the average blogger. Comment threads can be valuable learning tools if the discussion is civil and is not hi-jacked by those who would cause mschief..

  14. chris says:

    I’m embarrassed to have to admit that I haven’t recovered from the Little Ice Age.

  15. Michael 2 says:

    Thank you for this explanation. It makes very good sense.

  16. AnOilMan says:

    The last paper I read on the LIA showed that it may not have been a global phenomenon. Proxies showed something happened but not exactly the same effect or time frame in different parts of the world. Have you seen any papers that are different Anders?

  17. AnOilMan says:

    Ug! MWP! Medieval Warm Period… Not LIA.

    (I should just go have another nap now.)

  18. BBD says:

    AOM

    The evidence suggests that the MCA was *not* a global and synchronous warming event. This is sort of old news.

  19. ATTP,

    The idea of recovery from LIA is based on the possibility that internal variability is significant even on multicentennial time scale. Arguing against the recovery means arguing against such internal variability.

    I don’t think that anyone has been able to propose and justify any mechanism that would have the required properties, but then I don’t believe that anyone has been able to prove that the Earth system (and oceans in particular) cannot support such variability.

    You make the statement that changes in albedo are unlikely without presenting any justification. I would not consider that so obvious. Albedo is largely influenced by clouds and the most plausible explanations of significant long term variability do influence patterns of cloud cover. Changes in the state of oceans change also the state of the cloud cover, and through that the albedo.

    As long as we do not have fully satisfactory and generally accepted explanations of LIA and other historical periods, we should accept that relatively strong long term internal (unforced) variability is one of the alternatives that cannot be dismissed as partial explanation.

    John Mashey referred to Ruddiman. My impression is that his proposals have not reached wide acceptance, they have been noted as “interesting”, but not convincing as far as I have understood. Reading his writings has certainly not convinced me.

  20. Pekka,

    You make the statement that changes in albedo are unlikely without presenting any justification.

    Yes, I agree it’s possible. That’s why I didn’t say “impossible”, but said “unlikely”. It is only a blog post. I can’t explain everything in 5 paragraphs. As you say, it could be changes in cloud cover, but there is – as far as I’m aware – no evidence for this being the case. That doesn’t mean it’s impossible, though. I’m not ruling out that it could be something else, I’m simply pointing out that saying “recovering from the LIA” without suitable qualifications suggests a lack of understanding of what drives changes in our climate.

    we should accept that relatively strong long term internal (unforced) variability is one of the alternatives that cannot be dismissed as partial explanation.

    Do you really mean “unforced” as in no change in radiative forcing, or do you mean “unforced” as in no change in external forcing? If the latter, then I agree that internal variability could play a role if we could find some way in which it could produce a change in radiative forcing. However, I’ve again not seen a good argument as to this being possible and – as I understand it – the paleo evidence at the moment suggests that a significant contribution from internal variability (i.e., changes to our climate that are unassociated with changes in external forcing) is unlikely. I know there are D-O events that may be due to unforced variability, but these are thought to be due to an ice sheet instability, not changes in cloud cover (for example).

  21. BBD says:

    Peka

    The idea of recovery from LIA is based on the possibility that internal variability is significant even on multicentennial time scale. Arguing against the recovery means arguing against such internal variability.

    Or we can say that the “recovery” from the “LIA” was complete by ~1850 and the evidence-free contrarian argument collapses, as evidence-free contrarian arguments always do. We can also look at the calculated warming from the increase in GHGs ~1850 – present and see a physical explanation for all the warming that has occurred since then, further demonstrating that there is no multi-centennial internal variability involved.

  22. BBD,

    Why ~1850? Why not ~1940?

    As far as I know, there are numerous proposals for factors that have caused LIA either alone or usually in combination with some other factors, but no explanation has been free of problems or generally accepted as correct.

    The other relevant question is, how strongly and for how long can internal variability affect climate. For this question we don’t have any quantitative answers. Arguments have been proposed to limit the strength or persistence, but again no strong consensus exists. Models do not have such variability, but then this is an issue, where the models have little evidential power as the models cannot describe properly the behavior of the oceans. It’s not impossible that some ocean modes persist for long and have a significant influence on the climate.

  23. Pekka,

    The other relevant question is, how strongly and for how long can internal variability affect climate. For this question we don’t have any quantitative answers.

