24. August 2010

Menschen im Gespräch: Teil VI


Interview

mit

copyright Dr. Büntgen



Dr. Ulf Büntgen


In der heutigen Ausgabe von "Menschen im Gespräch" begegne ich Dr. Ulf Büntgen von der Eidgenössischen Forschungsanstalt für Wald, Schnee und Landschaft WSL in Birmensdorf/Schweiz (WSL). Dr. Büntgen ist dort als Scientific Research Assistant in der Paläo-Klimatologie-Gruppe im Bereich Dendro-Wissenschaften tätig (Gruppe Paläo-Klimatologie). Des Weiteren ist Dr. Büntgen Mitglied des "Oeschger Centre for Climate Change Research (Oeschger-Center)". Da Dr. Büntgens Expertise unbestritten ist, wofür wohl auch seine zahlreichen Veröffentlichungen sprechen (Liste der Veröffentlichungen), freut es mich besonders, dieses Interview zu führen.

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W.v.B.: Es ist schön, einen Wissenschafter begrüßen zu dürfen, der sein Augenmerk auf die Paläoklimatologie und im Speziellen auf mein Blogthema, die MWP oder genauer, die Klimavariabilität richtet.

Dr. Büntgen, ich möchte zu Beginn Einiges über den momentanen Stand in der Dendroklimatologie in Bezug auf langfristige Klimarekonstruktionen in Erfahrung bringen.

Wenn ich Städtenamen wie Hammerfest oder Kopenhagen anführe, so ist Ihnen sicherlich ersichtlich, dass es sich hierbei um Städte handelt, die auf dem 55 bzw. 70sten Breitengrad Nord gelegen sind. Für dazwischen liegende Breiten wurde in den letzten Jahren das sog. "Divergenz-Problem" nachgewiesen (Siehe Fußnoten). Was ist der aktuelle Stand bezüglich des "Divergenz-Problemes" in der Dendroklimatologie - gibt es Lösungansätze bzw. wie stehen Sie kritischen Anfragen gegenüber, in welchen u.a. auf die Möglichkeit von "nonlinear tree growth responses to temperature" und die damit einhergehenden Konsequenzen für die gängigen linearen Modelle hingewiesen wird (Cf., u.a. Löhle: A mathematical analysis of the divergence problem in dendroclimatology, in: Climatic Change. DOI 10.1007/s10584-008-9488-8.)?

Letztlich stellt sich die Frage, ob langfristige Klimarekonstruktionen, welche auf der Auswertung von Baumringdaten basieren, auch im wissenschaftlichen Sinne adäquat sind oder nicht. Was ist Ihr Resümee?


Dr. Büntgen: Erst einmal möchte ich mich bei Ihnen für die Einladung zum Interview und die damit verbundene, im Vorfeld zu leistende Recherche und Logistik bedanken. Genau wie meine, sagen wir "Vorredner" halte ich es auch für eine gewisse wissenschaftliche Verpflichtung oder gar Normalität, seriös über den Status Quo zu informieren. Die oft aus Forscherperspektive monierten Kommunikationsschwierigkeiten und daraus resultierenden Verzerrungen, können nur durch einen fair geführten Dialog, sowie einen respektvollen Umgang unter stetigem Abwiegen zwischen wissenschaftlicher Erkenntnisgewinnung, medialer Inszenierung und politischer Umsetzung vermieden werden. Damit möchte ich sagen, dass die politische Umsetzung wissenschaftlicher Ergebnisse sich stärker an deren Wahrheitsgehalt orientieren sollte, mögliche Unsicherheiten zu berücksichtigen hat und den komplexen Ablauf eines evolutionären Prozesses respektieren muss - die Interpretation vermeintlich finaler (und somit medial interessanter) Resultate ist keine Alternative.

Zur Kommunikationsthematik zwischen den drei Sphären "Wissenschaft, Medien, Politik" möchte ich folgendes anmerken. Themen rund um den Klimawandel sind längst in unserem Alltag etabliert, gehören zum festen Bestandteil tagespolitischer Meinungsbildung und werden zunehmend lobbyistisch instrumentalisiert. Spätestens bei ausführlicher Betrachtung fällt jedoch auf, dass der jüngst zu beobachtende Klima-Aktionismus auf einer nur teilweise fundierten Datengrundlage basiert. Ernüchternd muss also festgehalten werden: die in den Medien inszenierte und politisch geführte Debatte hat den wissenschaftlichen Kenntnisstand längst verlassen. Die jüngst entfachte Klima-Hysterie setzt die logische Abfolge von Erkenntnisgewinnung, Interpretation und Umsetzung ausser Kraft und gefährdet somit das Nachhaltigkeitsprinzip.

Nun zu ihrer Frage, respektive Behauptung und deren Einschätzung meinerseits: Über den Nachweis eines sog. - und vor allem unikausalen - "Divergenz-Problems" innerhalb der nördlichen Breiten, also in dem von Ihnen skizzierten zirkumpolaren Borealen Waldgürtel, oder zielt Ihre Beschreibung lediglich auf Skandinavien ab, ist mir keine Arbeit bekannt, welcher überzeugende Evidenzen für ein solches "Divergenz-Problem" entnommen werden könnten.  

Ein seit der zweiten Hälfte der 1990er Jahre teilweise angeführtes Phänomen, welches dann schnell unter dem Begriff der "Divergence" verbucht wurde und innerhalb der letzten fünf Jahre verstärkt in der Fachliteratur diskutiert - aber nicht erklärt - wurde, ist mit gewisser Vorsicht zu behandeln. In einem ersten Schritt muss - hier führt kein Weg vorbei - der Begriff, sprich das sujet - wenn nötig auch semantisch - definiert werden. Eine Dialektik wird unumgänglich. Danach erst lässt sich vielleicht eine halbwegs sinnvolle, sprich konstruktive Debatte führen.

