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  • Temperature Variations in Past Centuries and the so-called "Hockey Stick" « RealClimate
    and V Brovkin Assessing climate forcings of the earth system for the past millennium Geophys Res Lett 30 6 doi 10 1029 2002GL016639 2003 Bertrand C M F Loutre M Crucifix and A Berger Climate of the Last millennium a sensitivity study Tellus 54 A 221 244 2002 Briffa K R and T J Osborn Blowing Hot and Cold Science 295 2227 2228 2002 Briffa K R T J Osborn F H Schweingruber I C Harris P D Jones S G Shiyatov and E A Vaganov Low frequency temperature variations from a northern tree ring density network J Geophys Res 106 2929 2941 2001 Cook E R J Esper and R D D Arrigo Extra tropical Northern Hemisphere land temperature variability over the past 1000 years Quat Sci Rev 23 2063 2074 2004 Crowley T J Causes of Climate Change over the Past 1000 Years Science 289 270 277 2000 Crowley T J and T Lowery How Warm Was the Medieval Warm Period Ambio 29 51 54 2000 Esper J E R Cook and F H Schweingruber Low frequency signals in long tree line chronologies for reconstructing past temperature variability Science 295 2250 2253 2002 Gerber S F Joos P Brügger T F Stocker M E Mann S Sitch and M Scholze Constraining temperature variations over the last millennium by comparing simulated and observed atmospheric CO 2 Climate Dynamics 20 281 299 2003 Gonzalez Rouco F H von Storch and E Zorita Deep soil temperature as proxy for surface air temperature in a coupled model simulation of the last thousand years Geophys Res Lett 30 2116 doi 10 1029 2003GL018264 2003 Huang S H N Pollack and P Y Shen Temperature Trends Over the Past Five Centuries Reconstructed from Borehole Temperature Nature 403 756 758 2000 Jones P D K R Briffa T P Barnett and S F B Tett High resolution palaeoclimatic records for the last millennium Integration interpretation and comparison with General Circulation Model control run temperatures Holocene 8 455 471 1998 Jones P D M New D E Parker S Martin and I G Rigor Surface air temperature and its changes over the past 150 years Reviews of Geophysics 37 173 199 1999 Jones P D T J Osborn and K R Briffa The Evolution of Climate Over the Last Millennium Science 292 662 667 2001 Mann M E R S Bradley and M K Hughes Northern Hemisphere Temperatures During the Past Millennium Inferences Uncertainties and Limitations Geophysical Research Letters 26 759 762 1999 Mann M E Jones P D Global surface temperature over the past two millennia Geophysical Research Letters 30 15 1820 doi 10 1029 2003GL017814 2003 Mann M E Rutherford S Bradley R S Hughes M K Keimig F T Optimal Surface Temperature Reconstructions Using Terrestrial Borehole Data Journal of Geophysical Research 108 D7 doi 10 1029 2002JD002532 2003 Comments pop up 6 6 Responses to Temperature Variations in Past Centuries and the so called Hockey Stick 1 donald baker says 14 Dec 2004 at 10 42 AM Dear RC I am grateful to find this site For the most part it succeeds in presenting a scientific non political tone with lots of useful information As long as you maintain this non political stance the site can do great good Science is becoming more and more politicized and is in danger on certain subjects such as this of losing all credibility in the way that social and political science have done generally As a result we get The Day After Tomorrow and State of Fear both highly political the former hysterical as well and obviously partisan representations of issues If you can avoid such phrases as right wing or left wing you can contribute much to this important discussion Yours Don Baker 2 Steve Funk says 14 Dec 2004 at 2 12 PM Does anyone know what caused the medieval warming I would assume it is not anthropogenic Response In the modeling studies shown in Figure 2 above the relatively warm temperatures between AD 800 1200 or so are due to a combination of factors including 1 a relative lack of explosive tropical volcanic eruptions which can a substantial global cooling influence in comparison with later centuries and 2 relatively high estimated values of solar irradiance though solar reconstructions exhibiting very large century scale variability such as that used in the GKSS simulation shown in Figure 2 have recently been called into question see this article in Science by Foukal et al and references therein mike How do we know that these processes are not responsible for current climatic variation Response The same simulations referred to above indicate that natural factors such as volcanoes or solar variations are not sufficient to produce the observed 20th century forcing The observed 20th century warming ins these simulations can only be produced through the addition of anthropogenic forcing to the simulations mike How sensitive are estimates of historic temparature variation to changes lasting less than 30 years Response In general quite sensitive many historical and proxy climate indicators resolve year to year temperature variations quite well mike Why is the 6th century cooling a disaster which caused widespread starvation in the British Isles shown as a relatively small blip in the graphs Response This again gets at the important distinction discussed here see myth 2 between regional e g British and truly hemispheric or global scale temperature changes mike 3 Steve Funk says 14 Dec 2004 at 6 40 PM This again gets at the important distinction discussed here see myth 2 between regional e g British and truly hemispheric or global scale temperature changes http news independent co uk world science medical story jsp story 487550 There are probably much better sources but the sixth century cooling is variously attributed to a comet or Krakatoa neither of which happened in the western hemisphere Why would one assume it is just a regional phenomenon Response It is very difficult to to diagnose the potential climatic response to

    Original URL path: http://www.realclimate.org/index.php/archives/2004/12/temperaturevariations-in-past-centuries-and-the-so-called-hockey-stick/ (2016-02-13)
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  • False Claims by McIntyre and McKitrick regarding the Mann et al. (1998) reconstruction « RealClimate
    yield a reconstructed Northern Hemisphere mean temperature history quite similar to that of MBH98 and demonstrate skill against independent 19th century instrumental 2 data RE 0 39 0 33 respectively only moderately lower than the MBH98 result RE 0 51 The central hockey stick result of MBH98 is thus quite clearly seen to be robust to the specious criticisms raised by MM with regard to the centering convention used in the PCA of proxy data networks or the infilling of missing values in certain proxy series FIGURE 3 Comparison of MBH98 reconstruction from AD 1400 1980 with alternative reconstructions from AD 1400 1971 based on the direct use of the individual 95 proxy series available back to AD 1404 yellow and the 94 proxy series all 95 series except the St Anne series discussed above available back to AD 1400 green Shown for comparison is the instrumental NH annual mean record 1856 1980 red The MBH98 reconstruction was based on a stepwise approach employing increasingly more proxy data over time while the other two reconstructions for simplicity are performed with the same frozen proxy network over the entire interval Given that each of the criticisms of MBH98 raised by MM are demonstrably false one might well be led to wonder how MM using the MBH98 method and their putative corrected version of the MBH98 proxy dataset were able to obtain a reconstruction so at odds with the MBH98 reconstruction and virtually all existing reconstructions in particular in its apparent indication of anomalous 15th century warmth Rather than correcting the MBH98 proxy data set we demonstrate that the reconstruction of MM resulted instead from their selective censoring of key indicators from the MBH98 proxy dataset Indeed we are able to reproduce the MM reconstruction of anomalous 15th century warmth when the entire ITRDB North American data set and the Queen Anne series are censored from the proxy network Figure 4 These data in fact 70 of all of the proxy data used by MBH98 prior to AD 1600 were unjustifiably censored from the MBH98 dataset by McIntyre and McKitrick 2003 in their original analysis see Jones and Mann 2004 and Rutherford et al 2004 for a discussion MM in their more recent rejected submission to Nature instead filtered out the hockey stick pattern of low frequency variability in the North American ITRDB data through the incorrect PCA truncation described above which censors this pattern by retaining too few Principal Components series in the data As discussed above the MBH98 reconstruction and the variants of the reconstruction i e Figures 2 and 3 that address the various spurious criticisms raised by MM each pass statistical verification In stark contrast our reproduction of the MM reconstruction demonstrates that their reconstruction dramatically fails statistical verification see Figure 5 with an RE score 0 76 that is statistically indistinguishable from the results expected for a purely random estimate as a reminder RE 0 exhibits no skill and RE 1 is the average value expected for

    Original URL path: http://www.realclimate.org/index.php/archives/2004/12/false-claims-by-mcintyre-and-mckitrick-regarding-the-mann-et-al-1998reconstruction/ (2016-02-13)
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  • Weren’t temperatures warmer than today during the “Medieval Warm Period”? « RealClimate
    Jan 2008 at 9 08 PM and had relatively little impact on the global averages Dr Michael Mann addresses this here and for an example of a contrarian tactic that can make it appear otherwise or that the current 5 Hypography Science Forums Co2 Acquittal says 1 Feb 2008 at 1 35 AM by mitigating our impact And I know you like this place so yes I did look here RealClimate Here is an interesting article 6 Earth The Climate Wars Wildlife and Environment Forums says 20 Sep 2008 at 12 52 AM of the little ice age and medieval warm period and many other discussions of climate change RealClimate To summarise the ideas very simplistically I think this is what they say the little ice age and 7 Understanding the Basics of Global Holocene Climate Change Understanding Global Warming says 16 Mar 2009 at 8 57 PM magnitude of medieval warmth was weaker than that of today Dr Michael Mann also addresses this here and for an example of a contrarian tactic that makes it appear otherwise or that the current 8 Understanding Global Warming says 17 Mar 2009 at 1 46 PM magnitude of medieval warmth was weaker than that of today Dr Michael Mann also addresses this here and for an example of a contrarian tactic that makes it appear otherwise or that the current 9 Ben s Blog Blog Archive Head to head with a Cardinal says 24 May 2009 at 10 58 PM qualified in a way that I and the good Cardinal are not The medieval warm period wasn t warmer Warming didn t stop in 1998 But most appalling to me was this childish attempt to use his Site Google Custom Search Recent Comments What is the best description of the greenhouse effect Patrick Eriksson What is the best description of the greenhouse effect Kevin McKinney Anti scientists Carbomontanus What is the best description of the greenhouse effect Spencer Marvel et al 2015 Part III Response to Nic Lewis SteveS What is the best description of the greenhouse effect Chris Colose Blizzard Jonas and the slowdown of the Gulf Stream System doiknow What is the best description of the greenhouse effect James Powell Unforced Variations Feb 2016 Jim Galasyn What is the best description of the greenhouse effect Jack Barrett With Inline Responses Marvel et al 2015 Part III Response to Nic Lewis SteveS Marvel et al 2015 Part III Response to Nic Lewis steve s Marvel et al 2015 Part III Response to Nic Lewis Andrew Kerber Blizzard Jonas and the slowdown of the Gulf Stream System Hank Roberts Blizzard Jonas and the slowdown of the Gulf Stream System doiknow Marvel et al 2015 Part III Response to Nic Lewis MartinM Anti scientists Don McKenzie Marvel et al 2015 Part III Response to Nic Lewis Matt Skaggs Anti scientists mikeworst New On line Classes and Models Marcus Pages Acronym index Data Sources Categories Climate Science Aerosols Arctic and Antarctic Carbon cycle Climate