    It’s an important issue, but the last sentence of what you say is also important.

  24. ATTP,

    Perhaps I should add that I don’t really consider understanding internal variability to be of primary importance. it’s more important to know climate sensitivity, and there are other questions related to the dynamic behavior of climate when GHGs are added that are more important.

    Understanding internal variability helps in estimating the climate sensitivity, and may also help in figuring out what the future dynamics is likely to be, but these issue may be studied even with lesser understanding of internal variability.

    Another factor that reduces the importance of understanding the role of internal variability in the past warming is that making policy decisions should not require certainty, or even high likelihood of estimated outcome, it’s enough that the likelihood of relatively fast warming is not very small. Likelihood of 50% has almost the same policy implications as likelihood of 90%, and even a likelihood of 20% may have only little weaker influence on rational decision making that’s based on risk aversion.

    What we really would like to know is the high tail of possible values of TCRE.

  25. BBD says:

    Why ~1850? Why not ~1940?

    Because by ~1940, GAT was higher than it had been for ~1000y, and nobody I am aware of argues that the LIA lasted for 1000y.

  26. John Hartz says:

    The denier myth, Akasofu Proved Global Warming is Just a Recovery from the Little Ice Age is thoroughly debunked in detail by Dana’ Nuccitelli’s article, Akasofu’s Magical Thinking was Wrong posted on Skeptical Science.

  27. Andrew Dodds says:

    Pekka –

    The fundamental problem with a cloud-albedo long term shift is the fragility; the lifetime of water vapor in the atmosphere is (from memory) something like 3 days. That already makes it extremely hard for clouds to drive climate – there has to be a dynamic feedback at work (eg more clouds changing ocean circulation causing more clouds in turn) just to make such a process happen.

    Problem then is known perturbations – from ENSO to volcanic eruptions – that themselves exceed any possible magnitude for this hypothetical cloud-based process, and would surely overwhelm it (thinking back to the short water vapor lifetime).

    Processes based on Ice sheet growth an shrinkage, Carbon Dioxide or continental changes don’t have this problem, because they’ll continue to operate even if there is a short term ‘blip’ in the other direction.

  28. dana1981 says:

    Here’s the SkS rebuttal.

    This myth irritates me because as you say, it’s unphysical. I haven’t seen any evidence of significant internal variability on centennial timescales. Long-term climate changes are caused by external forcings, not by some magical unphysical ‘recovery’.

  29. BBD says:

    Clouds have two sides. At night, low marine cloud re-radiates IR down. Everybody seems to ignore this and thinks only of the upper surface albedo and SW reflection. It’s not that simple.

  30. Andrew,

    What I’m discussing is the possibility that the state of the ocean affects clouds and that affects albedo. The time scale is given by the ocean, not by the atmosphere.

  31. BBD says:

    So a really long-term (multi-centennial) change in SSTs in at least one major basin? Really large energy flux into or out of that basin over centuries… That sort of thing?

  32. BBD,

    A persistently different pattern of ocean currents that affects temperature gradients, atmospheric circulation and clouds. That’s what it might be.

    I don’t say, how likely that could be, but as long as phenomena of the scale of LIA are badly understood, we should not be too confident on anything less real proofs based quantitatively on solid principles.

  33. Steve Bloom says:

    “A persistently different pattern of ocean currents that affects temperature gradients, atmospheric circulation and clouds.”

    Which would leave a footprint. Is there one? Note that footprints of regional climate shifts, which are relatively common in the record, are apparent in a variety of proxies.

  34. LIA is a footprint.

  35. BBD says:

    Pekka

    I’m uncomfortable with the notion of spontaneous reorganisations of ocean circulation. As I understand it, small changes in forcing (volcanism; solar variability) triggered shifts in ocean circulation that caused regional climate change, but no significant global and synchronous warming/cooling events.

    This is why the terminology bothers me: MWP/MCA and LIA – they all come loaded with the sense of a major global and synchronous event that never really happened.

  36. BBD says:

    Pekka

    LIA is a footprint.

    But of what? Perhaps simply a little pre-anthropogenic slip towards the next glaciation? Orbital forcing at 65N is at a minimum. What might we expect?

  37. BBD says:

    pre-anthropogenic slip

    Pace John Mashey 😉

  38. Steve Bloom says:

    “LIA is a footprint.”