Zur Begrifflichkeit des "Divergenz-Problems" möchte ich folgende Textausschnitte anführen:
Evidence for reduced sensitivity of tree growth to temperature has been reported from multiple forest sites along the mid to high northern latitudes and from some locations at higher elevations. This alleged large-scale phenomenon reflects the inability of temperature sensitive tree-ring width and density chronologies to track increasing temperature trends in instrumental measurements since around the mid-20th century. In addition to such low-frequency trend offsets, resulting in warmer instrumental and cooler reconstructed temperatures, the potential inability of formerly temperature sensitive trees to reflect high frequency climate signals derives from some boreal and alpine sites. These two observations have recently been introduced as the 'Divergence Problem' (DP), with causes and scales being discussed in a veariety of recently published high-ranking peer-reviewed articles. If the DP turns out to be a real and widespread phenomenon (coincidentally) paralleling anthropogenic-induced changes of atmospheric composition as well as global warming, it would not only have a substantial effect on biomass productivity rates, with serious implications on carbon seequestration, but it would further question the overall ability of tree ring-based temperature reconstructions to capture earlier periods of putative warmth, such as the so-called Medieval Warm Period, and subsequently to model possible relations of forest ecosystems in a warming world.
In this regard, a comprehensive study synthesising the recent discussion of how trees at higher latitudes and elevations have responded to temperature variations (not a simple increase) over the past decades would be of multiple need and relevance. I believe this to be of particular importance, because low frequency offset between tree-ring chronologies (not single series) and the recent change in instrumental temperature readings (which might also contain bias) has been introduced as the "Divergence Problem (or Phenomena)" (DP) in tree-ring research (and more broadly in climatology and ecology). An ongoing debate on causes and scales of candidate reasons for the DP confuses the scientific literature. This is mainly related to the fact that any disassociation between tree growth and temperatures would limit the ability of tree-ring proxy data to reconstruct earlier periods that might have been as warm or even warmer than the late 20th century, and model possible reactions of forest ecosystems in a warming world. A potential reduction in climate sensitivity of formerly temperature-controlled habitats would further complicate estimates of future biomass productivitiy with implications for the global carbon cycle.
At the same time, innovative work has been put forward indicating that the DP may be attributed to methodological artifacts. These include inappropriate i) selection of sampling sites, ii) application of tree-ring detrending methods, iii) elimination of variance changes, iv) utilization of calibration technique, period and target data, as well as v) evaluation of seasonal response windows. Strategies for proper assessment of growth-climate relationships have now been published for the European Alps and Northern Eurasia. Since these studies are based on several thousand tree-ring series and independently demonstrated that DP is inexistent for a major mid-latitude mountain system and over large parts of high-latitude taiga forests, it hast been confirmed that no general underlying cause exists.
Publication of such kind of article should have the aim of opening the ongoing debate on the DP to adjacent disciplines dealing with aspects of plant growth and climatic change, such as biology, physiology and ecology. At the same time, a forthright discussion of those studies that initially propagated 'pitfall' related DP in light of new evidence would help overcoming actual constrains, and stimulate academics, industrial and applied plant scientists, students and researchers to rethink current perspectives of responses in a warming world.


In diesem Zusammenhang schätze ich als relevante und durchaus kritische Literatur folgende (meiner) Arbeiten ein:

Büntgen U, Schweingruber FH (2010) Environmental change without climate change? New Phytologist doi: 10.1111/j.1469-8137.2010.03342.x
Büntgen U, Wilson R, Wilmking M, Niedzwiedz T, Bräuning A (2009) The 'Divergence Problem' in tree-ring research. TRACE 7: 212-219.
Büntgen U, Frank DC, Wilson R, Carrer M, Urbinati C, Esper J (2008) Testing for tree-ring divergence in the European Alps. Global Change Biology 14: 2443-2453.
Visser H, Büntgen U, D'Arrigo R, Peterson A (2010) Detecting instabilities in tree-ring proxy calibration. Climate of the Past 6: 225-255.

Das Thema rund um "nonlinear tree growth responses to temperature" und die damit einhergehenden Konsequenzen für die gängigen linearen Modelle wird wohl am besten in Visser et al. 2010 (CP) angesprochen.

Dann steht noch mein Resümee aus. Natürlich sind die meisten langfristigen, auf Jahrring-daten (-breiten oder -dichten) basierenden Klima-Rekonstruktionen (Temperatur-) im wissenschaftlichen Sinne adäquat. Im wissenschaftlichen Sinne haben sie die Aufgabe den Stand der Forschung ehrlich zu reflektieren und beinhalten somit selbstverständlich keinen Anspruch auf Vollkommenheit. Wissenschaftliche Ergebnisse dürfen eben nicht als endgültig verstanden werden - siehe oben. Gute Arbeiten beinhalten darum auch eine, wenn möglichst umfassende und realistische Fehlerabschätzung. Speziell sei noch darauf hingewiesen, dass gerade aus Skandinavien die vielleicht besten regional-scale summer, i.e. June-August temperature reconstructions based on annually measurements of conifer maximum latewood density kommen. Eine Divergenz zwischen ansteigenden Sommertemperaturen und geringerem Jahrringwachstum ist in Skandinavien eh kein Thema, da die höchsten Temperaturen in den 1930er Jahren lagen (der Sommer von 1937 war extrem warm).

Fig. 1 (a) summer (June-July-August) temperature measurements of eight Scandinavian stations (+65° N) considering raw and homogenized data from the GHCN and GISS (32 series; orange), the Tornedalen composite back to 1816 (pink), and the gridded CRUTEM3v mean, averaged over 65-70° N and 20-30° E (red). The blue curve shows the gridded December-February winter mean, and corresponding linear trends (1860-2008) are described at the left side. (b) Twenty-five chronologies (green) and their mean (dark green). (c) Reconstructed summer temperatures back to 500 AD with their 20 warmest decades superimposed (horizontal bars).


Zu der hier gezeigten Figur haben wir unter folgendes geschrieben (Büntgen and Schweingruber 2010; NP):
Instrumental station measurements provide reliable information on northern Scandinavian temperature variability as far back as the early 19th century (Tornedalen; Klingbjer and Moberg, 2003). The warmest and coldest June-July-August means occurred in 1937 and 1902, respectively. Lines of evidence for significant long-term warming from cooler Little Ice Age to warmer recent summers, however, remain absent. A similar course is obtained from December-February winter means: cooler conditions before ~1910, warmth between ~1930-1950, cooling from ~1950-1980 and increasing temperatures from the 1980s to present (Fig. 1a). Linear trends of summer and winter temperatures are similarly free of any long-term trends. Independent of the data providers considererd (GHCN and GISS) and the versions used (raw and homogenized), station readings resemble grid-box means (CRUTEM3v; Brohan et al., 2006) of the past 150 years (Fig. 1a). The Abisko record used in Hallinger et al. correlates at 0.91 with gridded CRU summer temperatures back to 1869, and also does not indicate any long-term warming.
To best understand past changes in northern Scandinavian forest growth, we herein utilize a tree-ring network of 1,179 series containing ~190,000 annual maximum late wood density measurements from 25 conifer sites in Norway, Sweden, Finland and Russia  +65° N. This unique compilation appears to be most representative for vegetation dynamics across boreal Europe. Non-climatic age trends were removed from the raw density series using Regional Curve Standardization (Esper et al., 2003). Each of the site records perfectly tracks inter-annual to multi-decadal variations in northern Scandinavian summer temperature (Fig. 1b). Correlation with air temperature back to 1860 is 0.83.
A master chronology of 12 ring width and density chronologies from Swedish Torneträsk (Briffa et al., 1992; Grudd et al., 2002; Grudd, 2008), central Sweden (Gunnarson et al., 2010), Finnish Lapland (Helama et al., 2009), coastal Norway (Kirchhefer, 2001) and regional-scale networks (Gouirand et al., 2008; Linderholm et al., 2009; this Study) allow Scandinavian temperatures to be reconstructed back to AD 500 (Fig. 1c). Comparably warm summer occurred in the 8th century, between ~900 and 1200, in the 15th century, and again in the early-mid 20th century. The reduced warm/cool/warm amplitude associated with the Medieval Warm Period/Little Ice Age/Anthropogenic Era is indicative of internal ocean-atmosphere coupling and resulting climate inertia strong enough to override external forcing (Büntgen et al., 2010).


