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What we can learn from studying the last millennium (or so)

Filed under: — mike @ 15 May 2010 - (Español)

With all of the emphasis that is often placed on hemispheric or global mean temperature trends during the past millennium, and the context they provide for interpreting modern warming trends, one thing is often lost in the discussion: space matters as much as time. Indeed, it is likely that the regional patterns of past climate changes, rather than simple hemispheric or global mean temperature trends, will best inform our understanding of the dynamical mechanisms involved. Since much of the uncertainty in future projections relates to regional climate change impacts, it makes particular sense to focus on those changes in the past that involve regional changes and the underlying mechanisms behind them.

For instance, melting of the cryosphere (and consequent rises in sea level), subtle shifts in drought and rainfall patterns, and extreme events, are all regional effects that could be important threats to ecosystems and our environment. Such changes are often associated with phenomena like ENSO or the North Atlantic Oscillation. Yet there remain large uncertainties about how such mechanisms will respond to anthropogenic climate change.

There are a number of potential ways forward to improve our understanding. A first step is to look directly at the time-series of specific systems (like the ENSO index or the ocean temperatures in the North Atlantic) and try to extend them as far back as possible using proxy data. This gives more information on what the natural variations in these phenomena look like, and thus a better idea of how big a forced response would need to be before it could be reliably detected. Secondly, we can look to see if there is a relationship between various natural drivers of climate change (volcanic eruptions, solar variability or orbital forcing say) and any characteristics of these phenomena – amplitude, frequency or duration. Do volcanic eruptions appear to affect El Niño for instance?

For phenomena that need annual or decadal resolution data to be resolved, the last millennium or so is an obvious (and only) time period to be looking at for it is only for that period that there is sufficient paleo-data coverage of high enough temporal resolution. Other periods – such as the mid-Holocene 6000 years ago – are also useful, but the results are more long-term in nature (there is also a discussion of the value of different periods for reducing future projection uncertainty in this recent paper).

There are a number of different approaches to looking at reconstructions in recent centuries – some rely on high density regional networks (as seen in this recent paper by Guiot et al concerning European temperature trends for which they mostly used pollen data) and some rely on wider networks of more diverse proxies which aim to capture longer-range correlations to specific phenomena (such as the recent Mann et al (2009) paper).

When this is done, people usually find that while it was relatively cool in global mean temperatures from the 1400s to the 1800s known as the “Little Ice Age” and relatively mild in the 900s to 1300s interval ( sometimes termed the “Medieval Warm Period”). But the spatial reconstructions reveal, however, why such global terms can be quite misleading, and why alternative phrases such as the “Medieval Climate Anomaly” are being increasingly favored by the community. This latter terminology recognizes that while the interval displayed significant climate anomalies, they varied greatly, even in sign, from region to region. Many of the more profound climate anomalies, moreover, involve variables other than temperature, such as drought, rainfall, and atmospheric circulation. Though the medieval period is seen to be modestly warmer globally in comparison with the later centuries of the Little Ice Age (the peak global mean warmth is likely comparable to mid, but not late, 20th century warmth), some key regions appear to have in fact been colder, while other regions appear to have been warmer. Southern Greenland, for example, appears within uncertainties to have been as warm as today. However, much of the tropical Pacific was unusually cold, suggestive of the La Niña phase of the ENSO phenomenon (a similar conclusion was reached by Trouet et al (2009)). Thus even though some locations may have been as warm or warmer than today, the hemispheric mean appears not to have been.

Why does this matter? It matters because there are plenty of factors that can affect the overall mean temperature (solar variability, volcanoes, greenhouse gases, internal variability etc.) and so it is hard, given the uncertainties in the solar or volcanic reconstructions to precisely attribute the paleo changes in the global or hemispheric mean to these factors. But if we can look at more complex fingerprints of the changes, it might be possible to be more quantitative in those attributions since the spatial fingerprints of the different factors are easier to distinguish. Furthermore, if we can clearly tie the regional patterns to the different forcings, we might be able to use that information to inform regional projections under future conditions.

