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1. The theory of Anthropogenic Global Warming (AGW) holds that the warming trend since the end of the Little Ice Age (LIA) is unnatural and therefore human caused by way of fossil fuel emissions. The fossil fuel argument becomes relevant because the end of the LIA coincides with the Industrial Revolution such that the whole of the current warming event has occurred “in the industrial economy” making it useful to describe the current warming as “since pre-industrial times”.
2. Skeptics have argued that natural variability must be considered before the industrial economy is taken to be the cause of the recovery from the LIA because the timing of the glacial melt event in Europe (in particular the Colle Gnifetti) that is generally accepted as the marker between the LIA and the current warming period is controversial as described in related a post [LINK]. This controversy makes it possible for skeptics to point to the Medieval Warm Period (MWP) as an example of a warming event similar to the current warming that was not the creation of an industrial economy and therefore must have been a natural outcome of the known non-linear dynamical behavior of climate.
3. It is generally accepted and the paleo data show that there was a strong sustained warming period in Europe sometime during an uncertain period within the time span 800AD to 1400AD usually referred to as the MWP. There is a great deal of controversy, however, about the timing, the timescale, the geographical extent, and, most important of all, the degree of warming.
4. A bibliography of this topic consisting of eighteen papers written between the period 1994 to 2009 is presented below. The authors include prominent paleo climatologists such as Briffa, Mann, Cronin, Hughes, and Esper. The bibliography shows a general agreement of large uncertainties in the data such that the selection of the type of proxy data (eg tree ring, sediment, borehole, or climate model) and the geographical location where data were gathered strongly influences findings. It is uncertain whether it was global or localized in Europe and if so whether it was all of Europe or just Northern Europe. It is also uncertain as to exactly when the MWP occurred and for how long it lasted. Most of all it is uncertain as to exactly how warm it got specifically with respect to the current 20th century warming of “the industrial economy since pre-industrial times”.
5. Uncertainty of course creates controversy and given the large uncertainties involved in these studies and the large stake for the climate science argument for human cause that the current warming is “unprecedented in the last two millennia” , the MWP issue has generated a great deal of acrimonious debate. Here we argue that the controversy is partisan and sustained by the so called “Texas Sharpshooter” fallacy because uncertainty allows different researchers to pay more attention to the portion of the uncertainty band that supports their hypothesis.
6. In a related post [LINK] Professor Carl Wunsch explains that bias opportunity in this way: “From one point of view, scientific communities without adequate data have a distinct advantage because they can construct interesting and exciting stories and rationalizations with little or no risk of observational refutation. Colorful, sometimes charismatic, characters come to dominate the field, constructing their interpretations of a few intriguing, but indefinite observations that appeal to their followers, and which eventually emerge as “textbook truths.” 
7. The following characteristics are ascribed to one particularly notoriously data-poor field. (1) Tremendous self-confidence and a sense of entitlement and of belonging to an elite community of experts, (2) An unusually monolithic community, with a strong sense of consensus, whether driven by the evidence or not, and an unusual uniformity of views on open questions. These views seem related to the existence of a hierarchical structure in which the ideas of a few leaders dictate the viewpoint, strategy, and direction of the field. (3) In some cases a sense of identification with the group, akin to identification with a religious faith or political platform. (4) A strong sense of the boundary between that group of experts and the rest of the world. (5) A disregard for and disinterest in the ideas, opinions, and work of experts who are not part of the group, and a preference for talking only with other members of the community. (6) A tendency to interpret evidence optimistically, to believe exaggerated or incorrect statements of results and to disregard the possibility that the theory might be wrong. This is coupled with a tendency to believe results are true because they are widely believed, even if one has not checked (or even seen) the proof oneself”.
8. In another related post on this site we show that the commonly held belief in “the unprecedented warming of the Arctic” is not supported by the data [LINK] and in the bibliography below it becomes clear that the the uncertainty in the data is such that they can be interpreted both ways – yes the MWP was warmer than it is in the industrial economy and no it was not warmer than the industrial economy with some authors having the courage to acknowledge that large uncertainties mean that we don’t know and not that it provides the opportunity to interpret the data in a way that proves our hypothesis. And that implies that the paleo data do not show conclusively that the MWP was not as warm and that therefore the current warming is unprecedented and that therefore it must be human caused by way of fossil fuel emissions.
