28. Februar 2011

Changes on the North Icelandic shelf during the last millennium


Aus dem Abstract:
Warm and stable conditions with relatively strong influence of the Irminger current on the North Icelandic shelf are indicated during the interval AD 940-1300, corresponding in time to the Medieval Warm Period (MWP).

Viva Espania - Spanien und die MWP

Seit kurzem ist auf sciencedirect ein eingereichtes und akzeptiertes Manuskript von Martin-Chivelet et al., unter dem Titel Land surface temperature changes in Northern Iberia since 4000 yr BP, based on Delta13 C of speleothems als "Article in press" abrufbar.

Aus dem Abstract:
Main climatic recognized periods are: [...] (5) 1350-750 yr BP warm period (Medieval Warm Period) punctuated by two cooler events at ~ 1250 and ~ 850 yr BP;

16. Februar 2011

Ein Blick zurück - 1966

This epoch appears to show most of the same characteristics as the post-glacial epoch both in the Northern Hemisphere and in the Antarctic, only in less degree, perhaps because of its shorter duration.
The Arctic pack ice had melted so far back that appearences of drift ice in waters near Iceland and Greenland south of 70° N. were rare in the 800s and 900s and apparently unknown between 1020 and 1200, when a rapid increase of frequency began. This evidence hardly supports Brooks' suggeston that the Arctic Ocean again became ice-free during this epoch, though "permanent" ice was probably limited to inner Arctic areas north of 80° N. and possibly not including the Canadian Archipelago (to judge from occasional exploits there by the Old Norse Greenland colonist).[1] From the evidence of early Norse burials and plant roots in ground now permanently frozen in southern Greenland, annual mean temperatures there must have been 2°-4°C. above present values. It seems probable that sea temperatures in the northernmost Atlantic were up by a similar amount.
In western and mittle Europe vineyards extended generally 4°-5° latitude farther north and 100-200 metres higher above sea level than at present (Lamb, 1959). Estimates of the upper limits of the forests and of tree species on the Alps and more northern hills in central Europe range from 70 to 200 metres above where they now stand (Gams, 1937; Firbas and Losert, 1949). These figures suggest mean summer temperatures about 1° C., or a little more, above those now normal.
In North America archaeological studies in the upper Missisippi valley (approx. 45° N.) suggest a warm dry epoch, followed by a change to cooler, wetter conditions after A.D. 1300 (Griffin, 1961).
In lower latitudes Brooks (1949, p. 327, 355) names this as a wet period in central America (Yucatan) and probably in Indo-China (Cambodia). there is evidence of greater rainfall and larger rivers in the Mediterranean and the Near East (Butzer, 1958, p. 12). There is some evidence of a moister period in the Sahara from 1200 or earlier, lasting until 1550 (Brooks, 1949, pp. 330-8).
In southernmost South America the forest was receding rapidly to western aspects only, indicating a drier climate than in the previous epoch and more predominant westerly winds.
On the coast of east Antarctica, at Cape Hallett, a great modern penguin rookery appears from radiocarbon tests to have been first colonized between about A.D. 400 and 700, presumably during a phase of improving climate, and to have been occupied ever since (Harrington and McKellar, 1958). this tends to confirm the earlier assumption of explorers of the Bunger Oasis in east Antarctica of a period of marked climatic improvement about a thousand years ago, since which there has been only a modest reversion.
  Lamb, H. H.: SECONDARY CLIMATIC OPTIMUM (CIRCA A.D. 1000 to 1200),
in: The nature of certain climatic variations, in: Lamb, H. H. (editor):
The changing climate, selected papers,
London: Methuen 1966, 64f.


Es bleibt den Lesern überlassen, zu überprüfen, welche Aussagen mittlerweile als bestätigt gelten können und welche nicht. 

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[1] Glaciological and other studies of Arctic ice islands, and their presumed growth when formerly part of the Ellesmere Land ice shelf, have not so far been reduced to an agreed time scale (Crary, 1960, p. 34; Stoiber et al., 1960, p. 71). It seems most probable, however, that the ablation period in progress in the early 1950s began only about 40 years ago and that the total age of the ice is 620 years of less, implying growth during the Little Ice Age epoch and that net ablation prevailed before that, during the secondary climatic epoch, in the Canadian Archipelago.


Literatur
Brooks, C. E. P. 1949. Climate through the ages. 2nd ed. London, Ernest Benn.
Butzer, K. W. 1958. Studien zum vor- und frühgeschichtlichen Landschaftswandel der Sahara, Abh. math.-nat. Kl. Akad. Wiss. Mainz, no. 1.
Crary, A. P. 1960. Arctic ice island and ice shelf studies, Scientific studies at Fletcher's Ice Island, T-3: 1952-1955. Geophysical Research Papers (no. 63), vol. III, p. 1-37. Boston, Air Force Cambridge Research Center. (AFCRC-TR-59-232(3) ASTIA document no. AD-216815.)
Firbas, F.; Losert, H. 1949. Untersuchungen über die Entstehung der heutigen Waldstufen in den Sudeten, Planta (Berlin), vol. 36, p. 478-506.
Gams, H. 1937. Aus der Geschichte der Alpenwalder, Zeitschrift des deutsch. und österreich. Alpenvereins (Stuttgart), vol. 68, p. 157-170.
Griffin, J.B. 1961. Some correlations of climatic and cultural change in eastern North American prehistory, New York Academy of Sciences, Symposium on solar variations, climatic changes and related geophysical problems (Publication pending).
Harrington, H. J.; McKellar, I. C. 1958. A radio-carbon date for penguin colonization of Cape Hallett, Antarctica, N.Z. J. Geol. Geophys., vol. 1, p. 571-576.
Lamb, H. H. 1959. Our changing climate, past and present, Weather, vol. 14, p. 299-318.
Stoiber, R. E.; Lyons, J. B.; Elberty, W. T.; McCreahan, R. H. 1960. Petrographic evidence on the source area and age of T-3, Scientific studies at Fletcher's Ice Island, T-3: 1952-1955. Geophysical Research Papers (no. 63), vol. III, p. 78, Boston, Air Force Cambridge Research Center. (AFCRC-TR-59-232(3) ASTIA document no. AD-216815.)