Thus we can basically say that the warmer conditions of the Medieval era were tied to higher solar output and few volcanic eruptions and the cooler conditions of the Little Ice Age resulted from lower solar output and more frequent volcanic eruptions. But these drivers appear to have had an equally important, though more subtle, influence on regional temperature patterns through their impact on climate phenomena such as ENSO and the North Atlantic Oscillation. The modest increase in solar output during Medieval times appears to have favored the tendency for the positive phase of the NAO, associated with a more northerly jet stream over the North Atlantic. This brought relatively greater warmth in winter to the North Atlantic and Eurasia. A tendency toward the opposite negative NAO phase helps to explain the enhanced winter cooling over a large part of Eurasia during the later Little Ice Age period.

There is some model support for these patterns (see also instance Shindell et al, 2001) when the models include interactive ozone photochemistry to produce this dynamical response to solar forcing, but it is not captured in a simulation of the NCAR CSM coupled model which lacks those processes. Neither model simulation reproduces the apparent La Niña pattern seen in the Medieval temperature reconstructions:

Figure 1: Spatial pattern of mean temperature difference between the MCA and LIA periods (defined at the intervals AD 950-1250 CE and 1400-1700 CE respectively) compared with simulations of two different climate models forced with estimated differences in natural (volcanic and solar) radiative forcing between the two periods (Mann et al, 2009).

Other model simulations, however, using a climate model that exhibits a particular tropical Pacific mechanism, do reproduce such a response. In such models, the tropical Pacific counter-intuitively tends to the cold La Niña phase during periods of increased heating, such as provided by the increase in solar output and low volcanism of the Medieval era. If this response holds for the future, something that is still vigorously debated, it could imply a more La Niña-like response in the future. Most of the state-of-the-art climate models, e.g. those used in the IPCC Fourth Assessment, by contrast, suggest the opposite–a more El Niño-like future climate. The credibility of the models with regard to this phenomenon is not high, however, and lots more work is going to be needed (both on paleo-reconstructions and model improvements) before we can be confident in the future projections of changes in ENSO-like dynamics and mean state.

690 Responses to “What we can learn from studying the last millennium (or so)”

  1. 201
    Completely Fed Up says:

    Ambler, #193, however to turn a radiant intensity into a temperature reading at the surface you require a COMPUTER MODEL!!!

    Ergo, it’s wrong.

    (it’s easy to deny evidence as long as you don’t sweat changing your tune whenever convenient)

  2. 202
    Septic Matthew says:

    200, FCU: So, Septic, what do YOU say is causing the warming?

    I don’t know. I am not sure that the “warming” is ongoing or paused, and I expect that, because I am 63, I may never know.

    The blog called “climateprogress” had a proposal based on stabilization wedges about a year ago, but I didn’t read it until yesterday. I support investments in stabilization wedges because (a) they are mostly justifiable on multiple grounds, not just AGW and (b) since AGW might be true, it’s worthwhile to continue with some reasonable preparations.

  3. 203
    Doug Bostrom says:

    Frank Giger says: 19 May 2010 at 4:10 PM

    I say “decent” because the volume of herbicides required to kill kudzu makes one wonder if the cure isn’t as bad as the ill due to runoff into the water table.

    Veering wildly off-topic… It’s not really a scalable solution but I found that a truly tiny amount of RoundUp injected directly into the plant with a syringe will crisply and emphatically kill a enorumous specimen of Kudzu. Just a few mils will do it. I could never bring myself to spray the stuff willy-nilly.

  4. 204
    David B. Benson says:

    Completely Fed Up — I find I need to skip over your posts. Please reconsider your overall writing style.

  5. 205

    Slightly snarky of me, perhaps, Rod B, but I have to rejoin that Katrina was certainly quite significant, thank you, in a number of statistical areas–like mortality and economic loss and demographic structure (which seems to have been more-ore-less permanently altered.)