    As BBD implies, the paleo consensus is that if it’s anything coherent, it’s simply a slight and very straightforward Milankovitch-driven cooling, eccentuated in some times and places by volcanic eruptions. But your proposed “persistently different pattern” is not to be found.

  39. John Mashey says:

    BBD:
    For the overall view, see 2013 Tyndall Lecture
    I think that was well-received by the hundreds of scientists there, at least from sitting in the audience, but the book is of course more comprehensive. This work has come a long way from Plows, Plagues and Petroleum(2005) and quite a few other researchers have gotten involved fro ma wild variety of disciplines.

    Try Can natural or anthropogenic explanations of late-Holocene CO2 and CH4 increases be falsified?, i,e., Ruddiman, Kutzbach, Vavrus(2011)? Especially Fig 2 and Fig 6. Those don’t do LIA, but do the comparisons with other explanations, plus the human land footprint issue.
    That was one of those this special issue of The Holocene (paywall)..

    Also useful was one that appeared slighty before that special issue was Mitchell, et al(2011), “Multidecadal variability of atmospheric methane, 1000–1800 C.E.”

    but a little later was Sapart, et al(2012) (paywall), “Natural and anthropogenic variations in methane sources during the past two millennia”
    That ends: “It is thus likely that human activity contributed to variations in CH4 emissions to the atmosphere long before pre-industrial times.”

    Good science in progress, converging evidence, continuing reductions on uncertainty … and as always obligatory, more research needed :-))

  40. BBD says:

    John

    I’ve read Earth Transformed. Not sure Pekka has though.

  41. John Mashey says:

    BBD: good. Comments? What made sense? What didn’t? etc?
    (It’s hard for me to tell, because I’d been following this and corresponding with Bill for years, and watching hypotheses get challenged, adjusted, new data arrive, old data revisited and happy accidents of happening to meet people with missing pieces of the puzzles.)

  42. BBD says:

    John

    It all made sense. I have moved from being somewhat sceptical of the Ruddiman hypothesis to persuaded that it has merit.

    I particularly appreciated the comparison of MIS 1 GHG trends with MIS 5, 7, 9, etc in chapter 9. You brought this analysis up in comments here some months ago (you linked Ruddiman, Kutzbach & Vavrus 2011), which got my attention at the time. The alignment process was of particular interest, as was the insight into the real nature of MIS11. Chapter 20 was also illuminating. Excellent, highly recommended, especially to people such as (formerly) myself and Pekka who probably *think* they understand what Ruddiman is arguing but actually don’t.

  43. tallbloke says:

    ” the solar forcing has dropped in the second half of the 20th century and we’re still warming.”

    Hi Anders. It’s groundhog day again is it? 🙂

    Now that it has been acknowledged (By the more sensible warmists) that ‘the pause’ might have something to do with the clearly evident ~60 yr cycles found in ice cores, ice rafted debris, tree rings(!), stalactites and 100 other proxy indices, we need to revisit the question of the timescales on which the oceans absorb/release energy.

    The models are built with slabs of ocean which don’t take account of changing lunar declination which affects tidal flows on quite long timescales as the nodal cycle interacts with the apsidal cycle to produce the ~60 year cycle. (Keeling (yes, that keeling) & Whorf 2001).

    Solar energy entering the oceans (40% of it below 2m, 10% below 40m) can build up over long timescales if the Sun remains at above average activity levels. It did remains at above average activity levels all the way from around 1934 to 2003.

    So even if peak amplitudes were reducing (and the now recognized 15% overcounting by Waldmaier from 1945-1980 means they weren’t reducing much, the late C20th cycles were still well above the long term average from the depths of the little ice age, and the ocean heat content rose as a result.

  44. tallbloke,
    Try doing some actual physics. Cycles upon cycles tells me nothing about physical processes.

  45. tallbloke says:

    Hot Sunlight penetrates and warms seawater at depths of up to 100m and is mixed further down by internal tidal currents. Longwave radiation from the cooler sky can’t penetrate and warm the ocean beyond a few nanometres. The surface isn’t mixed down strongly, you have to get down to the vortices beneath wave troughs before that happens.

    There’s some ‘basic physics’ to think about.