W.v.B.: Zu Ihrer Forschungstätigkeit zählte, wie aus Ihrer Homepage entnehmbar (buentgen.com), in den letzten Jahren und zählt auch heute noch die Teilnahme an verschiedenen Projekten zur Bestimmung lokaler, regionaler und hemisphärischer Klimavariabilität. Ich meine hier insbesondere die Projekte ALP-IMP, MILLENNIA und MILLENNIUM.

Können Sie uns einen kurzen Überblick darüber geben, welche Einsichten Sie bezüglich der Existenz einer MWP im alpinen (ALP-IMP) und europäischen Raum und für die Nördliche Hemisphäre (MILLENNIA, MILLENNIUM) gewonnen haben und wie der aktuelle Stand der Forschung diesbezüglich, aus Ihrer Sicht  der Dinge heraus ist?


Dr. Büntgen: Der Vollständigkeit halber müssten hier auch noch einige andere Projekte angeführt werden, wie zB EuroTrans. Trotz der Gefahr einer Wiederholung, muss ich, einmal mehr auf die definitorische Genauigkeit des Begriffes MWP hinweisen. Dieser Begriff wurde - nageln Sie mich hier bitte nicht fest - von H.H. Lamb vor knapp 40 Jahren eingeführt.

Einige Temperaturrekonstruktionen zeigen in früheren, vorindustriellen Perioden ähnlich hohe Temperaturen wie sie im 20. Jhd. gemessen wurden. Diese Ergebnisse liegen für unterschiedliche Regionen vor. Für Europa und hier basierend auf Alpinen Jahrringdaten, zeigen viele Rekonstruktionen relativ warme Sommer kurz vor (und um) 1000 AD und dann noch einmal um 1200 AD und in der Mitte des 13. Jhd. Genau wie auch das 20. Jhd. durch eher wämere und kältere Perioden gekennzeichnet war, kann auch bei der MWP in keinem Fall von einer "homogenen" Klimaphase ausgegangen werden. Auch gab es wohl Phasen, in denen die Sommer kühler waren als vor und nach dieser Epoche (mit vorher meine ich dekadische bis mehrdekadische Intervalle zwischen 300 und 800 AD, mit nachher meine ich dekadische bis mehrdekadische Intervalle zwischen 1300 und 1900 AD). Über die räumliche Ausbreitung, die zeitliche Abfolge und die absolute Amplitude wissen wir heute noch nicht viel. Etwas mehr wissen wir über die als "Kleine Eiszeit" in die Literatur eingegangene Periode zwischen ungefähr dem 13. und 19. Jhd. - auch hier handelte es sich keineswegs um eine klimatisch homogene Phase, noch weniger Wissen haben wir bezüglich der vor der MWP liegenden Klimabedingungen, welche öfters als "Dark Age" bezeichnet werden, einem Begriff der historisch jedoch nicht fundiert ist.

In kurz, es besteht noch sehr viel Forschungsbedarf um die Klima, respektive Temperaturschwankungen der letzten zwei Jahrtausende zu verstehen. Dies gilt vor allem für das Verständnis über die Amplitude der Langfristtrends, deren räumliche Muster und die verantwortlichen Steuergrößen.


W.v.B.: In den vorangehenden Interviews wurden verschiedene, jedoch großteils sich deckende Aussagen bezüglich wissenschaftlicher Evidenz für die Existenz einer MWP in der Südlichen Hemisphäre, von meinen Interviewpartnern getroffen. Die Übereinstimmung lag meines Erachtens darin, dass meine Gesprächspartner der Meinung waren, dass es zum jetzigen Zeitpunkt wissenschaftliche Evidenz für die Existenz einer MWP/MCA sowohl für Südamerika, Australien und Neuseeland aber auch die Antarktis gibt (für Afrika konnte ich selbst einige Studien anführen, darunter auch eine "nordhemisphärische" Studie, an der Sie direkt beteiligt waren: Long-term drought severity variations in Marocco [pdf-Format, 560 KB]). Herr Dr. Büntgen, was ist Ihre Meinung hierzu?


Dr. Büntgen: Unser Verständnis klimatischer Bedingungen vor rund 1000 Jahren und noch früher ist für die südliche Hemisphäre nochmals um einiges schlechter! In Nordafrika sind wir übrigens noch sehr weit von der Südlichen Hemisphäre entfernt. Hemisphärische Studien stützen sich auch nur auf einige regionale Ergebnisse.


W.v.B.: Ich möchte Sie abschließend darum bitten, uns Ihre Einschätzung zur vorliegenden Evidenz für die Behauptung, dass die MWP ein globales Phänomen war, zu geben.


Dr. Büntgen: Gehen Sie von den möglichen Steuergrößen aus. Was bleibt übrig? Alles läuft auf dieser Skala auf die Sonne hinaus. Wenn dem so ist, müsste man sicher von einer globalen Anomalie ausgehen dürfen; natürlich durch interne Oszillationen modifiziert. Eine ausreichende Datengrundlage liegt aber leider noch nicht vor. Darum haben wir Arbeit.

Wir müssen es in Zukunft schaffen, die Proxy Datengrundlage zu verbessern, unterschiedliche Archive und Quellen optimal zu kombinieren und noch enger im Quervergleich mit Modellieren zu agieren.


W.v.B.: Ich danke für das Interview!


copyright W.v.B.

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Fußnoten

Das sog. "Divergenz-Problem", wie es sich in der Dendroklimatologie bezüglich Klimarekonstruktionen anhand der Auswertung von Baumringdaten stellt, wurde nach Esper et al. [1] erstmals 1995 von Jacoby und D'Arrigo, in ihrer Abhandlung "Tree-ring width and density evidence of climatic and potential forest change in Alaska"[2] für Alaska erkannt und erörtert. Verkürzt, aus "FURTHER DISCUSSION ON: TREE-RING TEMPERATURE RECONSTRUCTIONS FOR THE PAST MILLENNIUM ..."[3] entnommen, handelt es sich hierbei um folgenden Sachverhalt:
A number of tree-ring series indicate a divergence between tree growth and temperature at some northern site in recent decades.