    So the lack of attribution of Katrina to AGW is duly noted–but the episode is still a great example of what we may reasonably expect the future to hold under BAU scenarios. (And, sadly, even under more optimistic ones than BAU.)

  6. 206
    John Mashey says:

    re: kudzu, statistics, etc

    1) Global Change and Invasive Plants in Canada is a nice study by U of Toronto folks on the Northward march of kudzu. Se p.12 and pp.26-, with nice maps, relationships to temperature, etc.

    Not only do warmer winters let kudzu thrive, but it loves higher CO2.

    2) It is *much* easier to generate statistically-significant results from huge samples (like the geographical spread of things inhibited by cold that no one wants, like kudzu or bark beetles) than of relatively rare events like big floods and hurricanes.
    if people haven’t read IPCC AR4 WG II, Chapter 1, to see the vast number of indicators.

  7. 207
    Frank Giger says:

    CFU, go to post 93 for the study showing the drought was both within historical norms and had no markings of global warming forcing.

    When one sees increased evaporation and decreased rainfall it throws up a big red flag – as that is a really big marker for climate change.

    The drought in the SE USA didn’t display any of the markers that usually show when the climate itself is being altered.

    If you’re looking for 100% certainty in climate science, you won’t find it. That’s a bar set too high – and one usually put there by the other side of the arguments.

    @ Mr. Mashey: Yep, I agree that the slowly moving frost line allowing kudzu and other terrible stuff is far more persuasive in demonstration of climate change than a storm. We’ve all seen storms, including terrible ones. Floods aren’t new, either. But when one sees something new, something that couldn’t be there before due to cold, it is very powerful.

    @ Mr. Bostrom: Dang, I wish I’d of known that before going after a patch with machete and pickaxe!

  8. 208
    Jacob Mack says:

    This discussion on droughts is getting out of hand. With AGW and climate change in general there are regions that are usually hot and prone to droughts that will in fact become more moist due to localized weather and microclimate variations. We cannot just assume that there is or there will be worse droughts everywhere it is hot and dry to begin with due to changes in climate. Of course it is not unreasonable to assume this, but as we have seen over the last few decades alone the outcomes are not always what they are thought to be or ought to be. As the IPCC report states droughts may become worse,but rainfall and overall precipitation may increase in some regions as well. Recent studies have also shown that some crops thrive during hotter and dryer conditions. This is shown empircally in nature and in contrived experiments over the past several decades as recently highlighted in Scientific American.

    Oh on the CH4 issue in water; no not a serious issue. The oxidation of CH4 to C02 is more of favorable reaction in the atmosphere.

  9. 209
    Jacob Mack says:

    You guys do realize that even with a quantum computer we could not really predict many aspects of climate over the next 10-30 years. We can, however, make some good approximations: a low middle and high end time and qualitative estimate with some numerical figures that can be relatively accurate.

  10. 210
    Jacob Mack says:

    # 206 John Mashey so very true. Thank you for your post.

  11. 211
    Brian Dodge says:

    “A particularly nasty second order effect of warming that I do not wish on our Yankee friends. Frank Giger — 19 May 2010 @ 4:10 PM

    Assuming you are talking about the Kudzu [ku = earth, dzu (or tse, as is Lao Tse)= master; so, its chinese name means “master of the earth”] and not the toxic chemical human overrreaction, I think I would wish it on the willfully ignorant politicians obfuscating and delaying action to deal with global warming. Maybe if we planted some Roundup Ready(TM) kudzu to engulf the hallowed halls of Congress, it would get their attention?

  12. 212
    Hank Roberts says:

    > oxidation of CH4 to C02
    Remember, in sediment and water, oxidation is being done by methanogens — a biological process (the water is usually supersaturated). In air, I don’t know if organisms are pushing the reaction. But, enough, it happens.

