Ricostruzione del clima del passato - Qui i contributi

[highway to hell]

[highway to hell]

[highway to hell]

Chappice Lake, Alberta, Canada
Reference
Vance, R.E., Clague, J.J. and Mathewes, R.W. 1993. Holocene palaeohydrology of a hypersaline lake in southeastern Alberta. Journal of Paleolimnology 8: 103-120.


Description
The authors reconstructed a history of Holocene water level and chemical changes for Chappice Lake (50.17°N, 110.37°W), a shallow hypersaline lake located near the limit of the northern mixed-grass prairie in the northern Great Plains of southern Alberta, Canada.

The Medieval Warm Period was described as an interval of severe recurrent drought that was "likely related to higher temperatures" between AD 825 and 1125.

Comparison of the MWP droughts with those of the historical record reveals that the droughts of the past century "do not appear to match in severity or longevity" those of Medieval times, which suggests that the "higher temperatures" to which the MWP droughts were related were likely higher than those of the recent past.

[highway to hell]

Dog Lake, British Columbia, Canada
Reference
Hallett, D.J., Mathewes, R.W. and Walker, R.C. 2003. A 1000-year record of forest fire, drought and lake-level change in southeastern British Columbia, Canada. The Holocene 13: 751-761.



Description
The dendrochronological fire history of the forested area surrounding Dog Lake (50.77°N, 116.1°W) was compared with a high-resolution charcoal record derived from a sediment core extracted from the lake to reconstruct the region's fire history over the past 1000 years.

In addition, the authors constructed a proxy record of lake-level change based on accumulation rates of Chara globularis-type oospores in the lake sediment core.

These analyses revealed the presence of frequent forest fires and lowered lake levels during the Medieval Warm Period (AD 980-1270), which in the words of the authors support evidence of "warmer and drier climate than today."

[highway to hell]

Igaliku Fjord, South Greenland


Reference
Lassen, S.J., Kuijpers, A., Kunzendorf, H., Hoffmann-Wieck, G., Mikkelsen, N. and Konradi, P. 2004. Late-Holocene Atlantic bottom-water variability in Igaliku Fjord, South Greenland, reconstructed from foraminifera faunas. The Holocene 14: 165-171.


Description
The authors conducted a foraminifera, magnetic susceptibility and oxygen isotope multiproxy analysis of an ocean sediment core retrieved from Igaliku Fjord (60.70°N, 46.03°W), south Greenland, to obtain a paleohydrographic record of this region during the late Holocene.

According to Lassen et al.'s interpretation of the data, the Medieval Warm Period was a time of "relatively warm climate in terms of surface water temperature" between AD 885 and 1240, which was punctuated by enhanced mixing and wind stress that likely indicates sea ice was lacking during this time.

Based upon data presented in their Figure 3 on % distribution of selected foraminiferal species, δ18O and magnetic susceptibility, plus the relationship inferred from these data with temperature, we can conclude that current warmth has not yet reached the level of Medieval warmth.

[highway to hell]

Kootenay Valley, Southern Canadian Rockies, Canada
Reference
Hallett, D.J. and Hills, L.V. 2006. Holocene vegetation dynamics, fire history, lake level and climate change in the Kootenay Valley, southeastern British Columbia, Canada. Journal of Paleolimnology 35: 351-371.


Description
Working with a sediment profile obtained from Dog Lake, British Columbia (50°46'N, 116°06'W), the authors reconstructed the Holocene environmental history of Kootenay Valley in the southern Canadian Rockies, based on data related to fire frequency, drought and forest type and cover, all of which are influenced by temperature and precipitation.

This effort revealed that current global warming is only just beginning to orchestrate the return of the Valley landscape back to what it was like in medieval times (AD 800-1200).

[highway to hell]

Nebraska Sand Hills, Western North America
Reference
Sridhar, V., Loope, D.B., Swinehart, J.B., Mason, J.A., Oglesby, R.J. and Rowe, C.M. 2006. Large wind shift on the Great Plains during the Medieval Warm Period. Science 313: 345-347.


Description
The authors studied the orientation, morphology and internal structure of dunes in the easternmost portion of the Nebraska Sand Hills (in about the center of the state), where shallow core and outcrop samples indicate the dunes were formed some 800 to 1000 years ago.

