Michael Mann og The Hockey Stick

Hvis jeg bidrager med indlæg , henvender jeg mig til andre på klimadebat, men ikke til dig

Christoffer Bugge Harder skrev:

Jeg er egentlig personligt helt ligeglad med Mann selv - han er helt sikkert meget tyndhudet, ærekær og utilbøjelig til at rette fejl i sit eget arbejde, og jeg synes også, at han bare skulle have frigivet alle sine data + kode noget tidligere i hele forløbet, end han gjorde. Jeg betragter alle disse skænderier omkring hans person, eller detaljer i, hvem der sagde eller gjorde hvad i 1998, som distraktioner fra spørgsmålet om det mest sandsynlige temperaturforløb i fortiden.

Christoffer Bugge Harder skrev:

- men du er vel enig i, at de kun siger noget om temperaturerne på polerne? og i, at temperaturen på polerne er en dårlig(ere) proxy for temperaturerne overalt på kloden ifht. andre proxydata direkte fra lokale kilder. Hvordan...

kristofferszilas skrev:
De kan da ikke være vigtige i den sammenhæng da de på forhånd har valgt at afvise mulige naturlige mekanismer, som jo først er blevet foreslået fornyligt af Svensmark, Lindzen, Soon, Scafetta etc.

Kristoffer: jeg anerkender naturligvis, at iskernerne er en glimrende proxy - men du er vel enig i, at de kun siger noget om temperaturerne på polerne? og i, at temperaturen på polerne er en dårlig(ere) proxy for temperaturerne overalt på kloden ifht. andre proxydata direkte fra lokale kilder. Hvordan kan du så vedblivende påstå, at iskernerne er det eneste, man kan bruge til noget, når næsten intet andet end denne lokale proxy tyder på en stor middelalderlig varmeperiode?

Christoffer Bugge Harder skrev:
Kristoffer: jeg anerkender naturligvis, at iskernerne er en glimrende proxy - men du er vel enig i, at de kun siger noget om temperaturerne på polerne?

og i, at temperaturen på polerne er en dårlig(ere) proxy for temperaturerne overalt på kloden ifht. andre proxydata direkte fra lokale kilder. Hvordan kan du så vedblivende påstå, at iskernerne er det eneste, man kan bruge til noget, når næsten intet andet end denne lokale proxy tyder på en stor middelalderlig varmeperiode? Fordi, den viser det, du gerne vil høre?

På lige præcis dét område er jeg jo sådan set ikke amatør, men får mad på bordet netop fordi jeg kan tolke hvad isotopfraktioneringer betyder og hvor stor vægt man kan tillægge sådanne data.

Jeg forstår ganske udmærket meget af det du siger CBH og fandt faktisk din lister over sorte og hvide får i flokken af skeptikere ret interessant (da jeg så bedre kan argumentere med kilder, der i dine øjne ikke er sorte får), men her føler jeg dog at du tager munden lige lidt for fuld.

Og mht. historien: Mig bekendt har vi primært nogle beretninger fra forskellige dele af (Nord- og Vest)Europa fra Middelalderen, og hvis man kigger på f.eks en proxy som den meget anerkendte fra bunden af Torneträsk-søen i Nordsverige, så viser den netop også en stor middelalderlig varme. Men det gør proxyer fra mange andre dele af verden altså ikke i samme grad - og mig bekendt har vi slet ingen beretninger fra Afrika, Nord-Sydamerika, Australien og store dele af Asien så langt tilbage som 700-1000 år siden.

kristofferszilas skrev:
Uanset om Michael Mann bevidst har fusket med data eller simpelthen bare har valgt at se bort kilder som ikke viser hvad han ønsker, så kan man nu fuldstændig ignorere hans "hockey stick" temperaturkurve, fordi den tydeligvis ikke repræsenterer den virkelige verden.

It is unfortunate that my research, "An ikaite record of late Holocene climate at the Antarctic Peninsula," recently published in Earth and Planetary Science Letters, has been misrepresented by a number of media outlets.

Several of these media articles assert that our study claims the entire Earth heated up during medieval times without human CO2 emissions. We clearly state in our paper that we studied one site at the Antarctic Peninsula. The results should not be extrapolated to make assumptions about climate conditions across the entire globe. Other statements, such as the study "throws doubt on orthodoxies around global warming," completely misrepresent our conclusions. Our study does not question the well-established anthropogenic warming trend.

Selvfølgelig viser disse data at der eksisterede en middelalderlig varme periode efterfulgt af den lille istid, når det samme mønster ses på hver sin side af kloden og derfor ikke kan tilskrives lokale forhold. Det er på ingen måde en overtolkning..