  13. 213
    Thomas says:

    173 CFU: I don’t think that warmer necessarily implies that drought is worse. We know the atmosphere’s water vapor content is increasing as a result of rising temperature, and we expect, and I think observe that overall precipitation is increasing as well. So in order to be worse because of AGW a drought period must see an increase in precitation from warming that is less than the increase in evaporation due to the same cause. I don’t think that automatically follows. So droughts getting worse/more frequent, or high end precipitaion events getting worse are not obvious responses to rising global temperatures. In the precipitaion arena we need to demonstrate that the tails of the distribution are getting fatter -not just that the mean is shifting (to wetter). I know this is the expectation of many climate scientists, and that observations seem to indicate it is hapening, but some sort of understandable mechanism for why must exist.

  14. 214
    Jacob Mack says:

    # 212… yes Hank,but it is not as significant. CH4 is not converting in large amounts to C02

  15. 215
    Jacob Mack says:

    … in the water.

  16. 216
    Jacob Mack says:

    Hank still of interest though:


    Not be ignored, but not so important in terms of ocean acidity. Just like meathane in the atmosphere is important, but less so than actual direct C02 with such different residence times.

    As always thank you for your thoughtful and referenced post.

  17. 217
    John Mashey says:

    Perhaps we can avoid vague generalities of droughts and storms, given the reasons to believe there will be *more* droughts in some places in, *more* precipitation in other places, and it matters which si which. For the USA, I hope everyone is familiar with the US Global Climate Research Program’s Global Climate Change Impacts in the United States, 2009, a book well-written and illustrated for a general audience, and freely available online.

    Particularly useful are the ~5-pages for each region of the USA, describing expectations for that region.

  18. 218
    Edward Greisch says:

    188 Hank Roberts: THANK YOU! Now we know that what we have to deal with is paranoid personality disorder combined with coal company propaganda. That is a big advance. Now we have to look up how to deal with people who have paranoid personality disorder. That is a big step forward.

    You also told us that we must deal with the untrained mind in its own mindframe. That will be a big adjustment for some of us. We would rather train everybody to be scientists, which is not possible given the intelligence of the average person. [Nobody ever went broke by underestimating the intelligence of the average person.]

    Very well. We must deal with this information. RC must write unscientific but emotionally convincing articles from now on.

  19. 219
    Edward Greisch says:

    188 Hank Roberts: An example of an article that works better but isn’t scientific:

    “Global Warming Has Already Happened. In the mid 19th century, the Mississippi river froze over in the winter so you could drive on it at St. Louis. That’s how St Louis became known as the gateway to the west. Now the Mississippi river is ice-free at Davenport, Iowa most years. Cattaraugus County New York [Olean, Little Valley] got 450 inches [37.5 feet] of snow per year in the 1950s and 1960s. Now it gets only 96 inches of snow per year. Hurricane season starts in spring now. Hurricane season used to start in the fall. Tornados used to happen in the summer. Now they happen all year. “

  20. 220
    CRS says:

    This is the video for Dr. Norris’s colloquium presentation to Fermilab National Accelerator Laboratory, May 12, 2010.

    “Cloud feedbacks on climate: a challenging scientific problem”
Joel Norris, Scripps Institution of Oceanography

    Abstract: One reason it has been difficult to develop suitable social and economic policies to address global climate change is that projected global warming during the coming century has a large uncertainty range. The primary physical cause of this large uncertainty range is lack of understanding of the magnitude and even sign of cloud feedbacks on the climate system.

    If Earth’s cloudiness responded to global warming by reflecting more solar radiation back to space or allowing more terrestrial radiation to be emitted to space, this would mitigate the warming produced by increased anthropogenic greenhouse gases.

    Contrastingly, a cloud response that reduced solar reflection or terrestrial emission would exacerbate anthropogenic greenhouse warming.
    It is likely that a mixture of responses will occur depending on cloud type and meteorological regime, and at present, we do not know what the net effect will be.

    This presentation will explain why cloud feedbacks have been a challenging scientific problem from the perspective of theory, modeling, and observations. Recent research results on observed multidecadal cloud-atmosphere-ocean variability over the Pacific Ocean will also be shown, along with suggestions for future research.