  46. Concerned Citizen says:

    The fact that sunlight warms water so well is why the oceans absorb ~90% of the radiative imbalance. CO2 heats the cool skin layer over every square meter of the ocean surface. The same amount of sunlight warms the water below the skin layer, but less heat escapes up through the skin layer to the atmosphere.

  47. tallbloke says:

    The rate of evaporation (which removes latent heat and cools the surface as water vapour rises) is set by the rate of incident solar energy and by the atmosphere’s mass, which determines the surface pressure, not by fractional changes in its composition.

    Minnet’s ‘skin theory’ never made it through peer review. Even if it was correct it would have only operated under dead calm conditions, which almost never pertain in the real world. Even low speed breezes break up the ‘skin’ and rapidly increase the evaporation rate which utterly overpowers any feeble effect from increased co2. Wind is therefore proximate climate controller, since water vapour is the dominant radiative gas.

    Buoyant convection creates surface breezes and takes columns of water vapour rapidly skywards to the cloud tops, high above any notional ‘effective emission height’, where it radiates freely to space as it condenses. The planet’s dominant ‘greenhouse gas’ is an effective refrigerant; removing heat from the surface and conveying it to space.

    This is ‘basic physics’.

  48. You’ve rather lost me I’m afraid and appear to be heading in the direction of denying the existence of the greenhouse effect (or, rather, that it is a consequence of greenhouse gases). I have no interest in such a discussion of that is the direction you’re headed.

  49. verytallguy says:

    Tallbloke,

    rather than regale us with a whole set of left field theories of the Earth’s energy balance, supposed 60 year cycles etc, could you specify exactly in what way any of this is relevant to “recovery from the LIA”.

    Or are you actually simply agreeing with the OP that “recovery from the LIA” is not a meaningful way to explain current warming?

  50. Concerned Citizen says:

    If the evaporation rate isn’t affected by fractional changes in atmospheric water vapor (thus making it a feedback not a forcing), then what’s up with the Clausius–Clapeyron relation?

  51. BBD says:

    First, what VTG said.

    Second, the assertion that the surface skin layer is disrupted by wind is simply false. The intermolecular forces are universal and universally dominant at the surface skin. Basic physics. Energy can only cross the SSL by conduction, and the efficacy of conduction is set by the thermal gradient across the SSL. Evaporation only occurs at the top layer of the SSL. More basic physics.

  52. Steve,

    “LIA is a footprint.”

    As BBD implies, the paleo consensus is that if it’s anything coherent, it’s simply a slight and very straightforward Milankovitch-driven cooling, eccentuated in some times and places by volcanic eruptions. But your proposed “persistently different pattern” is not to be found.

    My point is that the question is, how LIA can be explained. In this case we are starting from an observation and searching for explanation. As long as we don’t have any specific explanation, we cannot search for anything specific in the footprint either. Therefore LIA is the footprint that we are trying to interpret and explain.

    The knowledge of its properties may be lacking, but it does not make sense to argue that we don’t have a footprint when a footprint is almost the only thing that we have.

    As I also have written, there are many proposals for explaining at least partially LIA, but there clearly isn’t any consensus, and no explanation has been put to severe test and succeeded in that.

  53. BBD says:

    Pekka

    Can we not lose sight of the fact that speaking of “the LIA” is potentially misleading? Especially when we qualify “the LIA” as “a footprint”, which makes it sound even more like a strong, synchronous and global in effect. Even more so when we wave away what Steve said using this language.

  54. BBD,

    When we make guesses about something we don’t understand, the guesses may always be misleading. As long as it’s not known what LIA is, it’s better to accept that no explanations that does not strongly contradict something else should be retained. The signal is strong enough to take seriously.

    It may be that it’s just a coincidental combination of several minor factors (forcings and moderate variability), but it could be also a sign of stronger natural variability driven by the dynamics of oceans.

  55. tallbloke says:

    Anders says: “denying the existence of the greenhouse effect (or, rather, that it is a consequence of greenhouse gases).”

    [Mod : Sorry, tallbloke, but that’s a little too close to denying the greenhouse effect to be of any interest.]

  56. BBD says:

    Except that the latter may well violate conservation of energy. It seems more *likely* that the former is correct. So that explanation is the one I place more weight on.

  57. BBD says:

    Incidentally, the changes in orbital dynamics (overarching) and the centennial-scale changes in atmospheric composition, volcanism and solar output aren’t “guesses”. When you combine loaded terms like “the” LIA and “a footprint” and “guesses”, you have begun to deploy rhetoric – the very thing you are supposed to be eschewing in favour of scientific objectivity.