9. Yet another issue is that the “unprecedented warming” is constrained to a comparison with “the last two thousand years”. What is the relevance of the number 2000? A more unbiased comparison would be with the beginning of the Holocene or with the previous interglacial the Eemian. In both of these cases we find much warmer temperatures without an industrial economy as shown in related posts [LINK] [LINK] [LINK] .
10. CONCLUSION: The unbiased interpretation of the MWP is that given the large uncertainty in the proxy data it is not possible to determine with sufficient certainty that the MWP was not warmer than today and that therefore the use of the MWP to support human cause in the warming since the end of the LIA is not possible because the data do not show conclusively that the MWP was not warmer than today.

[LIST OF POSTS ON THIS SITE]

BIBLIOGRAPHY

1. Hughes 1994:  Hughes, Malcolm K., and Henry F. Diaz. “Was there a ‘Medieval Warm Period’, and if so, where and when?.” Climatic change 26.2-3 (1994): 109-142.  It has frequently been suggested that the period encompassing the ninth to the fourteenth centuries A.D. experienced a climate warmer than that prevailing around the turn of the twentieth century. This epoch has become known as the Medieval Warm Period, since it coincides with the Middle Ages in Europe. In this review a number of lines of evidence are considered, (including climatesensitive tree rings, documentary sources, and montane glaciers) in order to evaluate whether it is reasonable to conclude that climate in medieval times was, indeed, warmer than the climate of more recent times. Our review indicates that for some areas of the globe (for example, Scandinavia, China, the Sierra Nevada in California, the Canadian Rockies and Tasmania), temperatures, particularly in summer, appear to have been higher during some parts of this period than those that were to prevail until the most recent decades of the twentieth century. These warmer regional episodes were not strongly synchronous. Evidence from other regions (for example, the Southeast United States, southern Europe along the Mediterranean, and parts of South America) indicates that the climate during that time was little different to that of later times, or that warming, if it occurred, was recorded at a later time than has been assumed. Taken together, the available evidence does not support a globalMedieval Warm Period, although more support for such a phenomenon could be drawn from high-elevation records than from low-elevation records. The available data exhibit significant decadal to century scale variability throughout the last millennium. A comparison of 30-year averages for various climate indices places recent decades in a longer term perspective.
2. Dean 1994: Dean, Jeffrey S. “The medieval warm period on the southern Colorado Plateau.” The Medieval Warm Period. Springer, Dordrecht, 1994. 225-241. Several questions concerning the Medieval Warm Period (MWP), an interval (A.D. 900 to 1300) of elevated temperatures first identified in northern Europe, are addressed with paleoenvironmental and archaeological data from the southern Colorado Plateau in the southwestern United States. Low and high frequency variations in alluvial groundwater levels, floodplain aggradation and degradation, effective moisture, dendroclimate, and human adaptive behavior fail to exhibit consistent patterns that can be attributed to either global or regional expressions of the MWP. There is some suggestion, however, that climatic factors related to the MWP may have modified the regional patterns to produce minor anomalies in variables such as the number of intense droughts, the occurrence of specific droughts in the twelfth and thirteenth centuries, the prevalence of low temporal variability in dendroclimate, and the coherence of some low and high frequency environmental variables and aspects of human adaptive behavior. These results suggest that the MWP does not represent warming throughout the world. Rather, it was a complex phenomenon that probably was expressed differently in different regions.
3. Stahle 1994: Stahle, David W., and Malcolm K. Cleaveland. “Tree-ring reconstructed rainfall over the southeastern USA during the Medieval Warm Period and Little Ice Age.” The Medieval Warm Period. Springer, Dordrecht, 1994. 199-212. A 1053-year reconstruction of spring rainfall (March-June) was developed for the southeastern United States, based on three tree-ring reconstructions of statewide rainfall from North Carolina, South Carolina, and Georgia. This regional reconstruction is highly correlated with the instrumental record of spring rainfall (r = +0.80; 1887-1982), and accurately reproduces the decade-scale departures in spring rainfall amount and variance witnessed over the Southeast during the past century. No large-magnitude centuries-long trends in spring rainfall amounts were reconstructed over the past 1053 years, but large changes in the interannual variability of spring rainfall were reconstructed during portions of the Medieval Warm Period (MWP), Little Ice Age (LIA), and the 20th century. Dry conditions persisted at the end of the 12th century, but appear to have been exceeded by a reconstructed drought in the mid-18th century. High interannual variability, including five extremely wet years were reconstructed for a 20-yr period during the late 16th and early 17th centuries, and may reflect amplified atmospheric circulation over eastern North America during what appears to have been one of the most widespread cold episodes of the Little Ice Age.