    Original URL path: http://www.realclimate.org/index.php/archives/2004/12/werent-temperatures-warmer-during-the-medieval-warm-period-than-they-are-today/ (2016-02-13)
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  • OK, But we do know it was warmer than present 6000 years ago, don’t we? « RealClimate
    palaeoclimatic reconstructions The emphasis on northern hemisphere palaeoclimate records as representing a global record is very problematic for those of us who live and work in the rest of the world In southern Australia the period 6 8 ka thousand years before present crater lakes that now show falling water levels were overflowing Reconstructions of climate indicate that the lake evaporation rainfall ration was above 1 and could hve been greater than 1 2 Palaeoclimates 3 51â 82 The current ratio in the region is about 0 8 If temperatures and solar radiation were at current levels at 6 8 ka this would require rainfall at 150 above today s volumes It is therefore likely that the climate at that time was cooler particularly in summer though the cloudy winters may have been warmer suggesting a more uniform seasonal cycle suppressing evaporation Rainfall was certainly higher at that time An abrupt shift at about 5 5 ka saw some 2 000 years of sustained drying In northern Australia at around 6 7 ka eastern Queensland reconstructions indicate warmer and drier conditions in southern Queensland and warmer and wetter conditions in the north www dsm unile it Bacheca IGCP437FinalConference AbstractBook Grindrod119 122 pdf the former from one site and the latter from multiple sites At that time animals in the montane tropics were restricted in their distribution due to higher temperatures suggesting that similar restrictions may occur under global warming Proceedings of the Royal Society Biological Sciences 270 1887 1892 track back through refs for palaeo work The past 200 years of land use change in Australia has compromised most hydrological proxies making reconstruction difficult We still don t have a good idea of what was going on in the Holocene in Australia but words like optimum developed to describe conditions in the cooler regions of the northern hemisphere confuse when applied globally Note also in reference to the climate of the past 2000 years south eastern Australia was relatively cool and wet until about the mid 19th century Journal of Hydrology 246 158 179 It is now as dry in the region as it was in the early Holocene and drier than it has been for most of the Holocene These changes precede the enhanced greenhouse effect If changes in rainfall projected by climate models occur it could become as dry in southern Australia as it was about 12 5 ka before present Simple pictures of how Earth s climate has behaved in the past rarely stand up to regional scrutiny We should not jump to the conclusion that largely northern hemisphere reconstructions show some kind of globally homogenised picture Palaeoclimatic reconstructions are sorely needed from the three southern continents Africa South America and Australia and from southern islands the latter mentioned to keep our Kiwi and Pacific friends happy 5 Understanding Global Warming Understanding Global Warming says 30 Dec 2007 at 3 55 AM see the links section The mid holocene warm period about 6000 years ago is a

    Original URL path: http://www.realclimate.org/index.php/archives/2004/12/ok-perhaps-recent-20th-century-warmth-is-anomalous-over-the-past-millennium-or-two-but-wasnt-it-warmer-during-the-holocene-optimum-some-6000-years-ago/ (2016-02-13)
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  • On Yet Another False Claim by McIntyre and McKitrick « RealClimate
    allow decision makers to scan your work and get the rational message contained within Keep up the good work sir D 2 Ken says 9 Jan 2005 at 2 19 PM All of this technical statistical jargon is over my head but I get the impression that the data on which the climate reconstruction is based is so sparse and uncertain that you can t draw any firm conclusions supporting either MM s or Mann s side of the debate Response Even without technical training or a statistical background you should have an adequate basis for discerning which of the two parties is likely wrong here Only one of the parties involved has 1 had their claims fail scientific peer review 2 produced a reconstruction that is completely at odds with all other existing estimates note that there is no sign of the anomalous 15th century warmth claimed by MM in any of the roughly dozen other model and proxy based estimates shown here and 3 been established to have made egregious elementary errors in other published work that render the work thoroughly invalid These observations would seem quite telling mike The question that I wish someone would address is What difference does it make CO2 is reaching levels that haven t existed for hundreds of millennia and it seems to me that on that time scale the climate a few hundred years ago provides no better basis for extrapolating future climate than does yesterday s weather Can any conclusion regarding future climate change be drawn from the hockey stick reconstruction and can any level of statistical confidence be placed on the conclusion Response Refer to this post and references therein for a detailed discussion of how comparisons of proxy based climate reconstructions with theoretical climate model simulations can inform our assessment of the role of both natural and anthropogenic factors in recent climate change It is often more instructive in this regard however to compare not just hemispheric mean series but the actual spatial patterns of estimated and observed climate changes in past centuries See e g the review paper Schmidt G A D T Shindell R L Miller M E Mann and D Rind 2004 General circulation modelling of Holocene climate variability Quaternary Sci Rev 23 2167 2181 doi 10 1016 mike 3 Bill Hemmer not on CNN says 9 Jan 2005 at 5 00 PM In considering the issue of earth warming I believe that the question of Is it or is it not occurring is irrelevant The more important question is it seems to me How should we behave In this I take my thinking from Pascal There are two realities and two behaviors paired to give us four scenarios 1 We behave as though earth warming is not occuring and it isn t 2 We behave as though earth warming is not occuring and it is 3 We behave as though earth warming is occuring and it isn t 4 We behave as though earth warming is occuring and it is If one thinks through the implications of each of these four scenarios our course of action seems clear What s the problem 4 Stephen Nodvin says 9 Jan 2005 at 11 07 PM Mike According to lay people that I talk to who have been influenced by the op ed pieces that you mention global warming has been disproved The MM story is indicative of a pattern in which industry and now our own government PR machines latch on to minority scientific articles to claim that an environmental issue has no basis The journal Science in the late 70s or early 80 s once published an article in which the author claimed that the major components of acid rain were weak acids The article should have failed peer review and never been published the scientist conducting the work titrated the samples in open air effectively measuring not only the weak acids in the samples but also the carbon dioxide from the room The work was plain wrong Nevertheless representatives of the power companies parroted the findings for several years to claim that acid rain was NOT related to industrial air pollution Stephen 5 Scott Robertson says 10 Jan 2005 at 12 40 PM Great Piece Mike If you are familiar with Tim Lambert he has spent much time discussing discrediting MM As an aside are you familiar with Positive Matrix Factorization PMF We are using it in lieu of PCA for samples we analyzed from a dust storm experiment in Asia It has several major advantages over PCA including that it doesn t produce negative non real results and you can incorporate uncertainty into the analysis so you can limit significance of low level or missing data Keep it up guys I have a brother in law who is a Fox News Talk Radio junky and he insists global warming is a myth yet has never read or seen a scientific article Truly unbelievable Response Thanks very much for your comment Scott Indeed there are other statistical approaches as you note to the problem of climate field reconstruction many of which are somewhat more sophisticated than and arguably preferable to PCA based approaches In this previous post here we discuss the results from a recent paper in press in Journal of Climate by Rutherford et al that uses the Regularized Expectation Maximization RegEM algorithm to reconstruct past temperature patterns from proxy data the results are remarkably consistent with past proxy based reconstructions using other e g PCA based methods Applications to reconstructions of patterns of past continental drought from tree ring data can be found here RegEM is an iterative approach for estimating the data covariance of an incomplete data set and imputing missing values in the process using what is sometimes referred to as ridge regression In this approach the main diagonal of the estimated data covariance matrix is inflated using a smoothing parameter determined by generalized cross validation GCV to insure that

    Original URL path: http://www.realclimate.org/index.php/archives/2005/01/on-yet-another-false-claim-by-mcintyre-and-mckitrick/ (2016-02-13)
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  • What If … the “Hockey Stick” Were Wrong? « RealClimate