Our only ikaite crystal
formed during LIA does indicate significant cooling relative to the
MWP.

Our most recent crystals suggest a warming relative to
the LIA in the last century, possibly as part of the regional recent
rapid warming, but this climatic signature is not yet as extreme in nature
as the MWP

our ikaite record builds the case that the oscillations of the
MWP and LIA are global in their extent and their impact reaches as
far South as the Antarctic Peninsula, while prior studies in the AP region
have had mixed results.

The Medieval period is found to display warmth that matches or exceeds that of the past decade in some regions, but which falls well below recent levels globally. This period is marked by a tendency for La Niña–like conditions in the tropical Pacific. The coldest temperatures of the Little Ice Age are observed over the interval 1400 to 1700 C.E., with greatest cooling over the extratropical Northern Hemisphere continents.

The instrumentally measured warming of about 0.6°C during the 20th century is also reflected in borehole temperature measurements, the retreat of glaciers, and other observational evidence, and can be simulated with climate models.

Large-scale surface temperature reconstructions yield a generally consistent picture of temperature trends during the preceding millennium, including relatively warm conditions centered around A.D. 1000 (identified by some as the "Medieval Warm Period") and a relatively cold period (or "Little Ice Age") centered around 1700. The existence of a Little Ice Age from roughly 1500 to 1850 is supported by a wide variety of evidence including ice cores, tree rings, borehole temperatures, glacier length records, and historical documents. Evidence for regional warmth during medieval times can be found in a diverse but more limited set of records including ice cores, tree rings, marine sediments, and historical sources from Europe and Asia, but the exact timing and duration of warm periods may have varied from region to region, and the magnitude and geographic extent of the warmth are uncertain.
It can be said with a high level of confidence that global mean surface temperature was higher during the last few decades of the 20th century than during any comparable period during the preceding four centuries. This statement is justified by the consistency of the evidence from a wide variety of geographically diverse proxies.

Less confidence can be placed in large-scale surface temperature reconstructions for the period from A.D. 900 to 1600. Presently available proxy evidence indicates that temperatures at many, but not all, individual locations were higher during the past 25 years than during any period of comparable length since A.D. 900. The uncertainties associated with reconstructing hemispheric mean or global mean temperatures from these data increase substantially backward in time through this period and are not yet fully quantified.

Very little confidence can be assigned to statements concerning the hemispheric mean or global mean surface temperature prior to about A.D. 900 because of sparse data coverage and because the uncertainties associated with proxy data and the methods used to analyze and combine them are larger than during more recent time periods.

Temperatures inferred from borehole data are not calibrated against the instrumental temperature record, thereby providing an independent measurement of past temperature.

BOREHOLES IN GLACIAL ICE
.......Ice sheet boreholes permit longer timescale temperature reconstructions because of the purity of ice, the great depth of the boreholes, and the opportunity for combination with isotopic information from the ice core itself. Ice borehole reconstructions have used several different methods, yielding similar results (Cuffey et al. 1995, Dahl-Jensen et al. 1998, Clow and Waddington 1999). The main assumption of these analyses is that the physical process of heat transfer in the ice is well understood.

As with continental boreholes, the ice boreholes give a temperature history that is the local surface temperature. For central Greenland (Cuffey et al. 1995, Cuffey and Clow 1997, Dahl-Jensen et al. 1998), results show a warming over the last 150 years of approximately 1°C ± 0.2°C preceded by a few centuries of cool conditions. Preceding this was a warm period centered around A.D. 1000, which was warmer than the late 20th century by approximately 1°C. An analysis for south-central Greenland (Dahl-Jensen et al. 1998) shows the same pattern of warming and cooling but with larger magnitude changes. Uncertainties on these earlier numbers are a few tenths of a degree Celsius for averages over a few centuries.

A borehole from Law Dome (Dahl-Jensen et al. 1999), in coastal East Antarctica, reveals a warming of approximately 0.7°C from the middle 19th century to present (uncertainty of approximately 0.2°C). This was preceded by a period of comparable
warmth centered on 1500–1600, a 1°C cooler period centered on 1300, and consistently warmer conditions prior to this (with temperature at A.D. 1 being approximately 1°C warmer than the late 20th century). There was no apparent warming during medieval times at this site. Uncertainties on these results for earlier time periods are on the order of a few tenths of a degree Celsius for averages over a few centuries.

Palle Miliam skrev:
Så eksistensen af MWP og LIA findes på trods af, at iskernedata fra Grønland samt Sydpolen kun udgør en lille del af dette materiale. Om det er fair at "udvande" iskernedata med disse andre proxy-data er jeg mere usikker på, det virker lidt som at blande æbler og pærer, men ekspertgruppen har talt!