13. Januar 2011

The Glaciers of Kilimanjaro

In ihrem neuen Artikel The Glaciers of Kilimanjaro nehmen Sherwood, Keith und Craig Idso Bezug auf die Arbeit: Is the decline of ice on Kilimanjaro unprecedented in the Holocene? (Pdf-Format, 5,7 MB), von Kaser et al., 2010.

Die Idso's schreiben, für uns von Interesse u.a.:
In addition, their analyses suggest that the mountain's plateau ice "may have come and gone repeatedly throughout the Holocene' and ...

Bei Kaser et al. finden wir in der Sektion "Summary and conclusions", unter Punkt 8:
The hypothesized ice history indicates that the plateau ice may have come and gone repeatedly throughout the Holocene.

Fairerweise muss man hier m.E. "the hypothesized ice history" erörtern. Es sollte hinzugefügt werden, dass sich Kaser et al. "Lake level records" (Lake Naivasha: Rainfall and drought in equatorial East Africa during the past 1,100 years [Abstract][1]; Lake Victoria: Solar variability and the levels of Lake Victoria, East Africa, during the last millennium [Abstract] [2]) bedient haben "in order to resolve a history of plateau ice on Kibo [Kilimanjaro's highest peak, W.v.B.]", wie aus Unterpunkt "A millennium-scale plateau glacier history and beyond" herauslesbar ist.

Aus einer Zusammenschau der Daten entwickeln sie dann ihre schlussendliche Interpretation in Punkt 7:
We interpret wet periods from lake level stands in East Africa as glacier growth periods on Kibo and postulate that after each wet period the shrinkage of plateau ice starts again. As a consequence, the application of the decay time period to the time series of lake level stands leads to the ice history of the Kibo summit plateau.

Bezüglich klimatischer Bedingungen während des Mittelalters kann man folgende Aussagen anführen:
Under both of our growth and retrait scenarios (Figure 11B and C) an absence of plateau glaciers before 1200 is shown and it is very probable that such extended dry periods have occurred several times throughout the Holocene (e.g. Bonnefille and Chalie, 2000)[3]. Besides the Naivasha record (Figure 11A), the reconstructed shallow water diatom (SWD, low percentages during high water levels) record in Lake Victoria (Figure 11D) supports the mentioned hiatus in plateau glaciation, showing above-average values (>= ~40%) after AD 1000 and a pronounced increase towards AD 1200 (Stager et al., 2005).
Kaser et al., 1087


Wenn wir hier die Abwesenheit des "plateau glaciers" um 1200 annehmen und die Zunahme in der Kleinen Eiszeit, gefolgt von einem "date of onset of the present shrinkage at around 1875 +/- 10 years" (1082), dies in Beziehung setzen zu den Aussagen von Wanner et al., 2008 (Pdf-Format, 3,63 MB):
... to the Little Ice Age (LIA; Wanner et al., 2000; Grove, 2004; Holzhauser et al., 2005; Matthews and Briffa, 2005)[4], which is the most recent and the most prominent of the periods of glacier advance during the Late Holocene, ...
Wanner et al., 1793

und
... So far, the LIA is the only period during the Holocene for which glacial advances have been identified in all parts of the globe ...
Wanner et al., 1801 


dann scheint mir unsere Definition einer MWP, gekoppelt - wo nötig - an ein Auftreten einer LIA im Untersuchungsgebiet, nicht so verkehrt zu sein.*

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[1] Verschuren D, Laird KR and Cumming BF (2000) Rainfall and drought in equatorial East Africa during the past 1,100 years. Nature 403: 410-414.
[2] Stager JC, Ryves D, Cumming BF, Meeker LD and Beer J (2005) Solar variability and the levels of Lake Victoria, East Africa, during the last millennium. Journal of Paleolimnology 33: 243:251.
[3] Bonnefille R, Chalie F (2000) Pollen-inferred precipitation time-series from equatorial mountains, Africa, the last 40 kyr BP. Global and Planetary Change 26: 25-50.

Aus dem Abstract:
The most characteristic feature of the synthetic curve is the changing variability through time.
During the last glacial period (30 to 15 kyr BP interval), our results indicate a ca. 450 mm/yr (32%) precipitation decrease relative to the present. Two maxima decrease periods are registered at ca. 19 kyr BP (700 mm/yr, ca. 45%) and between 18 and 16 kyr BP (608 mm/yr, ca. 42%). Several abrupt positive peaks of higher precipitation are noticed pre 35 kyr BP and ca. 22–20 kyr BP.
During the lower Holocene (10–7 kyr BP), the mean calculated precipitation estimate is 30 mm/yr (2%) below the present-day value, with several abrupt positive shifts; the maximum at ca. 8 kyr BP reaches 600 mm/yr (42%). Great variations between low and high precipitation values are expressed post ca. 4 kyr BP.

[4] Wanner, H., Holzhauser, H., Pfister, C., Zumbühl, H., 2000. Interannual to century scale climate variability in the European Alps. Erdkunde (Earth Scinece) 54, 62-69.
Grove, J.M., 2004. Little Ice Ages: Ancient and Modern. Routledge, New York.
Holzhauser, H., Magny, M., Zumbühl, H.J., 2005. Glacier and lake-level variations in west-central Europe over the last 3500 years. The Holocene 15, 789-801.
Matthews, J.A., Briffa, K.R., 2005. The 'Little Ice Age': re-evaluation of an evolving concept. Geografiska Annaler 87 A, 17-36.

* Es bleibt zu bedenken, dass es auch während der MWP zu "some glacier advances" gekommen ist, wobei die Frage gestellt werden muss, ob diese "advances" größer waren als jene während der LIA.

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ZUSATZ:  

16. Januar 2011

In einer Arbeit (Pdf-Format, 1,56 MB) aus dem Jahre 1994 wird "glacial geological Evidence" für eine weltumspannende MWP angeführt. Grove und Switsur konkludieren:
The first phase of the LIA began around the thirteenth century in all the regions for which there is evidence. The glacial phase preceding the MWP seems to have begun between the seventh and ninth centuries A.D. but is generally less securely dated and not dated at all in Canada. There is at least some indications of fluctuations in ice position in the course of the MWP in Norway, Alaska, and perhaps in extratropical South America and New Zealand, indicating that recession may have been interrupted by advances, perhaps of limited extent, as in the European Alps. The available evidence suggests that the MWP was global in extent and not uniform climatically. The glacial data needs to be considered in relation to that from other sources, but is of value in obtaining a more complete understanding of both the environment in the later medieval period and the possible causes of climatic change on the century time scale.
Grove, Switsur, 166