    that can be drawn range from a temperature decrease from 0 05 in AD 200 to 0 10 in 1980 along the upper edge of the envelope to a temperature increase from 0 50 in 200AD to 0 35 in 1980 The pitfalls of producing statistical judgements are endless however I just want to note that it is only right at the very very end within just a few years of 1980 that the instrumental record or any of the proxies first emerge above any of the possible projected lines that can be drawn through the uncertainty envelope I do seriously question the motives of the authors in including in bright red a smoothed instrumental record which is not smoothed using the same 40 year smoothing which was applied to the proxies In fairness for comparison purposes and because a reasonable estimate of climate cannot be made without applying such a broad smoothing to yearly data I do not feel it is appropriate for the figure to include a shorter term more noisy representation for the observations than are used to display the proxies So I dismiss the tail of the red curve I do this knowing full well that the numbers are real But I dismiss them because they cannot tell us anything about the true long term climate trends And having dismissed most of the blade of the hockey stick I am left with as I said more of a putter To summarize my perspective I see the GHG climate signal emerging but only just so I understand the spirit of the detection and attribution work which in theory allows us to project the current trends based on their demonstrated cause using model results But this is all theory and I fear that some of the detection and attribution work as well as the modeling work have suffered from not being double blind if you know what you re looking for you re much more likely to find it So from my perspective the on the ground trend is the kind of solid evidence that the average person will react to Inertia makes the public perception slow to turn But if the trend continues as IPCC projects and as nearly all reasonable scientists including myself expect then the changes will get their attention But I don t subscribe to the use of scare tactics even by those who are honestly scared I reject the hype in a recent article in the Independent about a point of no return for CO2 emissions When CO2 emissions are reduced climate will respond Meanwhile the intriguing affairs of human politics will surely produce more catastrophic outcomes from other causes unrelated to climate 5 John Bolduc says 27 Jan 2005 at 5 15 PM Regarding comment 4 I ve asked the following question under other topics but have not gotten a direct response Peter asserts his faith in humanity s instinct not to soil it s own nest I would generally subscribe to that faith myself The troubling aspect of climate change to me as a layperson is that as others have analogized it is like the frog placed in a pot of water that is brought slowly to boil The frog doesn t know that it should jump out My rudimentary understanding of the accumulation of greenhouse gases is that they persist for a relatively long time The problem with greenhouse gas emissions is that the problem is different in character compared to CFCs etc there is a relatively short time between action and result Therefore switching over to fusion or other non fossil fuel sources decades from now will put us in the position of significantly higher atmospheric concentrations of GHGs that will take a long time to dissipate through natural processes unless someone comes up with an effective sequestration method I suppose And given that people have a very poor understanding of the stocks and flows nature of GHG s see John Sterman s paper Cloudy Skies Assessing Public Understanding of Global Warming http web mit edu jsterman www I m not as comfortable as Peter that there will be timely breakthroughs in technological fixes public understanding and policy So my question again is how long can we afford to wait to reduce GHG emissions I m not asking this rhetorically Can those of you who model climate shed any light for us laypeople on what happens if we wait 10 20 50 or 100 years to reduce emissions Putting the effects of higher atmospheric concentrations aside if we double triple quadruple CO2 concentrations how long does it take to reduce those emissions I m assuming that if technological fixes come along that society will still have to deal with decades or centuries of climate change impacts before we return to normal levels of GHGs in the atmosphere Thanks 6 Mark Bahner says 27 Jan 2005 at 11 21 PM John Finn writes If anyone s running a book on Crichton v IPCC I ll have a couple of hundred on Crichton Gavin Schmidt replies I would recommend you not be so free with your lira There s a pretty easy way to resolve this matter In fact there is a non profit organization designed especially for such matters Long Bets Wigley and Raper Interpretation of High Projections for Global Mean Warming Science Vol 293 20 July 2001 assume a log normal distribution for the IPCC TAR projections and come up with the following temperature rise probability distribution for the year 2100 relative to 1990 see log all 1 1 29 degrees Celsius 5 1 68 degrees Celsius 25 2 42 degrees Celsius 50 3 06 degrees Celsius 75 3 78 degrees Celsius 95 4 87 degrees Celsius 99 5 61 degrees Celsius Michael Crichton s 0 8 degrees Celsius prediction as I understand it is for the 21st century So if we ratio that for the 110 years from 1990 to 2100 the value would be 0 8 110 100 0 88 degrees Celsius Let s take that as Crichton s 50 probability value and use the exact same ranges described by Wigley and Raper for the IPCC For example Crichton s 75 value would be 0 88 degrees Celsius 0 72 degrees Celsius which is 3 78 minus 3 06 or 1 6 degrees Celsius Completing the calculations for Crichton s predictions 1 0 89 degrees Celsius cooling 5 0 50 degrees Celsius cooling 25 0 24 degrees Celsius warming 50 0 88 degrees Celsius warming 75 1 60 degrees Celsius warming 95 2 69 degrees Celsius warming 99 3 43 degrees Celsius warming I will bet Gavin Schmidt or any other author on this website 200 on LongBets org that Michael Crichton s projections for temperature increases are more accurate than the IPCC assuming that the temperature being projected is average lower tropospheric temperature as measured by satellites With regard to the present debate about satellite measurements I m sure it will be resolved to the satisfaction of everyone by 2100 Just to make sure we all understand the terms of the bet suppose the lower troposphere warms by 1 60 degrees Celsius from 1990 to 2100 According to Crichton there s a 25 chance that the lower troposphere will warm by that much But according to the IPCC there is only about a 4 chance that the lower troposphere will warm by that little So if the lower troposphere warms by 1 60 degrees Celsius Crichton and I would win the bet Conversely if the warming was 2 69 degrees Celsius for example the IPCC and Dr Schmidt or whomever else is willing to bet against Crichton would win We ll call this bet the Julian Simon Memorial Bet I m waiting gentlemen And I m not kidding Mark Bahner environmental engineer Response There are at least three things wrong with this idea First and foremost neither you nor I will be around to collect so this appears a little pointless Secondly Crichton did not assume the same uncertainty as IPCC and in fact made his estimate to 7 or so decimal places That you would win if temperature changes were 1 6 twice his estimate but within the IPCC range seems a little perverse Thirdly we are not in the fortune telling business The reason why different projections are used is precisely because we have no clue how technology or economic growth are going to change things 100 years down the line However given a particular increase in greenhouse gases we can say reasonably precisely how much it will warm The current energy imbalance at the surface as demonstrated by the increasing heat content of the oceans implies there is at least a further 0 5 deg C surface warming in the pipeline As I said keep your money in your pocket gavin Response I m a bit more keen on this idea though it remains to be worked out Its been discussed on sci env recently But its a bit far out of RC s remit Go to sci env or my blog William Response There s another more serious problem with this bet All these scenarios are non mitigation scenarios i e they tell us what would happen if we did nothing to slow down or stop climate change We re unlikely to ever see the outcome of this hypothetical experiment I think it s very unlikely that humanity will sit and do nothing about the problem until 2100 I think it is quite possible perhaps even likely that warming by 2100 will be 1 6 degrees Celsius but not because Crichton was right but because people recognised he was wrong and implemented measures to halt the warming Stefan 7 Bruce Stram says 28 Jan 2005 at 9 56 AM You may well have addressed this at another place and another time but at least in this sequence I m left grasping You seem to argue explicitly that no evidence about what has happened in the past millenium is significant with regard to the conclusion that human caused global warming is upon us because we can come to that conclusion based upon analysis of what has happened over the past century This arguement would seen to hold as well for the whole history of the earth That seems to raise an obvious queston how is it that a relative eyeblink of time can be so definitive with regard to a warming trend that as far as I know has been significantly exceeded in the past Response The answer is very simple if you want to study the causes of 20th Century warming you need to look at 20th Century data And not just the climate response but more importantly the forcings Otherwise you cannot determine the cause E g to check whether solar variability may have caused 20th Century warming you need to look at 20th Century solar data Information on temperatures in the 15th Century doesn t help much to answer that specific question except for giving you some rather general information on possible climate system behaviour If you want to study the causes of a warming or cooling in period X of Earth history you study the data from period X Stefan Let me clarify this a bit Stefan is not saying we don t learn anything from paleoclimate records To the contrary we have learned a lot about how the system works A particularly good example is that of abrupt climate change as recorded in Greenland ice cores which few scientists thought was likely or even possible prior to those discoveries eric 8 Rick Watkins says 28 Jan 2005 at 3 39 PM Regarding 4 If anyone can suggest to me why I should not use these two examples as a model for how humanity will and should respond to our release of chemicals into the atmosphere I ll be happy to listen Cause and effect in the two examples given were readily apparent and the link between the two short and easy to grasp in the public mind in my daily experience this is far from true in the case of global warming Further the two examples required no personal