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Author Topic: How is past climate inferred from tree-rings and corals?  (Read 5657 times)

Keith

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Keith asked the Naked Scientists:
   
How can the earth's temperature be calculated with any accuracy from tree rings and coral, for example, methods used in the 'hockey stick' graph for past centuries.

Can we be sure they have not been underestimated? What is the margin of error?

What do you think?
« Last Edit: 06/01/2010 15:30:02 by _system »


 

Offline Mazurka

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How is past climate inferred from tree-rings and corals?
« Reply #1 on: 07/01/2010 14:53:10 »
The key issue when looking at the proxy data used to infer the paleoclimate is that no single technique is completely reliable of itself – broadly, dating of accurate “thermometers” (such as Oxygen isotope analysis) is generally not as good as accurate dating techniques (such as dendrochronology) and vice versa.  Actual human read thermometer records are only considered reliable for the past 150 years or so, but can help confirm the data towards the end of the proxy data sets.  The theories behind the different proxies often follow the principle of Uniformitarianism (The present is the key to the past) which underlies most of geology.

Mann et al’s “Hockey stick” is a meta- analysis of 16 (or so) (peer reviewed) paleoclimatic reconstructions, and uses complex statistical techniques to reconcile the widely varying error bars of each reconstruction.  It draws together tree ring analysis, isotopic analysis from calcite (e.g. coral, speleothems, foraminifera in sediments) and from ice cores, as well as analysis of air bubbles trapped within the ice (giving CO2 levels as well as atmospheric O).  Sediment cores (particularly of varve (rhymthite) deposits) It does not rely on any one particular study or technique.  If the peaks and troughs of the independently derived reconstructions did not correlate as well as they do, the Hockey stick would not have the prominence that the likes of the IPCC give it.

Dendrochronology is the study if patterns of tree rings and is often used to infer climate in the past and is often given as an example of a proxy as more people can grasp the concept of tree rings than Oxygen isotope analysis!  Strictly speaking, the patterns of tree rings only reflects local growing conditions not temperatures, but studies have shown the patterns to be quite consistent (at least regionally) which is why they are a useful proxy.  However the main advantage is the accuracy of dating – each ring represents a year.  Dendrochronology is very well constrained due to its use for archaeological purposes).   

Corals etc. are useful as thermometers as they are composed of calcite (CACO3) (and have growth rings) As the animal grows oxygen is taken up and locked into the mineral (with carbon and calcium) thus forming a record.  The ratio between O16 and O18 varies with temperature – I am fairly sure that the calibration of this thermometer is empirical (i.e. by experiment), but I could not give a figure for the error.  The measurement of the isotopic ratio is achieved using mass spectrometry and techniques to use smaller and smaller samples are constantly evolving.

Each of the proxies used are well documented within the journal literature, which leads to the problem that few people have sufficient understanding of each technique to be able to criticize  or otherwise reinterpret the data
 

Offline litespeed

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How is past climate inferred from tree-rings and corals?
« Reply #2 on: 07/01/2010 20:03:12 »
You wrote: "Each of the proxies used are well documented within the journal literature, which leads to the problem that few people have sufficient understanding of each technique to be able to criticize  or otherwise reinterpret the data."

"Using tree rings as a basis for assessing past temperature changes back to the year 1,000 AD, supplemented by other proxies from more recent centuries, Mann completely redrew the history, turning the Medieval Warm Period and Little Ice Age into non-events, consigned to a kind of Orwellian `memory hole' [22]. Fig.4 shows Mann's revision of the climatic history of the last millennium."

http://www.john-daly.com/hockey/hockey.htm

PS: I sense a potential false flag operation here. For one thing, you introduce quite an insult. And you must have known someone would call you on it. It took me all of about 90 seconds to find the commentary listed above. It makes me suspect you are setting up the hockey stick arguement just to have it knocked down or some reason or other.

PPS Kieth: Here is one climate URL that includes a number of studies that use proxies of various sorts to reconstruct temperature 2,000 years ago.

http://www.co2science.org/subject/r/summaries/rwpeuropenorth.php
« Last Edit: 07/01/2010 20:25:46 by litespeed »
 

Offline Mazurka

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How is past climate inferred from tree-rings and corals?
« Reply #3 on: 08/01/2010 11:48:09 »
I will not indulge myself in making an ad hominem attack on John Daly - this is not the place - and his reliance on "facts" and figures without fully checking them is well documented.

As I have said before, I do not like the "hockey stick" as it appears to overly simplify the science and is consequently easily criticised by the self proclaimed sceptics of the blogosphere, who are often repeating misinformation spread about by commercial lobbyists.  This is then spouted by the sensationalist end of the media and leads to the current situation, where most of the people working in the field of Climate Change are in broad agreement (See the IPCC report AR4 - which I consider to be a more reputable source than the blogosphere) yet the public - many of whom would cheerfully admit that they struggle with basic science - do not accept anthropogenic climate change.  In any case, the hockey stick does not rely on any one set of data.