In weiterer Folge wurde das Auftreten dieses Problems in mehreren Regionen der nördlichen Hemisphäre - D'Arrigo et al. sprechen im Groben von einem Auftreten zwischen dem 55 und 70sten Breitengrad Nord -,[4] von unterschiedlichen Forschern, mit differierenden Datierungsdaten für das erstmalige in Erscheinung treten (von den ~1950ern bis in die 1980er)[5] für Alaska u.a. von Taubes (1995)[6], Barber et al. (2000)[7], Lloyd und Fastie (2002), Davi et al. (2003)[8], Wilmking et al. (2004, 2005)[9], D'Arrigo et al. (2004, 2005)[10] und Driscoll et al. (2005)[11], für Canada von Wilson und  Luckman (2003)[12], für Japan von Yonenobu und Eckstein (2006)[13] und für Sibirien von Jacoby et al. (2000), bestätigt. Briffa et al. sprachen in ihrer Arbeit "Reduced sensitivity of recent tree growth to temperature at high northern latitudes, in: Nature (1998) 391: 678-682" gar von einer "widespread evidence for the DP" bezüglich Nord Amerika, Nord Europa und Sibirien, zu lesen bei Büntgen et al.[14] Jedoch, dies sei festgehalten, zeigen nicht alle Aufzeichnungen von Temperatur-sensitiven Baumringdaten Divergenz. D'Arrigo et al. "On the 'Divergence Problem' in Northern Forests: A review of the tree-ring evidence and possible causes, in: Glob. Planet. Change (2007). DOI: 10.1016/j.gloplacha.2007.03.004" führen auf Seite 9 hierzu  zumindest 13 Studien an. Folgten wir Briffa und dieses Phänomen würde hemisphärisch, wenn nicht im Weiteren gar für die südliche Hemisphäre  nachgewiesen, so scheint es, dass gängige dendrochronologische Klimarekonstruktionen basal an Aussagekraft verlieren würden bzw. prinzipiell als  ungeeignet eingestuft werden müssten. Büntgen et al. widersprechen einem bi-hemisphärischen Ansatz, wenn sie, mit Verweis auf die gängige Literatur schreiben: "Moreover, the current body of literature reveals that the DP does not exist at lower latitudes" (Büntgen et al.: The 'Divergence Problem' in tree-ring research, 2). Büntgen et al. weiter: "Therefore, the DP should not be thought of as an endemic large-scale phenomenon with one overriding cause, but rather a local- to regional-scale phenomenon of tree-growth responses to changing environmental factors including multiple sources and  species-specific modification" (Ibid., 2).

Es handelt sich beim Divergenz-Problem letztlich um eine Diskrepanz, welche in der gängigsten, neuen Untersuchung von D'Arrigo et al.[15] wohl zurecht wie folgt beschrieben werden kann: "
A number of recent tree-ring studies have addressed the 'divergence problem' in northern forests. It is defined herein as the tendency for tree growth at some previously temperature-limited northern sites to demonstrate a weakening in mean temperature response in recend decades, with the divergence being expressed as a loss in climate sensitivity and/or divergence trend.


Untenstehende Grafik  verdeutlicht die Diskrepanz zwischen instrumentell versus aus der Dendrochronologie rekonstruierten Temperaturdaten:

Fig. 3. Plot comparing recent tree-ring based Northern Hemisphere temperature reconstructions (Briffa, 2000; Esper et al., 2002; D'Arrigo et al.,2006) that extend into the 1990s with land based mean annual extra-tropical temperatures (20°–90° N — Brohan et al., 2006). The reconstructions have been scaled to the instrumental data over the common 1856–1992 period and the linear increase per decade calculated over the same period.

[Briffa: Annual climate variability in the Holocene: interpreting the message from ancient trees, in: Quaternary Science Reviews (2000) 19, 87-105. Esper et al.: Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability, in: Science (2002) 295, 2250-2253. D'Arrigo et al.: On the long-term context for late twentieth century warming, in: J. Geophys. Res. (2006) 111, D03103. DOI: 10.1029/2005JD006352. Brohan et al.: Uncertainty estimates in regional and global observed temperature changes: a new data set from 1850, in:. Res (2006) 111, D12106. DOI: 10.1029/2005JD006548. W.v.B.]


Es werden mehrere mögliche Ursachen für das Divergenz-Phänomen in der scientific community diskutiert. D'Arrigo et al. fassen zusammen:
These  possible causes include temperature-induced drought stress, nonlinear thresholds or time-dependent responses to recent warming, delayed snowmelt  and related changes in seasonality, and differential growth/climate relationships inferred from maximum, minimum and mean temperatures. Another  possible cause of the divergence described briefly herein is 'global dimming', a phenomenon that has appeared, in recent decades, to decrease the  amount of solar radiation available for photosynthesis and plant growth on a large scale. ... Additional potential causes include "end effects" and other  methodological issues that can emerge in standardization and chronology development, and biases in instrumental target data and its modeling. Although limited evidence suggests that the divergence may be anthropogenic in nature and restricted to the recent decades of the 20th century, more  research is needed to confirm these observations.
(Cf., Ibid, Abstract).