sacrifice or change of lifestyle on the part of the general population concerned or otherwise which seems to be unavoidable to curb emissions of GHG s unless a way can be found to deliver the goods to the world population while at the same time reducing emissions drastically can t see that somehow My little bit of pollution is insignificant even countries reason that way I m not going to going to whatever and miss out until so and so next door does I ve seen no letup in rampant consumerism and selfishness in the last 50 years Finally the lag as I understand it between cause and effect with respect to global warming will probably mean that by the time mankind in general and leaders in particular are concerned enough to do anything meaningful it will probably be too late Though I admire your faith in human nature I fear that it is sadly futile An excellent site just what was needed I think information regarding emerging climate trends and their correlation to climate models would be useful though understandably at this time tentative 9 dave says 28 Jan 2005 at 4 49 PM Re The main reason for concern about anthropogenic climate change is not that we can already see it although we can The main reason is twofold Of the emitted CO2 from burning of fossil fuels about 50 is absorbed by the ocean and terrestrial carbon sinks at present See Sarmiento and Gruber Although the residence time of a CO2 molecule in the atmosphere is about 5 years due to the photosynthesis respiration flux and the atmosphere ocean flux the actual time that the excess CO2 remains in the atmosphere is long it is thought to be about 100 years This is due primarily to the fact that CO2 is non reactive in the atmosphere and it takes quite a while for natural processes mostly in the ocean to scrub it out Also it thought that the easy carbon sinks are becoming saturated and will be less and less effective over time This means that a greater percentage of future emissions will stay in the atmosphere Now add to this some bad news about increased use of Coal as reported in New coal plants bury Kyoto Contains a quote from Gavin Coal is dirtier than oil or natural gas with respect to CO2 emissions in a ratio of about 3 coal 2 oil 1 gas The main culprits usual suspects are the U S China and India Now suppose the likely climate sensitivity for doubled CO2 is at the IPCC low end 1 5 C which it isn t it is probably at least 2 0 C Stefan says The IPCC gives the uncertainty range as 1 5 4 5 ºC Only if this is wrong and the true value is lower can we escape the fact that unabated emissions of greenhouse gases will lead to the warming projected by the IPCC Well given humankind s renewed eagerness to burn fossil fuels and their long lifetime in the atmosphere even a climate sensitivity below the low end estimate which no one believes or at the low end which is highly unlikely can still be overwhelmed by CO2 emissions going forward How about a tripling about 850 ppmv Anybody want to go for a quadrupling I see that some people here like to gamble 2 6 Too bad and how convenient they won t be around to see the results But future generations will As regards a fundamental faith in human nature We will not soil our own nest 4 I ll believe that when I see it 10 Jeffrey Davis says 28 Jan 2005 at 5 19 PM We will not soil our own nest I remember the PBS show which took Michael Palin to China He visited the filthiest grimiest city I ve ever seen Heartbreaking The Red Chinese make our rapacious extractives look sunshine fresh So suddenly Red China is going to turn green I don t think so And in all of the dismissals of GW I hear some high priest from Rapa Nui insisting that what the world needs is another stone head Woodsman don t spare that tree 11 Bill Hicks says 28 Jan 2005 at 5 44 PM Thank you for another interesting and usefull post It is important to remember now to get too bogged down in arguments over details when they don t affect the big picture John Finn and Mark Bahner thank you for an amusing end to a hard week of work Wow 2100 Remember Julian Simon was an economist who knew something about what he was talking about and Ehrlich didn t represent a consensus among the scientific community or even demographers You might want to be careful about relying on a work of admitted fiction when making predictions Response It is worth adding that Ehrlich has detailed a host of predictions that Simon would not bet on when Ehrlich offered Ehrlich did this not because he thought betting was a useful way to produce good science or good policy but to show that the emporor Simon has no clothes eric 12 Jo Calder says 28 Jan 2005 at 8 07 PM Stefan writes Carbon dioxide and other greenhouse gases are increasing rapidly in the atmosphere due to human activity This is a measured fact not even disputed by staunch climate skeptics But he should have written more carefully Indeed it appears that C02 levels are rising but to interpolate due to human activity implies that skeptics believe in an anthropogenic cause That s plain wrong So now let s consider the question of whether on geological timescales C02 trends anticipate or lag behind temperature trends Response It is unclear what you are saying is wrong here That CO2 levels are caused by something other than human activity or that the skeptics believe that CO2 levels are caused by something other than human activity In any case we ve adddressed both the issues of CO2 causes and lags and leads in previous posts on RealClimate See here and here eric 13 George Roman says 28 Jan 2005 at 8 52 PM Re 11 I have yet to hear any skeptic claiming that rising atmospheric CO2 is some sort of natural coincidence If they are I don t think they are a credible source See this post here from Dec 22 http www realclimate org index php p 87 14 dave says 28 Jan 2005 at 8 55 PM Re 11 anticipate or lag sorry this is too much fun I ll bet you guys at realclimate never anticipated this kind of response The comment assumes that there is no evidence that fossil fuel emissions add CO2 to the atmosphere and that CO2 increases in the atmosphere may lag some natural variability that nobody can identify Perhaps that variability is caused by Q waves from the Galaxy Ignorancia 331A An uncertainty in some ice core results is somehow translated into a statement about current CO2 emissions data in the 20th and 21st centuries which can be measured precisely However Q waves are not taken into account Now if you respond that seems ridiculous the true premise underlying the climate warming problem needs no defense On the other hand if you don t respond you leave the statement out there hanging in the wind for perhaps inhabitants of the galaxy alluded to above to pick up on Oh Earth to Jo Earth to Jo 15 dave says 28 Jan 2005 at 9 19 PM There was a timing problem When I said 11 in my comment 14 it turned out to be 12 from Jo Calder Well I m off to Galaxy Ignorancia 331A for a bit of rest And please look into those Q waves 16 Tom says 29 Jan 2005 at 10 46 AM Reponse to 4 If anyone can suggest to me why I should not use these two examples as a model for how humanity will and should respond to our release of chemicals into the atmosphere I ll be happy to listen I have to respond to this one too The reason I m concerned that we won t respond to this global threat in a rational way is that huge amounts of intellectual and writing talent are being exerted to make sure we don t This site is an important addition to the cause of pushing out accurate information I don t think the previous campaigns you mentioned to stop nuclear testing or limit CFC use had huge armies of paid writers and skeptics backed by organizations with political and economic interests in confusing and skewing the debate 17 tom says 29 Jan 2005 at 11 55 AM At some point it had to be pretty obvious to a significant number of the stone head people that the trees were going to be decimated But at a certain point tipping point it also probably became clear that the pooch was already screwed And so why not build a few more heads After all like our SUVs and trophy homes they were a big status symbol Given a lot of the commentary on this site I feel we may have already screwed our own particular pooch Even if you are a believer you reach the point where it all seems hopeless So why not get on the band wagon to hell and enjoy the ride Especially scary to me was the comment that excess CO2 remains in the atmosphere for 100 years What does this mean even assuming that we cut our emissions by say 50 Rearding the above comment in 9 I am still a bit confused with respect to the difference between residence time 5years and remain time of 100 years I would appreciate some clarification of that issue Response Residence time in the atmosphere refers to the average time before a molecule is reabsorbed into the ocean mixed layer However the ocean mixed layer equilibrates quickly to the atmospheric concentration but has a very slow equilibration time with the deep ocean So any anomaly in the atmosphere ocean mixed layer takes a long time to disappear This long time 100 yrs is essentially is the residence time for the atmopshere ocean mixed layer combination of reservoirs gavin 18 Peter J Wetzel says 29 Jan 2005 at 1 23 PM There is of course a wide spectrum of greeness among the governments of the world And there is an equally wide spectrum of suppression and or neglect of the populations they serve Governments do shift policy sometimes rapidly At other times change is agonizingly slow What is the most reliable way to convince each of the disparate governments to mandate reductions in CO2 emissions Governments act in their own best interests first and foremost they have as much instinct for self preservation as individual organisms do And like living organisms the strongest motivators for change are usually greed fear and pain Convince a government that CO2 reduction will cause them and in the case of democracies their populations less pain and suffering will provide them with greater security and safety and will improve their economic condition Frankly I don t think the pure science of the global warming issue has clearly identified enough of a global threat of pain and suffering nor of security and safety And the economic cost benefit ratio argument has always been a difficult one in the absence of cheap clean widely available alternative energy sources Pain and suffering Creeping sea level rise gets juxtaposed with tsunamis and with engineering marvels such as the Dutch Polders Encroaching drought in the sub tropics is juxtaposed with improved growing season length CO2 fertilization and potentially more precipitation for more polar nations Etc etc Security and safety The current consensus among scientists is that sudden catastrophic climate change is very unlikely The threat of a generally more volatile and uncertain insecure climate is perhaps the strongest argument science has been able to make And many governments have been convinced on the strength of these findings Yet the science in