Using a computer program, they then calculated what type of wind field had to have been operative during the MWP of AD 1000-1200 in order to produce the dunes, which are oriented quite differently from those that would form under today's wind regime (where air currents from the south in the spring and summer bring moist air from the Gulf of Mexico to the U.S. Great Plains) if the dune sand were free to move and not protected by prairie grasses.

Their work revealed that the spring and summer winds of the MWP would have had to have come primarily from the southwest, bringing much drier and hotter-than-current air from the deserts of Mexico, along with greatly reduced opportunities for rain.

In addition, they note that the drier and warmer conditions may have been even further "enhanced and prolonged by reduced soil moisture and related surface-heating effects," which, we might add, are not operative in our day to the degree they were 800 to 1000 years ago, as was demonstrated by still other of Sridhar et al.'s computer analyses.

The overall effect of these phenomena was suggested by them to have impacted much of western North America and the U.S. Great Plains.

[highway to hell]

Sierra Nevada, California, USA

Reference
Lloyd, A.H. and Graumlich, L.J. 1997. Holocene dynamics of treeline forests in the Sierra Nevada. Ecology 78: 1199-1210.


Description
3500-year histories of (1) treeline elevation fluctuation and (2) tree abundance were derived for five sites in the southern Sierra Nevada (~36.5°N, 108.25°W). The results indicated (1) synchronous increases in treeline elevation from AD 800 to 900 and (2) an episode of high tree abundance above the current treeline between AD 700 and 1200, which implies warmer-than-present temperatures during that period of time.

[Jadan]

Senti, Highway to hell, fa' prima a dare questo link, per piacere

http://www.co2science.org/scripts/CO...eB2C/Index.jsp

da cui è tratta la maggioranza delle tue citazioni (come questa)

http://www.co2science.org/scripts/CO...2_kootenay.jsp

Per la cronaca, questo sito, come il cugino GreeningEarth Society (GES), sostiene chge la CO2 fa un sacco bene perché aiuta la fotosintesi clorofilliana e quindi le piante e quindi il verdeggiamento del pianeta. La GES si sa perfettamente che è una filiazione della Western Fuel, i produttori di carbone. Di questa CO2 Science non sono stati trovati apparentamenti diretti anche se alcuni tra i loro fondatori furono anch'essi a libro paga della Western Fuel. In ogni caso, potrete comprare, in questo sito, un brillante video che mostra (a soli 12 dollari) come: "GOPE1_1992DVD - The Greening of Planet Earth: The Effects of Carbon Dioxide on the Biosphere
Is carbon dioxide a harmful air pollutant, or is it an amazingly effective aerial fertilizer? Explore the positive side of the issue in this half-hour documentary -- The Greening of Planet Earth - yours free today with a qualifying tax deductible donation of $12 plus shipping and handling. DVD burned in -R format, original release date 1992"

Dopodiché, gli articoli che loro citano sono certamente veri e indipendenti. Ma l'affastellamento casuale, il portare a confondere date e situazioni (come fa indirettamente hth) porta ad un rumore di fondo che non fa capire nulla.

[Enrico_3bmeteo]

Certamente, ma come dicevo poco sopra, se è MWP un riscaldamento tra l'800 e il 1000, non può essere MWP un riscaldamento avvenuto agli antipodi tra il 1250 e il 1500. Quando nel 1000 qui faceva caldo (è assodato) cosa faceva agli antipodi?

Ricordiamoci che noi parliamo di aumento medio globale delle temperature. Se in un posto ha fatto più caldo rispetto a oggi nel IX sec. e in un altro nel XIII sec. può benissimo essere che, in media e globalmente, OGGI faccia più caldo sia del IX che del XIII secolo globalmente intesi. Anche se, sia in un secolo che nell'altro, c'erano luoghi in cui faceva più caldo di ora.

Non so se riesco a spiegarmi ma è veramente difficile dialogare con le lenzuolate che separano un messaggio dall'altro...

Beh, è inteso che i riferimenti siano puntuali e non globali.
Io ho valutato l'attendibilità dei rilievi di Giraudi confrontandoli con le fasi di un ghiacciaio tipico alpino e da questo posso trarre delle conclusioni circoscritte all'arco alpino (ed appenninico). Se avessimo altri grafici con le oscillazioni glaciali di altri paesi potremmo fare lo stesso e poi alla fine riassumere il tutto in un quadro complessivo
Se mi tiri in ballo la temperatura media globale decade tutto perché la puntualità si perde nella generalizzazione tipica delle medie, quelle che io amo definire "la scienza madre dell'approssimazione"....