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[1] Esper et al.: Divergence pitfalls in tree-ring research, in: Climatic Change (2009) 94: 261.
[2] Jacoby und D'Arrigo: Tree-ring width and density evidence of climatic and potential forest change in Alaska, in: Glob Biochem Cycles (1995) 9: 227- 234.
[3] D'Arrigo et al.: FURTHER DISCUSSION ON: TREE-RING TEMPERATURE RECONSTRUCTIONS FOR THE PAST MILLENNIUM, Follow-up on the National  Research Council Meeting on "Surface Temperature Reconstructions for the Past 1000-2000 Years", March 2-3, 2006, Washington D. C., 1.
[4] Cf., Ibid., 2; der eigentliche Verweis geht auf Cook et al. und die Arbeit "Extra-tropical Northern Hemisphere land temperature variability over the past  1000 years, in: Quaternary Science Reviews (2004) 23: 2063-2074".
[5] Cf., u.a. Löhle: A mathematical analysis of the divergence problem in dendrochronology, in: Climatic Change, DOI 10.1007/s10584-008-9488-8. Er schreibt: "This problem is characterized by trees or assemblages of trees that showed a positive response to warming in the early part of the century showing a lessened or even negative response to warming in the period starting in the 1960s to 1980s". Lloyd und Fastie sprechen nach Löhle in  "Spatial and temporal variability in the growth and climate response of treeline trees in Alaska, in: Climate Change (2002) 58: 481-509" von den 1950ern  für Alaska; Jacoby et al. in "Long-term teperature trends and tree growth in the Taymir region of northern Siberia, in: Quaternary Research (2000) 53:  312-318" von den 1970ern für Sibirien.
[6] Taubes: Is a warmer climate wilting the forests of the north?, in: Science (1995) 267, 1595.
[7] Barber et al.: Reduced growth of Alaska white spruce in the twentieth century from temperature-induced drought stress, in: Nature (2000) 405, 668- 672.
[8] Davi et al.: Boreal temperature variability inferred from maximum latewood density and tree-ring width data, Wrangell Mountain region, Alaska, in:  Quaternary Research (2003) 60, 252-262.
[9] -Wilmking et al.: Recent climate warming forces contrasting growth responses of white spruce at treeline in alaska through temperature thresholds,  in: Glob. Change Biol. (2004) 10, 1724-1736.
-Wilmking et al.: Divergent growth responses in circumpolar boreal forests, in: Geophysical Research Letters (2005) 32, L15715. DOI: 10.1029/2005GL023331.
[10] D'Arrigo et al.: Thresh-olds for warming-induced growth decline at elevational treeline in the Yukon Territory, in: Glob Biochem Cycles (2004) 18 (3): GB3021, DOI: 10.1029/2004GB002249.
D'Arrigo et al.: Temperature variability over the past millennium inferred from northwestern Alaska tree rings, in: Climate Dynamics (2005) 24 (2-3).
[11] Driscoll et al.: Divergent tree growth response to recent climatic warming, Lake Clark, National Park and Preserve, Alaska, in: Geophysical Research Letters (2005) 32, L20703. DOI: 10.1029/2005GL024258.
[12] Wilson und Luckmann: Dendroclimatic reconstruction of maximum summer temperatures from upper tree-line sites in interior British Columbia, in: The Holocene (2003) 13, 853-863.
[13] Yonenobu und Eckstein: Reconstruction of early spring temperature for central Japan from the tree-ring widths of Hinoki cypress and its  verification by other proxy records, in: Geophysical Research Letters (2006) 33, L10701. DOI: 10.1029/2006GL026170.
[14] Cf., Büntgen et al.: The 'Divergence Problem" in tree-ring research, Seite 2 (abruf-/downloadbar [pdf-Format,  68 KB] unter: Divergence Problem).
[15] D'Arrigo et al.: On the 'Divergence Problem' in Northern Forests: A review of the tree-ring evidence and possible causes, in: Global Planetary Change (2007). DOI: 10.1016/j.gloplacha.2007.03.004.

16. August 2010

Menschen im Gespräch: Teil V


Interview

with

copyright Dr. Barron



Dr. John A. Barron


Today, I am proud to introduce Dr. John A. Barron as my next interview partner. Dr. Barron has a PhD in Geology and works as a researcher (Geologist, Micropaleontologist) for the US Geological Survey, Western Region, Southwest area (USGS Professional Pages). The scientific importance of his work lies inter alia in helping to "refine diatoms as a biochronologic tool for dating Cenozoic and Late Cretaceous rocks and sediments from polar waters and coastal upwelling regions like California. This biochronology was used to date tectonic events in California and the North Pacific, determine periods of widespread deep-sea hiatuses in the world's oceans, and detail the Cenozoic history of the Antarctic ice sheet"(cf., Synopsis of Research).

Dr. Barron has published dozens of papers in renowned scientific journals like the Journal of Paleontology or "Palaeogeography, Palaeoclimatology, Palaeoecology". In one of his papers: "Santa Barbara Basin diatom and silicoflagellate records suggests coincidence of cooler SSt with widespread occurrence of drought in the west during the past 2,200 years for PACLIM Proceedings volume" (="Santa Barbara ..."), Dr. Barron et al. found in the "assemblage data from the ODP 893A record evidence for a broad interval of generally cooler SSts between ~AD 800 and 1350, which corresponds to the Medieval Climate Anomaly (MCA)"(Abstract).
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W.v.B.: Dr. Barron, thank you for taking the time for this interview. Let me start the interview by focussing on your paper "Santa Barbara ...". Could you first, in general talk about the paper before you point out the main results?


Dr. Barron: The Santa Barbara Basin is unique, because it contains a varved sediment record that accumulated at fairly high sediment accumulation rates. It has a well developed time scale that is necessary for detailed correlation with the high-resolution time scales developed in ice cores and by tree rings. The aim of the paper was to determine paleoclimatic conditions during the Medieval Warm Period (MWP) and the Little Ice Age (LIA) in waters off California in order to test the hypothesis that cool-water, La Niña-like conditions occurred during the MWP and caused widespread drought in the southwestern parts of the US. Diatoms and silicoflagellates are phytoplankton and respond to surface water temperatures better that planktonic foraminifer, which live at greater depth ranges in the water column.


W.v.B.: Your paper shows evidence for a MWP/MCA. It seems to me that there is a lot of evidence for the existence of a MWP/MCA in the Northern Hemisphere and especially in North America. For all parts of North America and the US, peer-reviewed papers, presenting evidence for a MWP/MCA can be found (Alaska, Canada, Greenland and the US [without Alaska])[1]. During the last years, there has even been an increasing number of papers supporting the conclusion that there was a MWP/MCA in the US [2]. What is your view on the existence of a MWP/MCA in the Northern Hemisphere and especially in the US.


Dr. Barron: I believe that there is good evidence for a MWP/MCA in North America. For western regions dominated by Pacific weather that evidence stems from the persistence of La Niña-like conditions and the absence of strong El Niña events during the interval between about 900 and 1300 AD. The persistence of these conditions results in a stable North Pacific high pressure system being established during the late spring and summer at ~ 40 degrees North, causing northward deflection of storm tracks and drought throughout the southwestern US. Similar to the present summer of 2010, cooler conditions in coastal California coincide with warmer conditions throughout much of the eastern US. It is likely that high solar radiance during this period contributed to the persistence of these climatic conditions.


W.v.B.: Dr. Böhm responded to my question on the possible occurence of a MWP in the Southern Hemisphere in short: Yes, but less quantifiable. What ist your point of view on this topic?


Dr. Barron: My opinion is based on reading the literature. I know that there is good evidence for the persistence of La-Na-like conditions in Pacific coastal regions of South America during the MWP/MCA that resemble those of the western US. Mohtadi et al (2007) argue for a sustained northward shift of zonal systems (i.e., the South Westerly Wind belt and Antarctic Circumpolar Current) in the SE Pacific between ~1300 and 750 yr BP that contributed to reduced ENSO activitiy throughout South America.

Mohtadi, M., Romero, O., Kaiser, J., and Hebbeln, D., 2007. Cooling of the southern high latitudes during the Medieval Period and its effect on ENSO. Quaternary Science Reviews 26: 1055-1066.


W.v.B.: Recently I have read a paper, published by Graham et al.[ 3]. I encountered an interesting statement. The statement was: ... if not a "Medieval Warm Period", then a substantial "Medieval Climate Anomaly". What do you think, is there enough evidence for the claim that the Medieval Warm Period was indeed a global phenomenon or do you prefer the statement by Graham et al.?