4. Grove 1994: Grove, Jean M., and Roy Switsur. “Glacial geological evidence for the Medieval Warm Period.” The Medieval Warm Period. Springer, Dordrecht, 1994. 143-169. It is hypothesised that the Medieval Warm Period was preceded and followed by periods of moraine deposition associated with glacier expansion. Improvements in the methodology of radiocarbon calibration make it possible to convert radiocarbon ages to calendar dates with greater precision than was previously possible. Dating of organic material closely associated with moraines in many montane regions has reached the point where it is possible to survey available information concerning the timing of the medieval warm period. The results suggest that it was a global event occurring between about 900 and 1250 A.D., possibly interrupted by a minor readvance of ice between about 1050 and 1150 A.D.
5. Jones-Briffa 1998: Jones, P. D., et al. “High-resolution palaeoclimatic records for the last millennium: interpretation, integration and comparison with General Circulation Model control-run temperatures.” The Holocene 8.4 (1998): 455-471. Palaeoclimatology provides our only means of assessing climatic variations before the beginning of instrumental records. The various proxy variables used, however, have a number of limitations which must be adequately addressed and understood. Besides their obvious spatial and seasonal limitations, different proxies are also potentially limited in their ability to represent climatic variations over a range of different timescales. Simple correlations with instrumental data over the period since 1881 give some guide to which are the better proxies, indicating that coral- and ice-core-based reconstructions are poorer than tree-ring and historical ones. However, the quality of many proxy time series can be lower for earlier times. Suggestions are made for assessing proxy quality over longer periods than the last century by intercomparing neighbouring proxies and, by comparisons with less temporally resolved proxies such as borehole temperatures. We have averaged 17 temperature reconstructions (representing various seasons of the year), all extending back at least to the mid-seventeenth century, to form two annually resolved hemispheric series (NH10 and SH7). Over the 1901–91 period, NH10 has 36% variance in common with average NH summer (June to August) temperatures and 70% on decadal timescales. SH7 has 16% variance in common with average SH summer (December to February) temperatures and 49% on decadal timescales, markedly poorer than the reconstructed NH series. The coldest year of the millennium over the NH is ad 1601, the coldest decade 1691–1700 and the seventeenth is the coldest century. A Principal Components Analysis (PCA) is performed on yearly values for the 17 reconstructions over the period ad 1660–1970. The correlation between PC1 and NH10 is 0.92, even though PC1 explains only 13.6% of the total variance of all 17 series. Similar PCA is performed on thousand-year-long General Circulation Model (GCM) data from the Geophysical Fluid Dynamics Laboratory (GFDL) and the Hadley Centre (HADCM2), sampling these for the same locations and seasons as the proxy data. For GFDL, the correlation between its PC1 and its NH10 is 0.89, while for HADCM2 the PCs group markedly differently. Cross-spectral analyses are performed on the proxy data and the GFDL model data at two different frequency bands (0.02 and 0.03 cycles per year). Both analyses suggest that there is no large-scale coherency in the series on these timescales. This implies that if the proxy data are meaningful, it should be relatively straightforward to detect a coherent near-global anthropogenic signal in surface temperature data.
6. Mann-Bradley 1999: Mann, Michael E., Raymond S. Bradley, and Malcolm K. Hughes. “Northern hemisphere temperatures during the past millennium: inferences, uncertainties, and limitations.” Geophysical research letters 26.6 (1999): 759-762. Building on recent studies, we attempt hemispheric temperature reconstructions with proxy data networks for the past millennium. We focus not just on the reconstructions, but the uncertainties therein, and important caveats. Though expanded uncertainties prevent decisive conclusions for the period prior to AD 1400, our results suggest that the latter 20th century is anomalous in the context of at least the past millennium. The 1990s was the warmest decade, and 1998 the warmest year, at moderately high levels of confidence. The 20th century warming counters a millennial‐scale cooling trend which is consistent with long‐term astronomical forcing.