  21. 221
    Hank Roberts says:

    > a drought period must see an increase in precitation from warming
    > that is less than the increase in evaporation due to the same cause

    No, because that would assume no change in the local patterns — which is the question this topic is all about discussing.

    More extreme local precipitation events could put the water in fewer places and more rapidly so more runs off and less goes into groundwater. I think that’s a pattern expected from the paleo records, though I’ll have to poke around and see if my memory is correct or someone has addressed it.

  22. 222
    Hank Roberts says:

    This is what I was recalling, among others:
    “… Here, we analyse a 99-year record of hourly precipitation observations from De Bilt, the Netherlands, and find that one-hour precipitation extremes increase twice as fast with rising temperatures as expected from the Clausius–Clapeyron relation when daily mean temperatures exceed 12 °C. In addition, simulations with a high-resolution regional climate model show that one-hour precipitation extremes increase at a rate close to 14% per degree of warming in large parts of Europe. Our results demonstrate that changes in short-duration precipitation extremes may well exceed expectations from the Clausius–Clapeyron relation. These short-duration extreme events can have significant impacts, such as local flooding, erosion and water damage.”

    And for paleo work,
    “… a significant but transient decrease in precipitation at the onset of the PETM but a gradual return to pre-PETM levels by the end of the interval. The paleosols also show additional, although less dramatic, wet/dry cycles within the PETM interval that may correspond to precessional cycles that have been identified in the marine record of the PETM. This study counters interpretations of increased precipitation for Wyoming at this time and shows the importance of detailed case studies of continental strata to test climatic generalizations and models that have been developed for PETM precipitation patterns.”

  23. 223
    Completely Fed Up says:

    Thomas says:
    19 May 2010 at 9:57 PM

    173 CFU: I don’t think that warmer necessarily implies that drought is worse”

    I’m not saying that either, though that’s a LOT closer than some others are getting.

    Warmer means worse if nothing else changes.

    Therefore, to prove that AGW did NOT make that drought worse, you’d have to prove it didn’t make a difference IN THAT ONE SPECIFIC CASE.

    I.e. the one making the statement that the drought was NOT affected by AGW would have to show AGW did not cause any regional change or that the changes cancelled each other out.

    But the default is that it DID affect the drought.

  24. 224
    Completely Fed Up says:

    “With AGW and climate change in general there are regions that are usually hot and prone to droughts that will in fact become more moist due to localized weather and microclimate variations.”

    And this case is where there ARE generally hot and prone to droughts.

    Funny how nobody is noticing this and LEAPING to the conclusion I’m talking about all and every place and case.

    Isn’t it.

    “We cannot just assume that there is or there will be worse droughts everywhere it is hot and dry to begin with due to changes in climate”

    Yeah. Good job I’m not, isn’t it.

    Doesn’t stop people slamming a statement I’VE NEVER MADE.

  25. 225
    Completely Fed Up says:

    Frank Giger says:
    19 May 2010 at 8:02 PM

    CFU, go to post 93 for the study showing the drought was both within historical norms and had no markings of global warming forcing.”

    And says NOT ONE THING about whether it would be less severe if AGW didn’t happen.


  26. 226
    Completely Fed Up says:

    Septic Matthew says:
    19 May 2010 at 4:52 PM

    200, FCU: So, Septic, what do YOU say is causing the warming?

    I don’t know. I am not sure that the “warming” is ongoing or paused, and I expect that, because I am 63, I may never know.”

    Septicemia, the warming hasn’t paused.

    That you arent sure whether it has or not shows rather clearly how you are not willing to consider anything other than evidence that AGW isn’t happening and YOU are the one who is not open to ideas.

    You’ll never know not because you’re 63, but because you DO NOT WISH IT TO BE.

  27. 227
    CM says:

    CFU, I won’t play your word games. Anyone who has followed the thread will know how the drought discussion started, will know who has argued what based on cited sources, and will know to evaluate your representations of other people’s positions accordingly. If there’s nothing more of substance to discuss, we’re done.