this area still has much work to do Extreme events are notoriously the most difficult phenomena to correctly represent in climate models Progress is continuing apace and when I choose to use that word rather than if the science becomes more robust and when or if the corresponding climate trends toward volatility of weather emerge clearly from the background noise of natural daily weather then more and more governments will find motivation to act Hopefully breaks in alternative energy technology will accompany the progress in climate science That coupled with the increased urgency which will or might arise if climate becomes more obviously destructive should in my view eventually open the eyes of enough of the power brokers of the world so that meaningful policy actions will begin to be implemented 19 Eli Rabett says 29 Jan 2005 at 9 36 PM With respect to residence time in the atmosphere and mixed layer of the ocean most carbon cycle models I ve seen also include the biosphere and the residence time among these three is often given as much longer than 100 years but shorter than a millenium My question is what is the average time for the excess atmospheric CO2 produced by burning fossil fuels to mix into the deep ocean reservoir Response The mixing into the deep ocean is not a simple process it is enhanced by the rain of organic matter through the ocean column but decreased by the rain of carbonate shells It is enhanced too by the formation of deep water in the polar regions but slowed by the warming of the surface ocean The complexity of these processes mean that the decay of the CO2 impulse into the atmosphere ocean mixed layer cannot be characterised by a single exponential time scale The best estimates are for the average are therefore a little uncertain and range between 50 and 200 years gavin 20 Eli Rabett says 30 Jan 2005 at 4 34 PM If I understood your answer it is not the answer to the question I asked Perhaps I was not clear I accept that the mixing into the mixed layer is 50 200 years However the mixed layer being accessible to turbulent wave mixing with the atmosphere is essentially in contact with the atmosphere and over fairly short times equilibrates with it and other parts of the biosphere Moreover the amount of carbon in the three reservoirs the atmosphere mixed layer and the surface biosphere are approximately equal Given this situation a pulse of CO2 injected into the atmosphere will fairly quickly relax to 1 2 1 3 of the injected value This is what we have seen My question was how long it would take for the excess injected into the upper three reservoirs to relax into the much larger deep ocean reservoir Only when this happens will todays observed increase in CO2 mixing ratio decrease It was my impression that this was the 500 1000 year process admittedly it is not a single process but a complicated melange Just to be clear I am not talking about incorporation into rocks which will take much longer and be the final dilution Response Maybe I wasn t clear the mixing time for the atmosphere ocean mixed layer to come into equilibrium is short around 5 years the longer time scales of 50 200 years are for the mixing into the deep ocean and are thus the appropriate time scale for your question There is a long tail of the response 1000 years due to the slow mixing of deep ocean waters which makes it dificult to give just one number As you say the timescale for full sequestration is more like 50 000 years gavin 21 George Roman says 30 Jan 2005 at 8 51 PM This is a well written piece that covers the issue very well However the wording of the sentence the point of conducting science is to give an early warning rather than just wait until the facts are obvious to everyone is the weakest part of the entire argument Not surprisingly today I read an editorial by Mark Milke in the Calgary Herald exploiting this weakness for maximum rhetorical effect The editorialist removes the sentence from the context of the preceding text and then argues Readers should examine the above point by Rahmstorf because it contains shaky reasoning If it is indeed the case as McKitrick and McIntyre claim that the science behind Kyoto is not solid then to argue that the only thing that proves is that our data are not yet good enough assumes the very conclusion one is trying to reach He then goes on to say that McKitrick and McIntyre s article undermines claims about what caused late 20th century warming essentially ignoring most of the points made above by Rahmstorf I wrote a letter to the editor to try to expose some of these tactics and to more clearly convey the original message of this post It will be interesting to see if they publish it To give the editors some credit they did state at the bottom of the op ed piece that an associate of Michael Mann has promised to send a response to the McKitrick and McIntyre allegations in the coming days Ther position is outlined at http www realclimate org and http www climate2003 com 22 Bruce Stram says 31 Jan 2005 at 3 51 PM RE No 7 I m sorry but your response still doesn t have me quite with the program Indeed yes one must look to 20th century events approximately to understand 20th century outcomes But given what I understand to be true that greater warming has occured than in the distant past than is currently occurring how can we be so sure we are examining all the right 20th century events since these earlier warmings were clearly caused by events other than human driven carbon dioxide emissions 23 Jo Calder says 31 Jan 2005 at 7 09 PM Further to 12 and eric s response I m sorry if I wasn t clear enough in the second sentence by Stefan that I quoted this can only sensibly refer back to the whole of the first sentence giving the reading That carbon dioxide and other greenhouse gases are increasing rapidly in the atmosphere due to human activity is a measured fact not even disputed by staunch climate skeptics Further to 14 from dave I m happy for him that he has the power to measure ignorance from such a distance Further to 13 although I m not so sure that George s message is a response to mine a mode of explanation seen at What does the lag of CO2 behind temperature in ice cores tell us about global warming allows an appeal to some currently unknown process If we don t know what that process was how do we know that it isn t happening now or didn t start happening 50 years ago Regards Jo 24 Joel Shore says 1 Feb 2005 at 11 11 AM Directed to Jo 23 What the skeptics tend to dispute is that the warming seen in the 20th century is mostly due to anthropogenic causes and the rise in CO2 in particular They do not tend to dispute that the rise in CO2 is due to anthropogenic causes The evidence that the CO2 rise is due to humans is incontrovertable For one thing the timing with the industrial revolution is hard to dismiss as a coincidence especially since it is known that CO2 levels haven t been as high as they are now for at least 1 million years over which we have very good data from ice cores and likely for the last 20 million years Secondly the amount of the concentration rise is in line with the amount we know we are putting into the atmosphere through the burning of fossil fuels Actually the rise in CO2 concentrations is generally running at a rate of about 1 2 of what we know we are adding to the atmosphere showing that the biosphere and hydrosphere is able to take up some but not all of our additions Thirdly I believe that by looking at the different isotopes of carbon and or oxygen in the CO2 the additional CO2 can be attributed directly to the burning of fossil fuels Of course what is true is that as we warm the atmosphere the not well understood processes by which CO2 and methane seem to get released as the earth warms may come into play again and create a positive feedback If this occurs we will start to see an acceleration in the increase in concentration of CO2 and it could even start to rise at a rate greater than the amount we ourselves are directly adding to the atmosphere 25 George Roman says 1 Feb 2005 at 8 40 PM Another thought along these lines Suppose that scientists are completely wrong and that no future climate change will result from increased CO2 in the atmosphere of course we must ignore basic laws of atmospheric physics for this but let s for the sake of argument assume that this were true Then surely there would be no reason for concern over greenhouse gas emissions This might be true when only considering climate change impacts But CO2 is accumulating in the OCEAN also causing changes to ocean pH and biogeochemical cycles and thereby posing risks to marine ecosystems coral reefs and fisheries resources In recent years many papers in Nature have been published on this topic e g Kleypas 1999 Riebesell et al 2000 Caldeira and Wickett 2004 also see this paper http www stabilisation2005 com 55 Jerry Blackford pdf Decreased calcification of corals due to higher aqueous CO2 is already occurring and coral reefs are important for fisheries tourism and recreation coastal protection and pharmaceuticals It seems to me that this alone is enough reason for concern that greenhouse gas emissions pose risks to society and nature These risks are completely independent of any of the risks related to climate change One could even imagine a hypothetical situation where the scientific consensus stated that human induced climate change is of no concern but that the long term risks of CO2 accumulation in the ocean are reason enough for multi lateral international agreements to reduce CO2 emissions 26 dave says 1 Feb 2005 at 9 14 PM Re 17 19 20 and what I said in 9 Also it thought that the easy carbon sinks are becoming saturated and will be less and less effective over time This means that a greater percentage of future emissions will stay in the atmosphere There is an interesting discussion of this by Chris Jones et al of the Hadley Centre Impact of Climate Carbon Cycle Feedbacks on Emission Scenarios The paper is part of the current Avoiding Dangerous Climate Change conference happening now in England The discussion talks explicitly about how diminishing terrestrial and ocean carbon sinks over time require reduced CO2 emissions from fossil fuels land use to achieve stabilization goals at various levels e g 550 ppmv of CO2 in the atmosphere 27 Michael Gersh says 5 Feb 2005 at 12 57 PM The one thing you true believers in this debate refuse to acknowledge is that if we the rest of the human race are to take action what is your prescription for this action If we accept that human CO2 on the order of parts per billion causes

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  • Moberg et al: Highly variable Northern Hemisphere temperatures? « RealClimate