7. Briffa 2000: Briffa, Keith R. “Annual climate variability in the Holocene: interpreting the message of ancient trees.” Quaternary Science Reviews 19.1-5 (2000): 87-105. Over vast areas of the world’s landmasses, where climate beats out a strong seasonal rhythm, tree growth keeps unerring time. In their rings, trees record many climate melodies, played in different places and different eras. Recent years have seen a consolidation and expansion of tree-ring sample collections across the traditional research areas of North America and Europe, and the start of major developments in many new areas of Eurasia, South America and Australasia. From such collections are produced networks of precisely dated chronologies; records of various aspects of tree growth, registered continuously, year by year across many centuries. Their sensitivities to different climate parameters are now translated into ever more detailed histories of temperature and moisture variability across expanding dimensions of time and space. With their extensive coverage, high temporal resolution and rigid dating control, dendroclimatic reconstructions contribute significantly to our knowledge of late Holocene climates, most importantly on timescales ranging from 1 to 100 years. In special areas of the world, where trees live for thousands of years or where subfossil remnants of long dead specimens are preserved, work building chronologies covering many millennia continues apace. Very recently, trees have provided important new information about major modes of general circulation dynamics linked to the El Niño/Southern Oscillation and the North Atlantic Oscillation, and about the effect of large volcanic eruptions. As for assessing the significance of 20th century global warming, the evidence from dendroclimatology in general, supports the notion that the last 100 years have been unusually warm, at least within a context of the last two millennia. However, this evidence should not be considered equivocal. The activities of humans may well be impacting on the `natural’ growth of trees in different ways, making the task of isolating a clear climate message subtly difficult.
8. Crowley 2000: Crowley, Thomas J., and Thomas S. Lowery. “How warm was the medieval warm period?.” AMBIO: A Journal of the Human Environment 29.1 (2000): 51-55. A frequent conclusion based on study of individual records from the so-called Medieval Warm Period (∼1000-1300 A.D.) is that the present warmth of the 20th century is not unusual and therefore cannot be taken as an indication of forced climate change from greenhouse gas emissions. This conclusion is not supported by published composites of Northern Hemisphere climate change, but the conclusions of such syntheses are often either ignored or challenged. In this paper, we revisit the controversy by incorporating additional time series not used in earlier hemispheric compilations. Another difference is that the present reconstruction uses records that are only 900–1000 years long, thereby, avoiding the potential problem of uncertainties introduced by using different numbers of records at different times. Despite clear evidence for Medieval warmth greater than present in some individual records, the new hemispheric composite supports the principal conclusion of earlier hemispheric reconstructions and, furthermore, indicates that maximum Medieval warmth was restricted to two to three 20–30 year intervals, with composite values during these times being only comparable to the mid-20th century warm time interval. Failure to substantiate hemispheric warmth greater than the present consistently occurs in composites because there are significant offsets in timing of warmth in different regions; ignoring these offsets can lead to serious errors concerning inferences about the magnitude of Medieval warmth and its relevance to interpretation of late 20 th century warming.
9. Briffa 2001: Briffa, Keith R., et al. “Low‐frequency temperature variations from a northern tree ring density network.” Journal of Geophysical Research: Atmospheres 106.D3 (2001): 2929-2941. We describe new reconstructions of northern extratropical summer temperatures for nine subcontinental‐scale regions and a composite series representing quasi “Northern Hemisphere” temperature change over the last 600 years. These series are based on tree ring density data that have been processed using a novel statistical technique (age band decomposition) designed to preserve greater long‐timescale variability than in previous analyses. We provide time‐dependent and timescale‐dependent uncertainty estimates for all of the reconstructions. The new regional estimates are generally cooler in almost all precalibration periods, compared to estimates obtained using earlier processing methods, particularly during the 17th century. One exception is the reconstruction for northern Siberia, where 15th century summers are now estimated to be warmer than those observed in the 20th century. In producing a new Northern Hemisphere series we demonstrate the sensitivity of the results to the methodology used once the number of regions with data, and the reliability of each regional series, begins to decrease. We compare our new hemisphere series to other published large‐regional temperature histories, most of which lie within the 1σ confidence band of our estimates over most of the last 600 years. The 20th century is clearly shown by all of the palaeoseries composites to be the warmest during this period.