  28. 228
    Michael says:

    Dear Moderators

    A very very small number of regular posters reduce reading this forum to a painful experience as they try to belittle everyone with an alternative view in an attempt to show how smart they believe themselves to be. This greatly reduces the effectiveness of what I would believe to be the public outreach function of the blog. Why do you allow them to impact so negatively on your blog?

    Kind Regards


  29. 229

    Naindj 165: To link all this to human activities (the “A” in AGW) you need far more and this is why there are all these debates (Hockey Stick and MWP, cliamte sensitivity and feedbacks, etc…)

    BPL: There is no “debate” on this particular issue except among the ignorant. That CO2 is rising and that it’s of human technological origin was pretty well established in the 1950s.

  30. 230

    The AGW/drought connection can’t be figured out from assuming one vertical column. It’s an effect of the fact that under AGW, precipitation moves away from continental interiors and toward the coasts. Without the horizontal distribution, you can’t see it.

  31. 231
    Robert D says:

    There was a recent study in Oz about pan evaporation rates in Oz suprisingly not having increased in spite of the extra heat. It may have been due to more higher humidity air coming down from the north,. Sorry not to have the reference

  32. 232
    Steve Brown says:

    OT, Apologies. Latest Arctic sea-ice extent has just about hit the 2007 minimum for this month:


  33. 233
    Completely Fed Up says:

    “You guys do realize that even with a quantum computer we could not really predict many aspects of climate over the next 10-30 years.”

    The problem with quantum computers is working out how to program them. You need to set up your quantum states so that their energetic solution is the solution to the question “what is the climate doing”.

    This isn’t going to require one qbit per atom in the atmosphere, but it’s still a lot of qbits needed, and decoherence cannot happen until you’ve set the entire thing up.

    Just as with current machine state computers, there are some classes of problem that quantum computing cannot answer. The land of Panacea sank into the ocean of practicality a long time ago…

  34. 234

    OK guys, I’m struggling to understand some of the statements in Seager et al. 2009. Most confusing to me is this passage from the conclusions. The authors are comparing the robust linking of Southwestern drought to AGW, and refer to P (precipitation) and E (evaporation):

    “To the extent that P − E is reduced [in the Southeastern drought], the mechanism is different from that for the robust projection of reduced P − E in the Southwest which is driven by reduced P, with E reducing as soil moisture drops (Seager et al., 2007b). In the post 2005 drought, according to the NCEP Reanalysis, E dropped along with P, indicating that the recent drought was driven by a reduction of P and not by an increase of E.”

    OK, so E drops “along with” (presumably meaning “concurrently with”) P in the Southeastern case, while lagging in the Southwestern. But in both cases, this passage clearly says, the drought is driven by decreases in P. This is “different?” (It’s also bathetically obvious, if you lived through the drought–there was no question about the reality of “reduced P!”)

    Is there an editorial/auctorial lapse in clarity here, or am I just missing something?

    And if the characteristic difference was the time lag of decreases in E–and I can testify from personal experience just how dry the soil became in the “post 2005” Southeastern drought, soil texture in my back yard was reminiscent of wood ash–then what does this difference imply?

    Seager et al 2009 is very clear that the drought looks much like past (ie., pre-AGW) droughts and that from that perspective, there is no reason to suspect AGW as a cause. I don’t see any way to argue that, short of detailed re-examination of the data. Absent that, it’s “just a fact.”

    On the other hand, given the paper’s conclusion that the models do not reproduce the regional details robustly, it seems questionable to me how well-defined the profile of AGW’s influence on Southeastern drought in general actually is at present.

  35. 235

    Correction: “argue that” really should have been “argue *with* that.” (Penultimate paragraph.)

  36. 236
    Completely Fed Up says:

    “On the other hand, given the paper’s conclusion that the models do not reproduce the regional details robustly, it seems questionable to me how well-defined the profile of AGW’s influence on Southeastern drought in general actually is at present.”

    OK, how does the model’s inability to calculate exactly how much AGW did or did not affect the drought affect the scientific fact that warmer weather makes a drought worse?