Dr. Barron: I tend to argue with Graham et al. because strict use of the term Medieval Warm Period implies that temperatures were warmer everywhere on earth at the same time. This is climatically impossible, yet the documentation of anomalously warm conditions in Europe, the arctic regions, and the North Atlantic during the MWP/MCA are important observations that influence our understanding of the MWP/MCA.


W.v.B.: Thanks for the interview!


copyright W.v.B.

_____________________


Footnotes:

[1] Alaska:
Barclay et al.: Tree-ring crossdates for a first millennium AD advance of Tebenkof Glacier, southern Alaska, in: Quaternary Research 71 - 2009, 22-26.

Loso: Summer temperatures during the Medieval Warm Period and Little Ice Age inferred from varved proglacial lake sediments in southern Alaska, in: Journal of Paleolimnology 41 -2009, 117-128.

McKay et al.: Biogenic silica concentration as a high-resolution, quantitative temperature proxy at Hallet Lake, south-central Alaska, in: Geophysical Research Letters 35 - 2008, 10.1029/2007GL032876.

Loso et al.: A 1500-year record of temperature and glacial response inferred from varved Iceberg Lake, southcentral Alaska, in: Quaternary Research 66 - 2006, 12-24.

Canada:
Rolland et al.: Evidence for a warmer period during the 12th and 13th centuries AD from chironomid assemblages in Southern Island, Nunavut, Canada, in: Quaternary Research 72 - 2009, 27-37.

Cook et al.: Five thousand years of sediment transfer in a high arctic watershed recorded in annually laminated sediments from Lower Murray Lake, Ellesmere Island, Nunavut, Canada, in: Journal of Paleolimnology 41 - 2009, 77-94.

Zabenskie, Gajewski: Post-glacial climatic change on Boothia Peninsula, Nunavut, Canada, in: Quaternary Research 68 - 2007, 261-270.

Luckman, Wilson: Summer temperatures in the Canadian Rockies during the last millennium: a revised record, in: Climate Dynamics 24 - 2005, 131-144.

Greenland:
Vinther et al.: Climatic signals in multiple highly resolved stable isotope records from Greenland, in: Quaternary Science Reviews 29 - 2010, 522-538.

Lloyd: Late Holocene environmental change in Disko Bugt, west Greenland: interaction between climate, ocean circulation and Jakobshavn Isbrae, in: Boreas 35 - 2006, 35-49.

Lassen et al.: Late-Holocene Atlantic bottom-water variability in Igaliku Fjord, South Greenland, reconstructed from foraminifera faunus, in: The Holocene 14 - 2004, 165-171.

Dawson et al.: Late-Holocene North Atlantic climate 'seesaws', storminess changes and Greenland ice shee (GISP2) palaeoclimates, in: The Holocene 13 - 2003, 381-392.

US (without Alaska):
St. Jacques et al.: A 900-year pollen-inferred temperature and effective moisture record from varved Lake Mina, west-central Minnesota, USA, in: Quaternary Science Reviews 27 - 2008, 781-796.

Millar et al.: Late Holocene forest dynamics, volcanism, and climate change at Whitewing Mountain and San Joaquin Ridge, Mono County, Sierra Nevada, CA, USA, in: Quaternary Research 66 - 2006, 272-287.

Lund, Curry: Florida Current surface temperature and salinity variability during the last millennium, in: Paleoceanography 21 - 2006, 10.1029/2005PA001218.

Cronin et al.: Medieval Warm Period, Little Ice Age and 20th century temperature variability from Chesapeake Bay, in: Global and Planetary Change 36 - 2003, 17-29.

[2] Brunelle et al.: A 8000 year fire history from an Arizona/Sonora borderland ciénega, in: Journal of Arid Environments, Volume 74, Issue 4, April 2010, Pages 475-481.

*Barron et al.: Santa Barbara Basin diatom and silicoflagellate response to global climate anomalies during the past 2200 years, in: Quaternary International, Volume 215, Issues 1-2, 15 March 2010, Pages 34-44.

Schmeisser et al.: Modern and late Holocene wind regimes over the Great Plains (central U.S.A.), in: Quaternary Science Reviews, Volume 29, Issues 3-4, February 2010, Pages 554-566.

Hanson et al.: Megadroughts and late Holocene dune activation at the eastern margin of the Great Plains, north-central Kansas, USA, in: Aeolian Research, Volume 1, Issues 3-4, January 2010, Pages 101-110.

Knight et al.: A bimillennial-length tree-ring reconstruction of precipitation for the Tavaputs Plateau, Northeastern Utah, in: Quaternary Research, Volume 73, Issue 1, January 2010, Pages 107-117.

[3] The paper was titled "Tropical Pacific - mid-latitude teleconnections in medieval times", published in Climate Change 2006, DOI 10.1007/s10584-007-9239-2. The whole statement (p. 38) was: It (the nexus between long-established evidence for medieval drought in the western US ... and recent evidence suggesting a cool tropical Pacific, W.v.B.) also offers a partial response to the query posed by Hughes and Diaz more than a decade ago (Hughes and Diaz 1994 [Was there a "medieval warm period" and if so, where and when?, published in Climate Change 26 - 1994, 109-142, W.v.B.],) if not a "Medieval Warm Period", then a substantial "Medieval Climate Anomaly", a significant transient in Holocene climate that affected a large area of the Pacific Sector during medieval time, from about 500-1300 A.D.


Menschen im Gespräch: Teil IV



Interview

with

copyright Dr. Fowler



Dr. Anthony Fowler


Dr. Fowler is a Senior Lecturer at the School of Environment at The University of Auckland (Biography). His research activities over the last two decades have covered three fields: Palaeoclimatology, "Future climate change and climate change impact assessment and Hydrology" and water resource planning. Dr. Fowler has published numerous papers in well known peer-reviewed journals like The Holocene or the Journal of Quaternary Science

I am looking forward to get some new informations on longtime climate reconstructions for the Southern Hemisphere.

____________________


W.v.B.: Dr. Fowler, let's start with some questions on the occurence of the Medieval Warm Period in Australia and New Zealand.

During the last decade, several papers on longtime climate reconstructions for the Southern Hemisphere have been published. If we take a look on New Zealand, we can refer to the papers "Speleothem master chronologies: combined Holocene 18O and 13C records from the North Island of New Zealand and their palaeoenvironmental interpretation" (Abstract here: Speleothem master chronologies ...) and of course "Speleothem stable isotope records interpreted within a multi-proxy framework and implications for New Zealand palaeoclimate reconstruction" (Abstract here: Speleothem stable isotope ... ). On the other hand, Arrigo, Cook and Palmer found evidence for a MWP in an "Oroko Swamp silver pine tree-ring chronology", back in the year 2002, but also point to the fact that we must bear the following in mind:
The occurence 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.