6. Februar 2011

Drought duration and frequency in the U.S. Corn Belt

In der neuen Studie Drought duration and frequency in the U.S. Corn Belt during the last millennium (AD 992-2004) (Pdf-Format, ca. 890 KB), stellen sich die Autoren der nicht ganz unwichtigen Aufgabe, (to) "characterize the historic occurrence, duration, and frequency of drought events" (Stambaugh et al., 155).

Aus dem Abstract:
We used a new long tree-ring chronology developed from the central U.S. to reconstruct annual drought and characterize past drought duration, frequency, and cycles in the U.S. Corn Belt region during the last millennium. This is the first paleoclimate reconstruction achieved with subfossil oak wood in the U.S. and increases the current dendroclimatic record in the central U.S. agricultural region by over 500 years. A tree ring-width drought response function was calibrated and verified against monthly instrumental Palmer Hydrologic Drought Index (PHDI) during the summer season (JJA).


Für uns von Interesse, das Mittelalter. Wir finden dazu in der Studie folgende Aussagen:
In the thirteenth-century the most severe multidecadal drought in the entire record occurred during the 20-yr period 1229-1248 (Table 4). This drought pre-dates by several decades the end-of-century "Great Drought" of the Pueblo area in the southwest (Douglas, 1935). A drought almost twice as long occurred in the late 11th century (Table 4), but the approximately 61-yr drought in the late 12th century (ca. AD 1148-1208) appears to be the most significant drought of the entire reconstruction. This period corresponds to the single greatest megadrought in North America during the last 2000 years (Cook et al., 2007) and unmatched persistent low flows in western U.S. river basins (Meko et al., 2007). The region affected by this drought appears to have also included portions of the central U.S. (Laird et al., 1996). This drought marks the middle of the Medieval Warm Period  - an interval of warmer temperatures between approximately AD 800-1300 characterized by greater drought duration and frequency in the Northern Great Plains compared to more modern times.
Stambaugh, et al., 160f.


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[1] Douglass, A.E., 1935. Dating Pueblo Bonito and other ruins of the Southwest. Nat. Geog. Soc. Contrib. Tech. Papers.
[2] Cook, E.R., Seager, R., Cane, M.A., Stahler, D.W., 2007. North American drought: reconstructions, causes, and consequences. Earth Science Reviews 81, 93-134.
[3] Meko, D.M., Woodhouse, C.A., Baisan, C.A., Knight, T., Lukas, J.J., Hughes, M.K., Salzer, M.W., 2007. Medieval drought in the upper Colorado River Basin. Geophysical Research Letters 34, L10705.
[4] Laird, K.R., Fritz, S.C., Maasch, K.A., Cumming, B.F., 1996. Greater drought intensity and frequency before AD 1200 in the Northern Great Plains, USA. Nature 384, 552-554.

2. Februar 2011

Was haben Guizhou/Südwest China und Montana/Nordwest USA gemeinsam?

Aus dem Abstract des "Article in press", Climate and environmental changes during the past millennium in central western Guizhou, China as recorded by Stalagmite ZJD-21 von Tz-Shing et al., erfahren wir Interessantes aus der Zhijin Cave in Zhijin County in Guizhou. So schreiben die Autoren:
The ZJD-21 Delta18O record suggests: (1) dry/warm climates during AD 950-1100 (overlapping with most of the Medieval Warm Period, MWP, in Europe); (2) strengthening of the summer monsoon from the MWP toward the beginning of the Little Ice Age (LIA) at AD 1250; (3) realtively wet/cold conditions occurred between AD 1250 and 1500, shown by relatively light Delta18O values;


Ebenfalls möchte ich hier die Studie, Geomorphic and climatic change over the past 12,900 yr at Swiftcurrent Lake, Glacier National Park, Montana, USA (Pdf-Format, 1,5 MB) von MacGregor et al. vorstellen. 

Die Autoren führen aus:
A sediment core, representing a 12,900-yr record collected from Swiftcurrent Lake, located on the eastern side of Glacier National Park, Montana, was analyzed to assess variability in Holocene and latest Pleistocene environment.
MacGregor et al., Abstract


Für uns von Interesse die Analyse der "Period V: 1,3 ka to present":
The most recent period in the Swiftcurrent Lake core ist characterized by generally high %TOC, low C/N ratios and fine-grained sediment (Fig. 7). Period V brackets well-defined climate events including the Medieval Warm Period (MWP) and the Little Ice Age (LIA), and changes in the %TOC record are well-correlated with documented timing of climate changes defining the MWP and LIA. For example, %TOC is relatively high and stable between 1.3 and 0.7 ka, corresponding closely with slightly increased GISP 2 temperatures.

MacGregor et al., 87