23. Dezember 2010

Alaska und die MWP

In der letzten Ausgabe der Quaternary Science Reviews ist eine interessante Studie von Clegg et al., mit dem Titel Six millennia of summer temperature variation based on midge analysis of lake sediments from Alaska (pdf-Format, ca. 690 KB), erschienen. 

Die Autoren stellen einleitend fest:
We analyzed sediments of the past 6000 years from Moose Lake (61°22.45'N, 143°35.93'W, 437 m a.s.l.) in south-central Alaska for midge assemblages at decadal to centennial resolution. Here we report the results and use the assemblage data to derive mean July air temperature (TJuly) estimates using the transfer function of (Barley et al. 2006)[1]. The reliability of this approach at Moose Lake is evaluated by comapring midge-based TJuly estimates with weatherstation and treering-based temperature data from the same region (Davi et al., 2003)[2]. We then discuss temperature fluctuations at centennial and millenial timescales over the past six millennia and explore the factors causing these fluctuations.
Clegg et al., 3309.


Für uns von Interesse ist im Speziellen die Auswertung der gewonnenen Daten unter Punkt 4.3: "Temperature fluctuations over the past 2000 years: comparison with previous results". Wir lesen:
A number of recent paleoclimate studies in Alaska and elsewhere have focused on high-resolution reconstructions over the past two millennia (e.g. Hu et al., 2001; Loso et al., 2006; McKay et al., 2008)[3]. Comparisons of the TJuly record from Moose Lake with other Alaskan temperature records suggest that the regional coherency observed in instrumental temperature records (e.g. Wiles et al., 1998; Gedalof and Smith, 2001; Wilson et al., 2007)[4] extends broadly to at least 2000 cal BP. For example, climatic events such as the LIA and the Medieval Climate Anomaly (MCA; peak warmth around 1000 cal BP) occurred largely synchronously between our TJuly record from Moose Lake (Fig. 4C) and a Delta18O-based temperature record from Farewell Lake on the northwestern foothills of the Alaska Range (Hu et al., 2001; Fig. 4D).

Man kann also getrost behaupten, dass in vorliegender Studie Evidenz für die Existenz einer MWP gefunden wurde (ich betrachte nach wie vor die Bezeichnung Klimaanomalie in diesem Zusammenhang als irreführend; man möge sich Fig. 3 A, samt Beschreibung zu Gemüte führen; für mich ist hier ein deutlicher Temperaturanstieg von 600 A.D. mit einem Peak um 1200 A.D. ersichtlich).

Doch wie verhält es sich mit den anderen Studien; mit den Studien von Hu et al., 2001; Loso et al., 2006; McKay et al., 2008 und nicht erwähnten Arbeiten - verwendet von Ljungqvist, in seiner neuesten Studie - wie D'Arrigo et al. 2006 oder Loso 2009? 

Die älteste, erwähnte regionale Langzeitstudie für Alaska ist von Hu et al., erschienen unter dem Titel Pronounced climatic variations in Alaska during the last two millennia. Sie scheint, wie wir gleich im Zitat anführen werden, auch die erste hochwertige Langzeitstudie für Alaska zu sein, darum ist es zudem angebracht, hier zu beginnen.

Hu et al. gestalteten
multiproxy geochemical analyses of a sediment core from Farewell Lake (62° 33' N, 153° 38' W, 320 m altitude) in the northwestern foothills of the Alaska Range (Fig. 1A).
These analysis provide the first high-resolution (multidecadal) quantitative record of Alaskan climate variations that spans the last two millennia.
Hu et al., 10552.