Cook, E. et al.: Evidence for a 'Medieval Warm Period' in a 1,100 year tree-ring reconstruction of past austral summer temperatures in New Zealand, in:
GEOPHYSICAL RESEARCH LETTERS, VOL. 29, NO. 14, 12-1, 2002

(downloadable [pdf-format, 370 KB] here: Evidence for a 'Medieval Warm Period' ...)


What do you think, has there been done new research since 2002; briefly and concisely: What ist he "state of art" on tree-ring climate reconstruction for Australia and New Zealand? In the wider sense: Is there new evidence for the existence of a MWP in general for Australia and New Zealand, including other proxy data?


Dr. Fowler: From a local tree-ring perspective the Oroko Swamp study has not really been superseded and the cautious conclusions of Cook et al remain appropriate. Most recent New Zealand (NZ) work has focused on kauri tree rings in the context of ENSO reconstructions, rather that temperature. Our kauri chronology goes back almost 4000 years, but the species is not suitable as a temperature proxy. Those NZ species that are temperature sensitive do tend to show cooler conditions prior to the 20th century, but conclusions about whether that cooling was from an earlier warmer period largely rest with Cook and colleagues working in Tasmania and the NZ South Island west coast, and on other proxy data. We are currently assembling and synthesising the available evidence and, yes, there is evidence that NZ was somewhat warmer towards the beginning of the millennium, but not necessarily synchronised with the Northern Hemisphere. We are also seeing some evidence of a fairly abrupt transition into the period known as the Little Ice Age. It is worth noted here though that NZ is highly sensitive to changes in atmospheric circulation changes and that generalising to the scale of the Southern Hemisphere is decidedly problematic.


W.v.B.: If we cast a glance at South America, we will find new evidence for the existence of a MWP in this part of the world. In several papers, data from stratigraphy, pollen analysis, data from marine sediment cores, data from ice cores and other proxy data has become available and could be evaluated. Villalba et al. noted in the paper "Long-term multy-proxy climate reconstructions and dynamics in South America (LOTRED-SA): State of the art and perspectives", published in: Palaeogeography, Palaeoclimatology, Palaeoecology, 281 (2009) 175 (downloadable [pdf-format, 220 KB] here: Long-term multy-proxy ...):
The review and research articles in this special issue cover a variety of archives (historical documents, tree rings, ice cores, glacier fluctuations and lacustrine sediments) and suggest that, at least south of ca. 18°S. the spatial and temporal coverage of the data is adequate to develop synoptic multi-proxy reconstructions for the past ca. 400 years. Although additonal high-quality proxy records are still needed to resolve the finer temporal and spatial structure of past climate variations, the currently available data do provide a consistent picture of climate fluctuations at a large scale.


In my opinion, this is a step forward. What is your opinion on the evidence for a MWP in South America and the Antarctica - in short for the Southern Hemisphere in general?


Dr. Fowler: There is some great work coming out of South America and I note with interest evidence of abrupt transitions that bear some similarity to what we are seeing in NZ. As for the Southern Hemisphere as a whole - I think that the definitive synthesis work has yet to be done (and probably the data are not up to it), so I am reserving judgement!


W.v.B.: Dr. Fowler, is there any doubt about the accuracy of the statement, the MWP was at least a phenomenon that occured in the Northern Hemisphere? 


Dr. Fowler: I am fairly convinced by the evidence for a North Atlantic focus for what has been called the MWP, but am also intrigued by the differences in timing of warm periods across the hemisphere. So, the answer is yes, at least in places and, in an average sense, yes probably for the hemisphere as a whole. 


W.v.B.: Would you finally please summarize your opinion on the existence of a MWP (hemispheric and global). 


Dr. Fowler: NH local - I'm convinced. NH overall - yes, in an average sense (but with regional caveats). SH overall - uncertain, but probably warmer than the LIA, and probably more subdued than the NH. Global - I'm not sure it makes sense to combine the hemispheres given unknown forcing and the dramatically different character of the evidence. However, if the NH was warm and there is no convincing evidence for a cool SH then, I guess, global warmth is a logical conclusion. 


W.v.B.: I want to thank you for being my interview partner and wish you all the best for the future.

copyright W.v.B.
 

5. August 2010

Menschen im Gespräch: Teil III


Interview

with

copyright Ljungqvist


Fil. cand. Fredrik Charpentier Ljungqvist


Fredrik Charpentier Ljungqvist is Secretary for the Centre of Medieval Studies at Stockholm University and PhD student at the Department of History at Stockholm University (http://www.medeltid.su.se). One of his publications is listed in the branch NOAA Palaeoclimatology/Climate Reconstructions (Climate Reconstructions) on the famous side of the NOAA's National Climatic Data Center (NOAA Paleoclimatology Program). 

I am talking about the paper "Temperature Proxy Records Covering the Last Two millennia: a tabular and visual overview", which I have already mentioned (see the interview with Dr. Böhm.) Unfortunately you can only find the abstract and the data in txt-format (downloadable [txt-format, 1,3 MB], here: Temperature Proxy Records ...) but not download the paper as a whole as a pdf-file. I think this paper is very important and the reason for making this interview. Mr. Ljungqvist also published a book on climate changes during the last 10,000 years, and its inpacts on human history in Swedish last year at Sweden's major publishing house (Global nedkylning - Klimatet och människan under 10 000 ar).

____________________


W.v.B.: Mr. Ljungqvist, I am very glad and thankful for giving me the opportunity to conduct this interview with you. Could you please start with a short introduction. What was the starting situation and what did you do? On a second step you may answer the question, if you have found evidence for the existence of a Medieval Warm Period in Scandinavia, in the proxy data you added (Dalmutladdo, Lake Toskaljavri, Lake Tsuolbmajavri, Abisko Valley, Torneträsk, Lake Sjuodjijaure, Soylegrotta, Jämtland, Lake Spaime, Lake Laihalampi and Lake Flarken) or not. 


F.C.L.: My paper, "Temperature proxy records covering the last two millennia: a tabular and visual overview", was a review of available palaeotemperature proxy records from around the globe going back (at least) 2,000 years. All in all, 71 records were presented, visualized and discussed. (There area great many shorter records out there). The purpose with the work was to offer a good review of the data available of temperature changes in the last two millennia. Even for scholars in the field, it is easy to miss some records, and hence the need for a good review.

There are defiantly many evidences for the MWP in Scandinavia. From all sorts of data: tree-ring width and density, glaciological evidences, lake sediments, pollen records, stalagmites, lake varves and so on. 


W.v.B.: What were the results for Europe and the Northern Hemisphere? 


F.C.L.: It is most easy to firmly state the existence of the MWP on Greenland and in China. (The proxy data for China are actually better than for Europe.) The MWP is perhaps most clear there but we see it in almost all records from around the Northern Hemisphere, except for in a few drought-affected tree-ring records. So to answer your question, there was a warm period in both Europe and the Northern Hemisphere as a whole sometime between AD 800 and AD 1300 followed by generally cooler conditions until the 1900s. The peak of the MWP seems to have occured, in most places, just prior to AD 1000.