10. Briffa 2002: Briffa, Keith R., and Timothy J. Osborn. “Blowing hot and cold.” Science 295.5563 (2002): 2227-2228. Tree-ring records play an important role in reconstructing climate change patterns over the last millenium. In their Perspective, Briffa and Osborn highlight the report by Esper et al. of a largely independent record of widespread tree-growth variations across the extra-tropical Northern Hemisphere. Estimates of past temperature changes based on the record suggest that climate swings in the last 1000 years were greater than has yet been generally accepted.
11. Esper 2002: Esper, Jan, Edward R. Cook, and Fritz H. Schweingruber. “Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability.” science 295.5563 (2002): 2250-2253. Preserving multicentennial climate variability in long tree-ring records is critically important for reconstructing the full range of temperature variability over the past 1000 years. This allows the putative “Medieval Warm Period” (MWP) to be described and to be compared with 20th-century warming in modeling and attribution studies. We demonstrate that carefully selected tree-ring chronologies from 14 sites in the Northern Hemisphere (NH) extratropics can preserve such coherent large-scale, multicentennial temperature trends if proper methods of analysis are used. In addition, we show that the average of these chronologies supports the large-scale occurrence of the MWP over the NH extratropics.
12. Cook 2002: Cook, Edward R., Jonathan G. Palmer, and Rosanne D. D’Arrigo. “Evidence for a ‘Medieval Warm Period’in a 1,100 year tree‐ring reconstruction of past austral summer temperatures in New Zealand.” Geophysical Research Letters29.14 (2002): 12-1. The occurrence of the Medieval Warm Period (MWP) in the Southern Hemisphere is uncertain because of the paucity of well‐dated, high‐resolution paleo‐temperature records covering the past 1,000 years. We describe a new tree‐ring reconstruction of Austral summer temperatures from the South Island of New Zealand, covering the past 1,100 years. This record is the longest yet produced for New Zealand and shows clear evidence for persistent above‐average temperatures within the interval commonly assigned to the MWP. Comparisons with selected temperature proxies from the Northern and Southern Hemispheres confirm that the MWP was highly variable in time and space. Regardless, the New Zealand temperature reconstruction supports the global occurrence of the MWP.
13. Cronin 2003: Cronin, Thomas M., et al. “Medieval warm period, little ice age and 20th century temperature variability from Chesapeake Bay.” Global and planetary change 36.1-2 (2003): 17-29. We present paleoclimate evidence for rapid (<100 years) shifts of ∼2–4 °C in Chesapeake Bay (CB) temperature ∼2100, 1600, 950, 650, 400 and 150 years before present (years BP) reconstructed from magnesium/calcium (Mg/Ca) paleothermometry. These include large temperature excursions during the Little Ice Age (∼1400–1900 AD) and the Medieval Warm Period (∼800–1300 AD) possibly related to changes in the strength of North Atlantic thermohaline circulation (THC). Evidence is presented for a long period of sustained regional and North Atlantic-wide warmth with low-amplitude temperature variability between ∼450 and 1000 AD. In addition to centennial-scale temperature shifts, the existence of numerous temperature maxima between 2200 and 250 years BP (average ∼70 years) suggests that multi-decadal processes typical of the North Atlantic Oscillation (NAO) are an inherent feature of late Holocene climate. However, late 19th and 20th century temperature extremes in Chesapeake Bay associated with NAO climate variability exceeded those of the prior 2000 years, including the interval 450–1000 AD, by 2–3 °C, suggesting anomalous recent behavior of the climate system.
14. Mann-Jones 2003: Mann, Michael E., and Philip D. Jones. “Global surface temperatures over the past two millennia.” Geophysical Research Letters 30.15 (2003). We present reconstructions of Northern and Southern Hemisphere mean surface temperature over the past two millennia based on high‐resolution ‘proxy’ temperature data which retain millennial‐scale variability. These reconstructions indicate that late 20th century warmth is unprecedented for at least roughly the past two millennia for the Northern Hemisphere. Conclusions for the Southern Hemisphere and global mean temperature are limited by the sparseness of available proxy data in the Southern Hemisphere at present.