    We can’t model the properties of exotic metallic alloys. This doesn’t stop them from being made, because we know tantalum has an effect on mild-carbon steel, we just can’t model how exactly it does this.

  37. 237
    Completely Fed Up says:

    “perspective, there is no reason to suspect AGW as a cause.”


    I’m not saying AGW was a cause.

  38. 238
    Completely Fed Up says:

    CM says:
    20 May 2010 at 3:26 AM

    CFU, I won’t play your word games.”

    Uh, what “word games”?

    You complain that it cannot be said that AGW caused that drought.

    This is YOUR word game.

    I never said it.

  39. 239
    Completely Fed Up says:

    OK, peeps. A poll.

    Indicate which ones you agree with by listing as

    YES: # # #

    and which you disagree with by listing as

    NO: # # #

    1) AGW (climate change) creates warmer weather.

    2) Warmer weather makes drought more severe.

    3) Droughts are made more severe if you increase temperatures.

    Which ones are agreed as correct and which ones as wrong?

  40. 240
    Completely Fed Up says:

    “Not every weather event – even ones that make a very statistically noisy 18 month long one – is tied to Global Warming.”

    EVERY weather event is tied to global warming (climate change).

    Weather that increases precipitation by 2mm/hr affects a climate that is dry more than it affects a climate that is inundated.

    If you change the climate, the events you see are changed.

    If this were not so, you would be unable to see climates change.

  41. 241
    Completely Fed Up says:

    Further to 240, the issue may be clarified by noting that Frank has the process the wrong way around.

    Every event is affected by the climate.

    But no one event lets you tell how much the climate changed.

    Frank has it that weather drives climate. It’s the other way round. Weather is the expression of climate.

    A hand of cards is not defining pok er. But pok er DOES define what you get in a hand of cards.

    Hand == Weather.
    Pok er == Climate.

  42. 242
    Jeffrey Davis says:

    I don’t know. I am not sure that the “warming” is ongoing or paused

    Paused? For each successive month for the last several months, it has been the warmest 12 months on record. Paused/stopped wasn’t a good point to begin with, and now, even by its own criterion, is defunct.

  43. 243
    Jeffrey Davis says:

    Can I say that it’s strange to talk about the “cause” of something as non-discrete as weather? Where does “weather” happen? When does it begin and end? Does it end where it begins or does it move somewhere else? And if it moves somewhere else does it have a different cause there than where it began? Etc.

    As someone else has noted, the Earth is just getting warmer, and we’re getting the kind of weather that you get with a warmer Earth.

  44. 244
    Thomas says:

    Hank, 222,223 BPL 230, thanks for trying to provide an explanation for the (to me) unintuitive fattening of the tails problem. I am convinced the data shows it is a real effect. My problem is I lack a mechanism which I could plausably argue with a sceptic that provides this predicition with some degree of credibility. Perhaps no simple handwaving argument exists, and you can only ascertain it from a detailed simulation, or from observing natures solution to the changes in the system. But the problem of convincing the skeptic would then remain unsolved. I for one have high confidence in the global results of climate simulations (it will get warmer), but don’t find the low level details (location X will respond like XXX) to be have usable confidence. Now I can think of a few weak arguments, but I don’t think they are up to the challenge:

    (1) With warmer temps for a given ratio of P/E (which by Claudius-Clapteron we might expect to remain constant), drying will ocurr on a faster timescale. So for a fixed duration drought of a fixed intensity (change in the log(P/E)) the drying can progress further.

    (2) Changes in general circulation. If we can show that the tendency of planetary waves to intensify and/or get stuck in a pattern increases with global temperature then we have an explanation for the fat tails problem. This might be due to how close the natural wavelength is to a resonence (an integer times the wavelength equals 2 time pi, i.e. teleconnections tend to reinforce regional signals). But, a priori one wouldn’t expect that this is more than 50% likely.

  45. 245
    Dappledwater says:

    Kevin McKinney #234 – as clear as mud. As you point out, reads as self-contradictory.