    trend that we are forcing it to have I think the money is on 2011 2020 being warmer than 1990 2000 But if it isn t it won t make me think we missed something or have some fundamental lack of knowledge Of course various pundits will claim the scientists had it wrong but I guess we will cross that bridge if and when we come to it Accepting that climate is the statistics of weather over some reasonable time period I want to ask about something that climate modelling does not seem to address thresholds Since there has been noticeably increased warming over the last 20 years or so and less so before that is it possible that the forcing from GHGs in the atmosphere surpassed some threshold around then and that there is some feedback outside the normal physics affecting water vapor that has kicked in to create the observed warming See this data for example GHG levels were also quite high in 1984 344 5 ppm CO2 as measured at Mauna Loa but the warming has risen dramatically since then We are at about 379 ppm CO2 now awaiting the official measurement due soon Naively it looks like something happened Response I very much doubt it See my response to 11 below eric 11 Peter J Wetzel says 16 Feb 2005 at 10 47 PM Dave 10 Let s suppose just suppose that we have just recently encountered the onset of a Dansgaard Oeschger event These events produce rapid climate warming which is followed by a gradual steady cooling over many years centuries in fact Superimposed on this event might be a true anthropogenic signal However the D O event itself is a phenomenon that is clearly within the bounds of natural variability Although there is little solid confirmation of the cause of these sudden D O warmings one of the most plausible explanations is based on a parallel phenomenon which well known in the atmosphere The oceanic boundary layer is driven by the identical thermodynamic processes that drive the atmospheric boundary layer In the ocean there are dual buoyancy mechanisms salinity and temperature which drive boundary layer instability In the atmosphere there is fundamentally only one dominant driving mechanism temperature although modulated by water vapor and the latent heat released by condensation into clouds The equivalent to a D O event in the atmospheric boundary layer occurs when large amounts of potential instability accumulate and the instability is then released in the form of violent thunderstorms The phenomenon which delays the release of instability while it continues to accumulate is known as a cap a stable layer which contains the instability until a threshold is reached at just one most unstable location Then the vast reservoir of instability is released explosively This is a process with large hysteresis Instability accumulates far beyond an equilbrium state before its release is triggered The system then restores itself to a relaxed stable state as the instability is released And once this stable state is established a new reservoir of pent up instability begins to accumulate In the atmosphere these processes operate on time scales of hours to days The oceanic boundary layer the thermocline with its much more vast reservoirs of heat operates on much longer time scales I contend that a likely trigger of the D O events is the same sort of local break of an instability cap which initiates an intense episode of Deep Water formation either in the North Atlantic or in the Antarctic seas the latter is more likely the case in today s climate Over decades to centuries this instability is released cold water which previously lingered at the surface is sequestered in the deep ocean and the circulation gradually subsides just as the explosive development of initial thunderstorms following the first breach of the atmospheric cap is followed by a succession of weaker and weaker storms As I said let us suppose that such a D O event was triggered some time in the last two decades If so then the prognosis as defined by the course of natural variability would be for a short period of rapid warming followed by a gradual steady cooling Since D O events are a fairly common feature of past climates there certainly is precedent Since our observational record is far to short to have documented any D O event previously we could be blindsided by the onset of such a phenomenon And since we happen to be in an unprecedented period of increasing CO2 it would be exceedingly difficult to separate such an uncommon natural event a D O warming from the anthropogenic signal What we think we know as climate scientists pales miserably in comparison to all the phenomena we have never observed with modern technology Response I would agree with much of this comment though I m far less sanguine about our understanding of D O events and I m far less convinced of their relevance to modern climate It seems increasingly clear that D O events must involve major sea ice changes and there is not much sea ice left by comparison with what was present during the glacial period 20000 years ago when these events happened so D O events are increasingly unlikely in the future I would also add that the prediction made by 11 about what a D O event would look like is based on the Greenland ice core records and the picture of abrupt warming slow cooling picture comes from the data on millennial timescales It does not begin to describe what did actually happen globally during these events and is even less relevant to what might happen in the future In any case the key point is that there is no evidence to suggest we have crossed any sort of threshold Comment 10 implies there are aspects of the data which suggest this That is an eyeball approach to the data which is not backed up by any sort of analysis So to 10 I would say again that the answer is firmly no there is no evidence for any sort of threshold being crossed eric 12 John Finn says 17 Feb 2005 at 7 44 AM I think the money is on 2011 2020 being warmer than 1990 2000 But if it isn t it won t make me think we missed something or have some fundamental lack of knowledge Of course various pundits will claim the scientists had it wrong but I guess we will cross that bridge if and when we come to it Hang on a minute This site has consistently maintained that past climate variability had occurrred within a fairly narrow range It s argued against the MWP and LIA or their extent It supports the hockey stick representation of shallow fluctuations of climate over the past 1000 years and points to the sudden unprecedented increase in 20th century temperatures as evidence that increased CO2 levels are the main contributory factor Now apparently if the next 20 years fail to show the expected level of increase then it s all down to natural variability Never mind the continuing increase in atmospheric CO2 what happened to the 0 5 degree rise due to equilibrium response to recent levels of GHGs I d like to remind you we are now 15 years into the 1990 2100 period when everything is about to unfold In 2020 we will be 30 years along the journey This will be 32 years after James Hansen told the world that global warming was happening now If the evidence in 2020 is no stronger than it is to day then you ve got at least 2 things wrong Climate sensitivity which I believe is about 1 3 of your claims and past climate variability which is probably wrong anyway Apart from that you ll be spot on Response I would agree that the money is on 2011 2020 being warmer than 1990 2000 I would somewhat disagree that if it isn t there is no problem Assuming Co2 levels increase roughly as expected then it would I would guess plucking numbers somewhat out of the air be better than 95 certain that the latter period would be warmer What I think Eric was saying was that even given that there is a small chance that natural variability could cause a downturn That is the science point of view But from a political point of view if there was a decade long downturn in temperature and all the attribution analyses showed it to be a temporary downswing with more warming expected later no one would listen William Response Yes William has properly interpreted my point Another way to look at this is that while I won t be that suprised if 2011 2020 is not warmer than 1990 2000 I will be very surprised if 2000 2030 is cooler than 1970 2000 The data show that natural variabilty on decadal timescales is non trivial but that on multidecadal timescales it becomes much more trivial Having said all that this is a little off the cuff One could quibble with the detais My point is merely this natural variability exists and looking at individual decades in not really long enough eric 13 dave says 17 Feb 2005 at 1 44 PM Re 10 11 I merely raised the possibility of a threshold being crossed wondering whether there was any serious analysis that supported it Of course I ve seen the often used IPCC TAR result here showing that modelling results combining natural and anthropogenic forcings reproduce 20th century global mean surface temperature anomalies relative to the 1880 to 1920 mean This would seem to imply that no threshold has been crossed However Richard Alley writing for a general audience in Scientific American Nov 2004 in an article entitled Abrupt Climate Change spends a lot of time talking about thresholds changes of state using his canoe tipping analogy He states that many thresholds may still await discovery And we know how Wally Broeker feels about this So I was wondering what the thinking was about thresholds with respect to the current warming trend on this blog I also think that paleoclimate scientists tend to see more possibility of abrupt climate changes than climate modellers do Also re the current warming trend Hansen and Sato expect 2005 to surpass 1998 due to a weak El Nino superimposed on the general anthropogenic forcing in this GISS press release So we know which way they re betting If this indeed comes to pass and the warming grows in subsequent years there may come a point when the signal exceeds the known forcings and feedbacks as currently modelled At which point it might be decided that transient climate sensitivity is too weak or some unknown threshold has been crossed Of course this is all just pure speculation on my part And hey why not Finally when I raise the possibility of some undetected threshold in the current warming signal that doesn t mean I believe it Response Thresholds stuff is quite difficult On the one hand they may be there awaiting discovery OTOH there is no good sign of them at the moment One example is the THC shutdown which was predicted by simpler models but tends not to occur in the more sophisticated models coupled AOGCMs OTTH although these models are capable of showing some emergent behaviour ie stuff not built into them this may be limited HadCM3 vegetation for example shows a speed up of atmos CO2 in mid C21 when the Amazon forests start to decline due to climate change But if the thresholds were related to GHG levels we would have noticed because CO2 etc are continuously monitored The bottom line is probably that whilst we should take the idea of sudden changes seriously and investigate possibilities within the scientific arena it wouldn t work to take such stuff out into the wide world without a rather better basis Analogies are nice but they aren t evidence just guides to