This autumn I will come out with a new paper, with an improved quantitative mulit-proxy temperature reconstruction for the extra-tropical Northern Hemisphere covering the last 2,000 years. That reconstruction shows the same pattern: a Roman Warm Period, a Dark Age Cold Period, a Medieval Warm Period, a Little Ice Age and, finally, the modern warming. In an on-going project, together with other scholars, I looked into even more data and the pattern is still there. Some of these results were presented at a large international conference in May. 


W.v.B.: Do you think that the MWP was a global phenomenon? 


F.C.L.: Yes, I defiantly think so. It was a global, or near global, phenomenon. We have best evidence for the MWP in China, Europe, Iceland and Greenland, and parts of North America but there are now also some data showing the MWP in, for example, the Himalayas, New Zealand, South America and the Antarctic Peninsula. 


W.v.B.: Would you finally please summarize the gained results and your opinion on the existence of a MWP for my readers. 


F.C.L.: Most likely, the MWP was one of the many warm climate cycles during the last 10,000 years. Those climate cycles, with a multi-centennial warm period approximately every 1,000-1,500 year, are often called "Bond Cycles" and are of still unknown origin although changes in solar forcing probably play a part. The MWP is a historical reality. The 10th and 11th centuries were, most probably, at least as warm as the 20th century as a whole.

Whether any period was as warm, globally, as the last 20 years is impossible to answer with the data presently available. It looks like that some parts of earth experienced peak MWP warming more intensive that the present one but that other parts of earth now experience mean temperatures above any during the MWP. In comparing the MWP with the present warming, it is important to keep in mind that the occurrence of a global MWP (and other warm periods in the past) does not at all rule out that the present global warming partly or largely are due to anthropogenic greenhouse gas emissions from the burning of fossil fuels.  


W.v.B.: I want to thank you for being my interview partner. I wish you continued good luck and still another beautiful summer. 


F.C.L.: Have a continued good summer yourself! 

copyright W.v.B.
 

1. August 2010

Nachtrag

Es hat mich sehr gefreut, mit Dr. Böhm das Interview zu führen.  In der Nachbetrachtung möchte ich zumindest einen Punkt herausheben. 

Dabei handelt es sich um die Aussage, dass die MWP im alpinen Raum (für weite Gebiete Mitteleuropas) "deutlich in zwei besonders warme Phasen im 10. und im 12. Jahrhundert getrennt durch ein kühleres 11. Jahrhundert zerfällt."

Im ALP-IMP-final-report (abruf-/ downloadbar [pdf-Format, 5,7 MB] unter: http://www.zamg.ac.at/ALP-IMP/downloads/ALP-IMP-final-rep-public.pdf) finden wir auf Seite 21, die von mir schon als Verweis dienende Grafik von Büntgen et al. 2006.

FIG. 6. (a) Alpine summer temperature reconstruction with the orange and blue boxes denoting the 10 warmest and coldest decades, respectively, and the smoothed red line being a 20-yr low-pass filter. Temperatures are expressed as anomalies w.r.t. 1901–2000. (b) High-frequency comparison between the MXD (red; this study) and RW (blue; Büntgen et al. 2005a) RCS chronologies. Records were normalized over the 951–2002 common period. The 51-yr moving correlations (black) indicate their temporal relationship, with the horizontal line denoting the 95% significance level, corrected for lag-1 autocorrelation (Trenberth 1984). (c) Length fluctuation (m) and 50-yr average mass balance (gray) of the Great Aletsch glacier (Haeberli and Holzhauser 2003).

Hier wird der Temperaturanstieg bis 970 A.D., gefolgt von einem Temperatur"sturz" bis ca. 1046 A.D. und der darauf folgende Anstieg bis ans Ende des 12 Jhs - kurz unterbrochen -  deutlich sichtbar (natürlich unter Vorbehalt). Diese Messungen decken sich in etwa mit den Aussagen von Dr. Böhm. Es bleibt die Frage, warum es im Zeitraum einer, für das untersuchte Gebiet generell feststellbaren Erwärmung, zu einer doch ansehnlichen Abkühlungsphase kommen konnte.

Büntgen et al. führen in ihrer Arbeit als möglichen Erklärungsansatz die Anzahl der "Sunspots" an. Sie verweisen auf Solanki et al.: Unusual activity of the Sun during recent decades compared to the previous 11,000 years, erschienen in: Nature, 431, 1084–1087.

Ich möchte hierzu auf Usoskin et al. und ihrer Arbeit "SOLAR ACTIVITY OVER THE LAST 1150 years: DOES IT CORRELATE WITH CLIMATE? (abruf-/ downloadbar [pdf-Format, 560 KB] unter: http://www.mps.mpg.de/dokumente/publikationen/solanki/c153.pdf) verweisen.

Figure 1. legt nahe, dass die Anzahl der "Sunspot numbers" im Zeitraum von ca. 950- 1050 A.D.  stark zurückgegangen ist, wobei es sich hierbei nicht um direkte Beobachtungen handelt, sondern um Rekonstruktionen aus 10 Be Konzentrationen in Eisbohrkernen (Näheres, siehe Bildunterschrift zu FIG. 2):


Ersichtlich auch aus Figure 2. der Arbeit "Millennium-Scale Sunspot Number Reconstruction: Evidence for an Unusually Active Sun since the 1940s" (abruf-/ downloadbar [pdf-Format, ca. 200 KB] unter: http://cc.oulu.fi/~usoskin/personal/Sola2-PRL_published.pdf)

FIG. 2 (color). Time series of the sunspot number as reconstructed from 10Be concentrations in ice cores from Antarctica (red) and Greenland (green). The corresponding profiles are bounded by the actual reconstruction results (upper envelope to shaded areas) and by the reconstructed values corrected at low values of the SN (solid curves) by taking into account the residual level of solar activity in the limit of vanishing SN (see Fig. 1). The thick black curve shows the observed group sunspot number since 1610 and the thin blue curve gives the (scaled) 14C concentration in tree rings, corrected for the variation of the geomagnetic field [20]. The horizontal bars with attached arrows indicate the times of great minima and maxima [21]: Dalton minimum (Dm), Maunder minimum (Mm), Spo¨rer minimum (Sm), Wolf minimum (Wm), Oort minimum (Om), and medieval maximum (MM). The temporal lag of 14C with respect to the sunspot number is due to the long attenuation time for 14C [19].

Die Anzahl der Sunspots und die Sonneneinwirkung in generi für diesen Zeitraum ist zweifellos ein weiter zu verfolgender Ansatz, um die klimatischen Schwankungen von damals (mit) zu erklären.