    You need to watch that.

  58. Pekka,

    As long as it’s not known what LIA is, it’s better to accept that no explanations that does not strongly contradict something else should be retained.

    I’m slightly confused as to what this other explanation is. I agree that we shouldn’t reject anything that is consistent with the available evidence, but saying that something could be partly/wholly responsible for the LIA is not quite the same as providing a mechanism that is consistent with the evidence.

  59. ATTP,

    No it’s not, but what’s the mechanism that’s known to be consistent with the evidence?

    In Bayesian reasoning something that’s both specific and agrees with the evidence gets support relative to non-specific that agrees with the evidence. When no explanation is particularly specific (i.e. formed using know knowledge related to LIA observations, but allowed to use specific information about forcings) Bayesian reasoning cannot support any of the alternatives. My understanding is that proposals that involve solar variability and volcanism have some specificity, but not much as it has not been possible to predict LIA from such data.

    Here we have a phenomenon that does not provide much specific support to any of the alternative explanations in the Bayesian spirit. The phenomenon tells that something non-negligible has taken place. Thus it gives generic support to all explanations, but cannot differentiate between them. It provides a reason to consider also the alternative that relatively strong unforced internal variability has contributed significantly. Providing a reason to consider is nothing like a proof, only what the expression says.

  60. Pekka,
    I’m not an expert, but it was my understanding that there is evidence that TSI was lower and evidence that volcanic activity was higher. Both of these would influence temperatures. They may not explain it all, but they may be sufficient to largely explain it. That doesn’t mean that some other mechanisms weren’t operating (in fact, there probably were other mechanisms). The argument I would make is that we can explain the lower temperatures during the LIA as being a due – in part – to reduced TSI and increased volcanic activity.

    However, this is rather diverting from the point of the post. The point of the post was mainly to argue that suggesting that we’re somehow recovering from the LIA – in a bouncing back kind of way – doesn’t really make physical sense. Our climate responds – on multi-decade timescales at least – to changes in forcings. The radiative forcing today is greater than during the LIA partly due to reduced volcanic activity, partly due to increased TSI, and mainly due to our emission of GHGs.

  61. ATTP,

    Yes. There’s very likely some solar influence and some volcanic contribution. As far as I have understood, it’s difficult to explain very much by them, perhaps not impossible, but difficult and require additional arguments to reach a sufficient strength. Therefore the explanations provided by them are not specific enough (the extra factors needed are not known) to lead to strong support in the Bayesian reasoning. Something additional unknown seems to be required. That something might be connected to forcings, but only might.

    This is not totally beside your original post. I agree that what’s related to LIA has probably been over for at least 100 years, but it’s not totally excluded that the same mechanism that made LIA as strong as it was has had a contribution also to the more recent warming. That’s not likely, but that’s possible. I mentioned 1940 in one earlier comment, because it makes more sense to speculate that the same mechanism would have contributed to the maximum around 1940.

    I don’t present such speculation to propose that that would be the case. I present it to show that there’s an infinity of possible speculation that makes sense from some point of view. Such speculations have equally little weight when they are presented in support of strong AGW as they have when they are presented against that. Accepting that there are uncertainties on all these details does not remove the evidence for AGW. In my view the main reports of IPCC WG1, and presently AR5 in particular, tell a balanced view. Additional studies that support deviation from that in either direction should be taken as new material that may ultimately lead to some modification in the picture, not as something presently of same weight as the assessment of IPCC WG1.

  62. John Hartz says:

    Pekka Pirilä

    Back in the days when theologans argued the question, “How many Angels can fit on the head of a pin?”, were they engaged in Bayesian reasoning?

  63. catweazle666 says:

    And Then There’s Physics says: “Try doing some actual physics.”

    You people are funny!

  64. catweazle,
    As I might have mentioned before, I don’t really mind people being condescending. Condescending and right is okay. Condescending and horribly wrong, however, is just irritating.

  65. jsam says:

    On behalf of us old engineers everywhere I can only apologise.

  66. John Hartz says:

    Here’s an interesting article about a new finding about what caused the recovery from the last “Big Ice Age”:

    The last ice age Phys.org, July 3, 2014

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