Sie gelangten hierbei zu folgenden Resultaten:
The relatively warm climate A.D. 850-1200 at Farewell Lake corresponds to the Medieval Climatic Anomaly, a time of marked climatic departure over much of the planet (9,32)[5]. 
Our SWT reconstruction at Farewell lake indicates that although the 20th century, represented by the uppermost three samples, was among the warmest periods of the past two millennia, two earlier intervals may have been comparably warm (A.D. 0-300 and A.D. 850-1200). These data agree with tree-ring evidence from Fennoscandia, indicating that the recent warmth is not atypical of the past 1000 years (33, 34).[6]
[Von Interesse Fig. 3.; die durchschnittlich höheren Temperaturen im Mittelalter gegenüber der Kleinen Eiszeit sind meines Erachtens claire and distincte ablesbar, W.v.B.]
Hu et al. 10555.


Loso et al. finden in ihrer 2006, in Quaternary Research publizierten Arbeit A 1500-year record of temperature and glacial response inferred from varved Iceberg Lake, southcentral Alaska [pdf-Format, ca. 1 MB]) ebenfalls Evidenz für die Existenz einer MWP im Untersuchungsgebiet vor.

Sie führen folgendermaßen ein:
In this paper, we present a new 1500-yr-long varve record from iceberg lake, a proglacial lake in the heart of southern Alaska's icefileds.  ... Even taking into account the evidence for early onset o the Little Ice Age in southern Alaska around 1200 A.D. (Wiles et al., 2002)[7], this chronology clearly spans a complete warm-cold-warm cycle, including clear expressions of both the LIA and the MWP that preceded it.
Loso et al., 13.


Um letztlich konkludieren zu können:
Relative to the entire period of record, temperatures suggested by the chronology were lowest around 600 A.D., increased slowly and sporadically from then until reaching a relatively stable value in the Medieval Warm Period between A.D. 1000 and 1250, cooled again towards a sustained period of Little Ice Age cold between A.D. 1500 and 1850, and have increased dramatically since then.
Loso et al., 23.


Als nächste Arbeit wollen wir On the long-term context for late twentieth century warming (pdf-Format, 920 KB) von D'Arrigo et al., erschienen in Journal of Geophysical Research, anführen. D'Arrigo et al. "develop two new tree-ring-based reconstructions of NH temperatures that address several priorities recommended for the generation of such records" (2). Die in Alaska liegenden Proxydaten, welche weit genug zurückreichen, weisen eine MWP auf, klar ersichtlich in Table 1., an den Daten "Seward" und "Coastal Alaska", abzulesen in Figure 2. Generell in Bezug auf die NH konkludieren D'Arrigo et al. nachstehend:
We have presented STD and RCS NH temperature reconstructions for the past 1259 years. In so doing, we have addressed several recommended priorities [Esper et al., 2005b][8] for the development of large-scale reconstructions (see above). ... The NH RCS reconstruction displays pronounced variability, including significant "MWP" and "LIA" departures.
D'Arrigo et al, 11.


Auch die Studie von McKay et al., Biogenic silica concentration as a high-resolution, quantitative temperature proxy at Hallet Lake, south-central Alaska (pdf-Format, 1,35 MB), erschienen in Geophysical Research Letters, zeigt eine MWP an. Die Autoren schreiben:
BSi-inferred summer temperatures were warmer than the long-term (2ka) average (3.1 °C) from ca. 100 to ca. 500 AD, before decreasing rapidly to ca. 2°C by 600 AD (Figure 3). This cooling coincides with a period of glacial advance in the Chugach Range, and the Kenai and St. Elias Mountains [Wiles et al., 2008][9]. After ca. 600 AD, summer temperature gradually icreased from 2 to ca. 4°C by 1300 AD, and remained above the 2 ka average until ca. 1500 AD.
Mc Kay et al., 5


Die letzte Studie, Summer temperatures during the Medieval Warm Period and Little Ice Age inferred from varved proglacial lake sediments in southern Alaska, (pdf-Format, 620 KB) erschienen im Journal of  Paleolimnology, kann als "Update" zu seiner Studie von Loso aus dem Jahre 2006 gesehen werden und zeigt ebenfalls, klar ersichtlich, eine MWP in der Studiengegend an. Im Abstract fasst Loso zusammen:
A Medieval Warm Period is evident from 1000 to 1100 AD, but the temperature reconstruction suggest it was less warm then recent decades - an observation supported by independent geological evicence of recent glacier retreat that is unprecedented over the period of record.
Loso, 117.