thinking William 14 Peter J Wetzel says 17 Feb 2005 at 7 43 PM The undetected threshold scenario that I posed in 11 could be more generally characterized as a shift in the thermo haline circulation THC caused by an emergent new source region for deep water formation The most obvious D O Dansgaard Oechsger events in the vicinity of the LGM Last Glacial Maximum 20 000 years ago represent extreme examples of this scenario The interactions feedbacks between THC shifts and sea ice and glacial calving as indicated by ice rafted debris in deep sea core records would tend to seriously magnify the climate changes compared to what would be expected from a similar THC shift today But I d like to move beyond the discussion of simple thresholds There is another class of slightly more subtle climate shifts that could be driven by long time scale ocean circulation processes which fall beyond the scope of observational recognition The most tangible example of how such a shift might appear is the apparent recent emergence last half of the 20th century of a much more intense oscillation in and particularly a much more intense positive phase of the Arctic Oscillation AO which is closely paralleled by the North Atlantic Oscillation NAO The class of hypothetical climate shifts to which I allude involve fundamental changes in the frequency and amplitude of known oscillatory behavio u r of the atmosphere ocean cryosphere system and the potential emergence of new oscillatory behaviors We should not limit ourselves to being vigilant for the crossing of thresholds The undiscovered effects that may emerge as we proceed with our uncontrolled experiment in anthropogenic forcing run the gamut from modulation of the frequency and duration of relatively short term well characterized cycles such as ENSO El Nino Southern Oscillation AO and the Pacific Decadal Oscillation through long term oscillations which have always been a part of the natural variability of the Earth system but which operate on such long time scales that they have not been properly recognized or understood e g D O type events to the emergence of completely new modes of oscillation as climate is influenced by anthropogenic processes The statement there is no evidence of such emergent changes about which I ve speculated only means that vigilant climate scientists have not yet identified the possible evidence perhaps because it does not exist and my speculation is wrong or because it has either been overlooked or is still in the early stages of emergence 15 dave says 17 Feb 2005 at 7 47 PM I hope you re receptive to a follow up on this interesting subject I couldn t agree more with your remarks I wasn t trying to be alarmist Alley s article really does emphasize non linear state changes I was puzzled by the related to GHG levels we would have noticed statement I take threshold to mean that GHG forcing could alter some natural variability like the Arctic Oscillation which spends more and more time in its positive mode increasing heat transport from mid latitudes or ENSO more frequent or stronger El Nino events There tends to be an emphasis here at realclimate org on the hockey team results Mike prefers this term for example this posted subject Moberg et al And while the hockey team shows the late 20th century and current warming to be unusual it is not as important in my view as the current warming trend itself Why Well because that s just how people work Humans work on human timescales which are short I know that s not the scientific view but that s reality So for example here s some bad luck from the GISS press release cited in 13 and given the U S refusal to participate in Kyoto or otherwise make meaningful GHG reductions Compared to the average temperatures from the 1951 to 1980 period the largest unusually warm areas over all of 2004 were in Alaska near the Caspian Sea and over the Antarctic Peninsula But compared to the previous five years the United States as a whole was quite cool particularly during the summer emphasis added So when Eric says Still this doesn t mean the period 2001 2010 or 2011 2020 will also be as warm We ll see citing some possible natural variablity which masks a longer term climate warming trend I want to say we re doomed doomed This point of view guides my posts I m being as honest as I can be Response I assume that by your we re doomed statement you are saying you worry that in the event it is not warmer in 2001 2010 than 1990 2000 that this will damage our ability to get the word out I think that is true but it doesn t change the fact that climate is highly variable and it might just work out that way The natural world doesn t always cooperate in a way that makes things easy to explain Now on the thresholds business you should be aware that I m probably more conservative on this than most in the paleoclimate research community especially those like Alley and me that have worked mostly on ice cores looking at longer timescales My take on this is that the case that big non linear responses can happen is strong but that the case that they will happen in the near or medium term is weak I would echo William s comment that this is an interesting and important area of scientific study but I do not think it is well enough understood that it makes sense to spend a lot of energy warning people about it The reality of global warming is to paraphrase IPCC very well understood and very very likely and that is a statement few would disagree with On the othe hand

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  • “New Analysis Reproduces Graph of Late 20th Century Temperature Rise” « RealClimate
    McKitrick 19 Foxcheck says 12 May 2005 at 10 03 PM Hocky Stick graph vindicated The hockystick graph that has been widely identified as the symbol of Global Warming has been reproduced by NCAR or National Center for Atmospheric Research using their own homebrew code This being a U S agency and all its hard for anyone to cry 20 Adam Algar says 13 May 2005 at 8 15 AM This is extremely interesting but shouldn t we wait until the paper makes it past peer review before lauding its praises 21 Hank Roberts says 13 May 2005 at 1 48 PM The New Yorker articles mentioned can be read by backtracking from here http www newyorker com online content 050425on onlineonly01 22 Terry says 14 May 2005 at 10 25 PM I m new to all this so maybe you can answer some newbie questions 1 Why does the hockey stick use the surface temperature record for its most recent data and not the satelite data Response Because the overlap between the proxy records many of which only go up to 1980 and the satellite record which starts in 1979 is too short Plus the satellite record is not measuring surface air temperature which is the key variable that people are trying to reconstruct gavin 2 Why doesn t the hockey stick use tree ring data to estimate the most recent temperature trend What would it look like if it did Response Over the recent period all the proxies including tree rings are calibrated to the recent data and so it looks the same 23 Terry says 15 May 2005 at 10 55 AM Another newbie question I was just noticing the scale on the vertical axis of the hockey stick It shows variation of about 0 4 degrees over a 1 000 year period for the smoothed series How does this compare to long term temperature variation since the ice age How much have temperatures changed since the last ice age Over how long a period did it change Is the temperature over the last 1 000 years unusually stable Thanks Response The global temperature change at the peak of the last ice age about 20 000 years ago were about 5 to 7 deg C colder than the present Most of that seems to have warmed up by around 11 000 years ago at which point temperatures in the mid to high latitude Northern Hemisphere were actually warmer because of the orbital configuration However truly global estimates of the mean temperatures through the Holocene is extremely difficult and has only really been attempted for last couple of millennia gavin 24 Jeff Norman says 15 May 2005 at 11 20 AM I am a bit confused by Gavin s responses to 22 I thought all of the prehistorical temperature reconstructions were plotted with temperature anomalies on the y axis Since all of the proxy sources are located at different latitudes and elevations and therefore have different ambient temperature conditions this is the only way they can be compiled If the temperature anomalies for the individual proxy sites can be compiled this way why can t they be compared to the temperature anomaly data from any of the MSU data sets Given that the absorption of IR energy in the troposphere and subsequent surface warming resulting from reduced heat transfer from the surface is supposed to be what s driving surface global warming I would have thought that seeing this clearly presented would be important I was also under the impression that the IPCC had concluded that the global warming recorded in the first half of the 20th century was mostly natural and the warming observed in the second half of the 20th century starting in 1975 was mostly anthropogenic If this is true then this means the proxy record should only be able to show the anthropogenic warming between 1975 and 1980 When I look at the Wikipedia Image 1000 Year Temperature Comparison it looks like all the proxy reconstructions show more warming in the first half of the 20th century than the second half of the 20th century Shouldn t it be a priority to update the major proxy records to the end 20th century to clearly demonstrate the anthropogenic impact Jeff Response You are correct It is the anomaly temperature that is calibrated against However it does not matter which satellite record you are looking at none of them are the surface air temperature lower tropopshere is not the same thing and you still have an overlap problem You can even argue that the 100 years of instrumental record are still not sufficient to calibrate the proxies for multidecadal and longer variability the relatively few years available from the satellites which of course have calibration issues themselves is not going to help Updating the proxy networks to the present day would of course be helpful as would adding to the number of long multi centennial records gavin 25 per says 15 May 2005 at 7 45 PM Hi I am a little bit confused by all the triumphalism and I can t see what the fuss is about The results that Wahl Ammann get are just about identical to those reported by M M indeed their major conclusion that omission of the Bristlecone pines leads to the 15th century temperature statistics lacking significance has also been noted by M M Given that their results are so very similar to M M why all the hoo haa yours per Response Not sure what you are referring to that we have said but the reason why this is noteworthy is because it underlines our comments made earlier that the PC normalisation issue that was the basis for the M M 2005 paper is irrelevant and that the reconstructions only diverge and get worse