15. Moberg 2005: Moberg, Anders, et al. “Highly variable Northern Hemisphere temperatures reconstructed from low-and high-resolution proxy data.” Nature 433.7026 (2005): 613. A number of reconstructions of millennial-scale climate variability have been carried out in order to understand patterns of natural climate variability, on decade to century timescales, and the role of anthropogenic forcing^{1,2,3,4,5,6,7,8}. These reconstructions have mainly used tree-ring data and other data sets of annual to decadal resolution. Lake and ocean sediments have a lower time resolution, but provide climate information at multicentennial timescales that may not be captured by tree-ring data^9,10. Here we reconstruct Northern Hemisphere temperatures for the past 2,000 years by combining low-resolution proxies with tree-ring data, using a wavelet transform technique¹¹ to achieve timescale-dependent processing of the data. Our reconstruction shows larger multicentennial variability than most previous multi-proxy reconstructions^1,2,3,4,7, but agrees well with temperatures reconstructed from borehole measurements¹² and with temperatures obtained with a general circulation model^13,14. According to our reconstruction, high temperatures—similar to those observed in the twentieth century before 1990—occurred around AD 1000 to 1100, and minimum temperatures that are about 0.7 K below the average of 1961–90 occurred around AD 1600. This large natural variability in the past suggests an important role of natural multicentennial variability that is likely to continue.
16. Hunt 2006: Hunt, Barrie G. “The medieval warm period, the little ice age and simulated climatic variability.” Climate Dynamics 27.7-8 (2006): 677-694. The CSIRO Mark 2 coupled global climatic model has been used to generate a 10,000-year simulation for ‘present’ climatic conditions. The model output has been analysed to identify sustained climatic fluctuations, such as those attributed to the Medieval Warm Period (MWP) and the Little Ice Age (LIA). Since no external forcing was permitted during the model run all such fluctuations are attributed to naturally occurring climatic variability associated with the nonlinear processes inherent in the climatic system. Comparison of simulated climatic time series for different geographical locations highlighted the lack of synchronicity between these series. The model was found to be able to simulate climatic extremes for selected observations for century timescales, as well as identifying the associated spatial characteristics. Other examples of time series simulated by the model for the USA and eastern Russia had similar characteristics to those attributed to the MWP and the LIA, but smaller amplitudes, and clearly defined spatial patterns. A search for the frequency of occurrence of specified surface temperature anomalies, defined via duration and mean value, revealed that these were primarily confined to polar regions and northern latitudes of Europe, Asia and North America. Over the majority of the oceans and southern hemisphere such climatic fluctuations could not be sustained, for reasons explained in the paper. Similarly, sustained sea–ice anomalies were mainly confined to the northern hemisphere. An examination of mechanisms associated with the sustained climatic fluctuations failed to identify a role for the North Atlantic Oscillation, the El Niño-Southern Oscillation or the Pacific Decadal Oscillation. It was therefore concluded that these fluctuations were generated by stochastic processes intrinsic to the nonlinear climatic system. While a number of characteristics of the MWP and the LIA could have been partially caused by natural processes within the climatic system, the inability of the model to reproduce the observed hemispheric mean temperature anomalies associated with these events indicates that external forcing must have been involved.Essentially the unforced climatic system is unable to sustain the generation of long-term climatic anomalies.
17. Sridhar 2006: Sridhar, Venkataramana, et al. “Large wind shift on the Great Plains during the Medieval Warm Period.” Science 313.5785 (2006): 345-347. Spring-summer winds from the south move moist air from the Gulf of Mexico to the Great Plains. Rainfall in the growing season sustains prairie grasses that keep large dunes in the Nebraska Sand Hills immobile. Longitudinal dunes built during the Medieval Warm Period (800 to 1000 years before the present) record the last major period of sand mobility. These dunes are oriented NW-SE and are composed of cross-strata with bipolar dip directions. The trend and structure of the dunes record a drought that was initiated and sustained by a historically unprecedented shift of spring-summer atmospheric circulation over the Plains: Moist southerly flow was replaced by dry southwesterly flow.