  46. 246
    Completely Fed Up says:

    “So for a fixed duration drought of a fixed intensity (change in the log(P/E)) the drying can progress further.”

    It can also make the reversion to wet clime harder: if the ground is too hard baked, the rainfall will skoosh right out to sea, causing a flood on the way, whereas if there were a little moisture to keep the pores in the ground open, more would have seeped in and

    a) not added to the flood
    b) would be available next time the day didn’t rain

  47. 247
    Completely Fed Up says:

    “Can I say that it’s strange to talk about the “cause” of something as non-discrete as weather?”


    However, can I say it’s strange to talk about talking about the cause of something when nobody has talked about the cause of something?

  48. 248

    CFU (#236), I’m not arguing–I’m asking. Statements are there to help those who may be able–and inclined–to help me, to diagnose my confusion better!

    That said, you seem to be talking past me. Yes, a drought in warmer weather will be worse than one in cooler weather, all other factors being equal. And yes, by extension, AGW should worsen drought via what Seager et al. call “increased E.” In that, you and Seager et al (and me, for that matter) all seem to be agreeing, if I have this right. (That implies a “yes” to your three propositions in #239, by the way.)

    But what Seager et al. are saying is that, according to the best data that they have, the Southeastern drought did not differ from historical droughts. That’s not a theory, that’s what they found when they looked at the data. It wasn’t drier, it wasn’t longer. And that’s what I find hard to argue with–unless one is prepared to do a thorough analysis of the same and/or related data. And I’m not equipped to do so.

    (BTW, that’s not contradicting AGW theory–for one thing, IIRC, regional warming in the Southeast for the decades around the turn of the 20th century was such that we haven’t yet exceeded the anomalies recorded then. IOW, AGW hasn’t–yet!– driven temps in this region past historical norms, so why would we expect droughts to show a different profile? That doesn’t negate the reasonable expectation that those norms WILL be exceeded, and quite likely not so far in the future, either.)

    What you don’t seem to get about Seager et al. is that they are, in part, doing attribution. They are doing that largely by comparing what the models come up with when driven by historical SSTs to what the instrumental record says. What they found was that: 1) results were not very robust across model runs; and 2) there was only a weak correlation between the two, and that for the winter half of the year only. Hence, the drought “lacks an AGW profile”–or however they phrased it.

    My reading of this would be rather that we may not have a very good idea yet of what that AGW profile specifically for drought in the US Southeast may actually be–and I would have thought that that was somewhat in support of your general points about how AGW may have affected the drought, since otherwise one would probably be led to accept the Seager et al conclusion quoted at face value.

    I’ve responded at some length, CFU, but I truly am not trying to argue any position. I’m not “challenging” Seager et al., nor relying upon it as an “authority” to support some “point.”

    I am interested in trying to grasp what they are saying about an event that was very vivid for me, and consequently to understand what that event really has to say about the greatest crisis of our day–AGW. (For one thing, my personal domestic water supply is currently at risk due to the legal repercussions of that drought!–along with that of the rest of the population of the county I live in, of course. It’s out of the news right now because reservoirs are full, but the political system remains dysfunctional in dealing with a clear and present danger to the health and wealth of this area.)

    That’s why I asked about that bit from the conclusion that I found confusing. (Any ideas, anyone?)

  49. 249
    SecularAnimist says:

    Kevin McKinney wrote: “… we may not have a very good idea yet of what that AGW profile specifically for drought in the US Southeast may actually be …”

    Have you perused the chapter on the Southeast in the US Global Climate Research Program’s Global Climate Change Impacts in the United States, 2009, which John Mashey referenced in comment 217?

  50. 250
    Hank Roberts says:

    Thomas, 244, you’re restating the original post — that’s the point of the thread here, that it’s the regional details of the past millenium we need, in order to have a better idea what we can learn from that past period.

    As to “the data shows it is a real effect…. I lack a mechanism … that provides this predicition with some degree of credibility.” — It’s been observed; it’s happening. As my old doctor used to say, “In theory, theory and reality are the same, but in reality, they often differ.”