if you start removing data Thus the argument can move on from the methodology of MBH98 to more important issues gavin 26 Terry says 16 May 2005 at 9 21 AM Re 23 Wow There was a lot of variation in temperatures over the past 20 000 years A change of 5 to 6 degrees over 9 000 years is about 0 6 degrees per thousand years You imply that there have been significant temperature swings since then also By contrast the hockey stick shows virtually no change over 1 000 years in the mean temperature and only a small variation in temperature over the past 1 000 years 0 2 degrees variation from the mean ignoring the recent uptick in temperatures Does this recent stability sound at all suspicious to you A priori wouldn t you expect to see temperature swings more along the lines of Espers Wahl Ammann and M M Soon and Baliunus etc To put it differently which is more likely that temperatures were unusually stable over the past 1 000 years or that the hockey stick is generated by a weak model unable to detect significant temperature swings from the tree ring record To strenghten your case you should address this frontally After all it is the heart of the debate i e is a recent rise in temperatures of about 0 5 degrees so unusual that it must be anthropocentric If different models give different results which should we believe It would help tremendously if you could show or at least explain why we should choose the model that produces highly stable temperatures over the ones that show significant fluctuations Moreover it would help if you could show why we should expect this temperature stability to continue into the future As I mentioned in another message the most powerful argument you could make would be to calibrate your model from data that doesn t include the recent warming and show that it can predict the recent warming That should be easy to execute That would show that your model has the power to predict large temperature swings Since your model shows recent temperaturs to be far above anything seen in the last 1 000 years the recent tree ring record must be completely off the charts Every year we should be seeing observations never before seen in the 1 000 year tree ring history You should be able to see this in the data with the naked eye A couple of quick analyses should solve this question once and for all Do it Let s prove that humans are causing this warming Response There seem to be a lot of misunderstandings in the above The last 20 kyr includes the end of the last ice age which was indeed a large change but as you note rather slow It also includes large swings D O type stuff within the last ice age Those don t appear in any record from since the end of the ice age ie the holocene You say A priori wouldn t you expect to see temperature swings to which the answer is No why should you expect this a priori You have to go by what the records show Which is that the holocene has indeed been an unusually stable period You seem to be a bit confused about the reconstructions W A largely reproduce MBH S B don t have a reconstuction neither technically do M M The swings in the Esper record are larger than the MBH version but still show now to be unusually warm and the recent warming to be unusually fast And this is only one small component of D A William 27 SteveF says 16 May 2005 at 1 07 PM Ice core records suggest the idea of Holocene stability 8 2ka even aside but this notion is somewhat mythical There are plenty of records particularly from low latitudes that demonstrate rapid and reasonably large scale big enough to cause significant ecosystem responses climate changes during the Holocene However its a fallacy to suggest that just because temperatures have changed in the past we can t be responsible for temperature changes now Furthermore the fact that climate changes in the Americas and the Middle East during the Holocene appear to have aided the decline of civilisations is an important cautionary tale 28 Jeff Norman says 16 May 2005 at 1 59 PM Re 27 While it is true the decline of cultures civilizations is associated with climate change these declines are generally associated with periods of cooling I am not aware of a culture civilization that declined as climate warmed There are numerous examples of cultures civilizations thriving during warmer periods I am not aware of any civilizations that did not occur during the Holocene 29 Dan Allan says 16 May 2005 at 2 43 PM Regarding post 28 I am not aware of a culture civilization that declined as climate warmed First there is very little in the way of archaeological data about cvilzations during the transition from the last glaciation to the Holocene There is therefore no record of any civilation disappearing for any clearly known reason So the statement proves nothing Since sea surfaces rose by roughly 400 feet since the peak of the last ice age due to melting of glaciers it is quite possible that a great many civilizations did decline or perish due to warming and in fact perished so thoroughly that there is no trace of them It is also possible that the sea level rise was gradual enough that the impact was mitigated on low population hunter gatherer societies that simply moved to higher ground Unfortunately New York London and Tokyo are not low population hunter gatherer communities that can be easily uprooted I m not an economist but I would have to guess that cost of moving all of the world s modern cities that are close to sea level to higher ground would be rather significant 30 Jeff Norman says 16 May 2005 at 4 10 PM Re 29 I remain unaware of any culture civilization that declined as climate warmed There are numerous examples of cities that have had to deal with rising or lowering sea levels To my knowledge none of these resulted in the decline of a culture civilization Current examples include Venice and New Orleans 31 Stephen Berg says 16 May 2005 at 5 06 PM Re 30 A great read on the topic is The Long Summer by Brian Fagan He tells the story of how civilisation is impacted by climate Oh and if climate change continues unchecked sea levels will rise enough to devour Venice and New Orleans not to mention Rio de Janeiro Shanghai and many hundreds of other cities that are located on coastlines at low elevations Millions of people will have to relocate leaving behind their livelihoods and dreams which will cause the greatest refugee crisis the world has ever seen I hate to sound the doomsday alarm but the Pentagon also mentioned this in their report http www ems org climate exec pentagon climatechange pdf The executive summary http www ems org climate pentagon climatechange pdf The full report 32 Steve Bloom says 16 May 2005 at 6 15 PM Re 28 Try the Anasazi Strictly speaking it was the lack of rainfall that did them in but this was during the medieval warm period Even subtle changes in climate hotter or colder seem able to change rainfall patterns in ways that are very bad for human societies in particular those in already marginal climates Recently I was talking with a water manager for a local district here in the SF Bay Area and he told that sediment records in Lake Tenaya roughly in the middle of the western slopes of the Sierra Nevada mountains showed a multi year period in the 1200s with no sedimentation whatsoever meaning essentially no precipitation This was part of the same drought that got the Anasazi A repeat of this entirely natural event would be a large scale catastrophe for the western U S to say the least Two years of such a complete drought would largely eliminate all natural water supplies for a population in the 50M range Now let s see what can we humans do to bring on another such event as soon as possible Speaking of California the geologic record proves that we get great earthquakes once every 200 years or so on the major fault sections and are close to overdue at this point The fact that public policy in CA continues to be directed toward packing as many humans as possible onto the major fault zones speaks for itself and does not augur well with regard to our ability as a society to deal coherently with climate change 33 Peter Hearnden says 17 May 2005 at 4 48 AM Re 30 Ahhh so you admit it s warming then Jeff you just don t see it as a problem That s progress of a sort Re The Holocene surely it s a time where global temperatures show no sign until now of excluding the rise out of the ice age before that s the cry temperatures rising Indeed the opposite seems to be the case if my memory of the appropriate wiki page s serves me correctly So it s unlikely any civilisations have faced problems associated with rising temperatures until now 34 SteveF says 17 May 2005 at 9 05 AM Actually civilization collapse in the Holocene is often related to changes in precipitation to more arid conditions rather than temperature Hodell D A Curtis J H and Brenner M 1995 Possible role of climate in the collapse of classic Maya civilization Nature 375 391 394 Cullen H M deMenocal P B Hemming S Hemming G Brown F H Guilderson T and Sirocko F 2000 Climate change and the collapse of the Akkadian empire Evidence from the deep sea Geology 28 379 382 Huang C C Zhao S C Pang J L Zhou Q Y Chen S Li P H Mao L J and Ding M 2003 Climatic aridity and the relocations of the Zhou culture in the southern Loess Plateau of China Cliamtic Change 61 361 378 Anyway the point I was making seems to have been missed in post 28 namely and simply that significant climate changes can have adverse effects on human populations Furthermore given that global warming doesn t mean that the whole world is simply going to get warmer rather climate changes will be complex pointing out that civilizations in the past may not have been affected by warming is something of a red herring 35 Joseph O Sullivan says 17 May 2005 at 10 39 AM Re 9 Steve Bloom the environmental group blog I refered to is the NRDC s actionblog Some of the comments from commentors but not from the NRDC that support the skeptic s claims about the hockey stick and Dr Mann are here http blog nrdcactionfund org archives 2005 05 judges are not html more and http blog nrdcactionfund org archives 2005 05 greenpeaceful c html more A book that discusses climate change effects on human history is Climate History and the Modern World by Hubert Lamb It s not too technical and it s a good summary of this topic 36 Jeff Norman says 17 May 2005 at 9 57 PM Re 32 Steve So there was a medieval warm period and it did have climate impacts outside of Europe and the North Atlantic I thought so Re 33 Peter Global temperatures increased during the 20th century I have never doubted this Perhaps you are confusing me with someone else If as Steve says there was a global medieval warm period there must have been a time before that when it was less warm there was Therefore there must have been a period just prior to the medieval warm period when global temperatures increased I hope this helps with your progress 37 Michael Jankowski says 18 May 2005 at 10 14 AM Re

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