In seiner Ausarbeitung der Temperaturrekonstruktion schreibt Lobo weiters:
Temperatures were generally higher than average from 950 to 1350 AD, and after a brief rise around 1850 AD reached their highest levels after 1950 AD.
Loso, 125.


Letztlich konkludiert Loso folgendermaßen:
Second, the Medieval Warm Period is evident in the record, and  based on that and other evidence I suggest that it most clearly and consistently manifest itself in southern Alaska during the eleventh century AD.
Loso, 127.


Halten wir fest:
Die neue Studie von Clegg et al. (2010) kann als eine von vielen Studien angesehen werden, die Evidenz für die Behauptung liefert, dass es auch in Alaska eine MWP gab.

Frühere Studien von Hu (2001), Loso et al. (2006) und  Mc Kay et al. (2008) sowie bei Clegg et al. nicht angeführte, jedoch von Ljungqvist  (2010) verwendete, weitere Studien von D'Arrigo et al. (2006) und Loso (2009) zeichnen ein ähnliches Bild, wenngleich des Öfteren von einer Klimaanomalie gesprochen wird.

__________

[1] Barley, E.M., Walker, I.R., Kurek, J., Cwynar, L.C., Mathewes, R.W., Gajewski, K., Finney, B.P., 2006. A northwest North American training set: distribution of freshwater midges in relation to air temperature and lake depth. Journal of Paleolimnology 36, 295-314.
[2] Davi, N.K., Jacoby, G.C., Wiles, G.C., 2003. Boreal temperature variability inferred from maximum latewood density and tree-ring width data, Wrangell Mountain region, Alaska. Quaternary Research 60, 252-262.
[3] Aufgeführte Studien werden im Lauf des Posts erörtert.
[4] Wiles, G.C., D'Arrigo, R., Jacoby, G., 1998. Gulf of Alaska atmosphere-ocean variability over recent centuries inferred from coastal tree-ring records. Climatic Change 38, 289-306.
Gedalof, Z., Smith, D.J., 2001. Interdecadal climate variability and regime scale shifts in Pacific North America. Geophysical Research Letters 28, 1515-1518.
Wilson, R., Wiles, G., D'Arrigo, R., Zweck, C., 2007. Cycles and shifts: 1,300 years of multi-decadal temperature variability in the Gulf of Alaska. Climate Dynamics 28, 425-440.
[5] Stine, S. (1994) Nature (London) 369, 546-549.
Broecker, W. S. (2001) Science 291, 1497-1499.
[6] Briffa, K. R., Bartholin, T. S., Eckstein, D., Jones, P. D., Karlen, W., Schweingruber, F. H. & Zetterberg, P. (1990) Nature (London) 346, 434-439.
Briffa, K. R., Jones, P. D., Bartholin, T. S., Eckstein, D., Schweingruber, F. H., Karlen, W., Zetterberg, P. & Eronen, M. (1992) Climate Dyn. 7, 111-119.
[7] Wiles, G. C., Jacoby, G.C., Davi, N.K., McAllister, R.P., 2002. Late Holocene glacier fluctuations in the Wrangell Mountains, Alaska. Geological Society of America Bulletin 114, 896-908.
[8] Esper, J., R. J. S. Wilson, D. C. Frank, A. Moberg, H. Wanner and J. Luterbacher (2005b), Climate: Past ranges and future changes, Quat. Sci. Rev., 24, 2164-2166.
[9] Wiles, G. C., D. J. Barclay, P. E. Calkin, and T. V. Lowell (2008), Century to millennial-scale temperature variations for the last two thousand years indicated from glacial geologic records of southern Alaska, Global and Planetary Change, Volume 60, Issues 1-2, January 2008, Pages 115-125.