18. Loehle, Craig. “A 2000-year global temperature reconstruction based on non-treering proxies.” Energy & Environment 18.7 (2007): 1049-1058.  Historical data provide a baseline for judging how anomalous recent temperature changes are and for assessing the degree to which organisms are likely to be adversely affected by current or future warming. Climate histories are commonly reconstructed from a variety of sources, including ice cores, tree rings, and sediment. Tree-ring data, being the most abundant for recent centuries, tend to dominate reconstructions. There are reasons to believe that tree ring data may not properly capture long-term climate changes. In this study, eighteen 2000-year-long series were obtained that were not based on tree ring data. Data in each series were smoothed with a 30-year running mean. All data were then converted to anomalies by subtracting the mean of each series from that series. The overall mean series was then computed by simple averaging. The mean time series shows quite coherent structure. The mean series shows the Medieval Warm Period (MWP) and Little Ice Age (LIA) quite clearly, with the MWP being approximately 0.3°C warmer than 20th century values at these eighteen sites.
19. Loehle, Craig, and J. Huston McCulloch. “Correction to: A 2000-year global temperature reconstruction based on non-tree ring proxies.” Energy & Environment 19.1 (2008): 93-100.  A climatic reconstruction published in E&E (Loehle, 2007) is here corrected for various errors and data issues, with little change in the results. Standard errors and 95% confidence intervals are added. The Medieval Warming Period (MWP) was significantly warmer than the bimillennial average during most of the period 820 – 1040 AD. The Little Ice Age was significantly cooler than the average during most of 1440 – 1740 AD. The warmest tridecade of the MWP was warmer than the most recent tridecade, but not significantly so.
20. Esper 2009: Esper, Jan, and David Frank. “The IPCC on a heterogeneous Medieval warm period.” Climatic Change 94.3-4 (2009): 267-273. In their 2007 report, IPCC working group 1 refers to an increased heterogeneity of climate during medieval times about 1000 years ago. This conclusion would be of relevance, as it implies a contrast in the spatial signature and forcing of current warmth to that during the Medieval Warm Period. Our analysis of the data displayed in the IPCC report, however, shows no indication of an increased spread between long-term proxy records. We emphasize the relevance of sample replication issues, and argue that an estimation of long-term spatial homogeneity changes is premature based on the smattering of data currently available.  The Proxy Record: Proxies shown in the AR4 include an ice core record from W Greenland (Fisheret al. 1996), a multi-proxy record from E Asia (Yang et al. 2002), and six treering records representing: SW Canada (Luckman and Wilson 2005), W USA (Lloyd and Graumlich 1997), N Sweden (Grudd et al. 2002), NW Russia (Hantemirov and Shiyatov 2002), N Russia (Naurzbaev et al. 2002), and Mongolia (D’Arrigo et al. 2001). The tree-ring chronologies are all combinations of samples from living trees with relict/historical material. These temporally overlapping data have been matched using ‘cross-dating’, a technique introduced in the early 20th century into dendrochronology (Douglass 1929), so that every ring is assigned a calendar year. Application of this method allowed for the development of continuous, millenniumlong tree-ring chronologies, even though the single trees included in such a record may have only reached ages of 200 years or less. The Greenland ice core proxy integrates several δ18O timeseries that were first detrended to remove non-climatic long-term trends and then combined to a single record. The regional multi-proxy record from E Asia combines a total of nine different proxies, including tree-ring, ice core, peat bog, lake sediment, and documentary data. As some of these proxies are not annually resolved, decadal variance is substantially reduced in the combined time series. Importantly, all tree-ring records shown in AR4 were detrended using a method known as ‘Regional Curve Standardization’ (RCS; Esper et al. 2003). This technique allows the preservation of centennial scale climate variability, even if the tree-ring chronologies are composed of relatively short segments of material from living trees and historical wood (Cook et al. 1995). Application of RCS is essential to reconstruct the low frequency spectrum of temperature variability including long-term cooling trends (Esper et al. 2004) and thus necessary to estimate the spatial extent of MWP (Broecker 2001).