The comment I made about understanding of the different datasets is that I would be surprised if many academics (let alone the general population)would be comfortable making highly technical criticism outside of their own expertise.  In other words, I suspect a professor specialising in dendroclimatology, whilst being perfectly able to understand a peer reviewed paper on (for example) oxygen isotope analysis of an ice core, may not consider themselves a suitable to peer review a paper on the subject.

Personally (and possibly because I do not understand the science to a sufficient level) I am sceptical about temperature figures derived from tree rings - tree ring patterns reflect annual growth rates which may be affected by other factors such as drought, land management practices or more recently "acid rain". However this scepticism does not extend to unqualified denial of mans effect on the atmosphere.

   
 

Offline Madidus_Scientia

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How is past climate inferred from tree-rings and corals?
« Reply #4 on: 08/01/2010 12:38:12 »
A quote from the link litespeed pasted:

Quote
Climate alarmists contend that the degree of global warmth over the latter part of the 20th century was greater than it has been at any other time over the past one to two millennia, because this contention helps support their claim that what they call the "unprecedented" temperatures of the past few decades were CO2-induced. Hence, they cannot tolerate the thought that the Medieval Warm Period of a thousand years ago could have been just as warm as, or even warmer than, it has been recently, especially since there was so much less CO2 in the air a thousand years ago than there is now. Likewise, they are equally loath to admit that temperatures of the Roman Warm Period of two thousand years ago may also have rivaled, or exceeded, those of the recent past, since atmospheric CO2 concentrations at that time were also much lower than they are today.

This is bullrubbish. A straw man argument. As it has been pointed out to you before climate scientists know very well that CO2 is not the only driver of climate.

They do however believe that it is the major factor in current global warming.

For those who have a spare 7 minutes this youtube video explains it well.
« Last Edit: 08/01/2010 12:45:24 by Madidus_Scientia »
 

Offline litespeed

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How is past climate inferred from tree-rings and corals?
« Reply #5 on: 09/01/2010 18:53:43 »
Hi Mazurka,

Just a couple of points. First, I don't know this guy Daly and will withdraw use of his commentary on the hockey stick for now. However, I found this excellent discussion of the debate in Wikepedia. http://en.wikipedia.org/wiki/Hockey_stick_controversy.

Further, as you point out, the hockey stick guys Mann et al [MBH] used a variety of proxy matterial however:

"The report found that MBH method creates a PC1 statistic dominated by bristlecone and foxtail pine tree ring series (closely related species). However there is evidence in the literature, that the use of the bristlecone pine series as a temperature proxy may not be valid (suppressing "warm period" in the hockey stick handle); and that bristlecones do exhibit CO2-fertilized growth over the last 150 years (enhancing warming in the hockey stick blade)."

The above commentary seems incomplete to me. This is because what seems most suppressed in the MBH graph is the severity of the little ice age [LIA]. Further, although brislecone pines "do exhibit CO2-fertilized growth" that is an independent variable from temperature. The most obvious conclusion is some of these issues are anything but settled.

Also note the reconstructed graph shows the climate has been warming on and off since at least 1700, with a large spike beginning in about 1900. However, not everyone agrees this spike is primarily due to human CO2. I have seen at least two or three commentaries that contend most [2/3] of the 20th century warming was caused by increased solar activity that did two things. First, it provided warming in and of itself.

Second, increased solar output reduces cosmic rays with a corresponding reduction in low level cloud formation. They contend low level clouds reflect more back up then back down. I have not provided citations for a couple of reasons. As stated in another thread, I am only about 1/3 of the way through my cosmic ray investigation. Further, I don't have the bookmarks organized.

I will point out, however, sunspot cycle 24 has still gone missing. This might be bad for crops. At least in England  :) http://www.bioedonline.org/news/news.cfm?art=686

JUST FOR GRINS:
"River Thames frost fairs were held on the Tideway of the River Thames at London between the 15th and 19th centuries when the river froze over. During that time the British winter was more severe than now, and the river was wider and slower.

"During the Great Frost of 1683–84, the worst frost recorded in England,[1][2][3] the Thames was completely frozen for two months, the ice 11 inches (28 cm) thick at London. Solid ice was reported extending for miles off the coasts of the southern North Sea (England, France and the Low Countries), causing severe problems for shipping and preventing the use of many harbours.[4] Near Manchester, the ground was frozen to 27 inches; in Somerset, to more than 4 feet."

http://en.wikipedia.org/wiki/River_Thames_frost_fairs

 

Offline Mazurka

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How is past climate inferred from tree-rings and corals?
« Reply #6 on: 11/01/2010 11:26:17 »
As I previously indicated, I am not entirely happy with the use of tree rings as temperature proxies as I am not sufficiently knowledgeable about the state of the art in dendro studies

It is easy to pluck out quotes that criticize this, or indeed any other, aspect of climate science. Subsequent paragraphs  in the wiki not quoted...
"The result of fixing some of the alleged errors in the overall reconstruction does not change the general shape of the most recent part of the reconstruction.
Similarly, studies that use completely different methodologies also yield very similar reconstructions".

Given that instrumented temperature data of increasing accuracy is available from 1850 or so, which is around the time that fossil CO2 became the dominant industrial fuel source, I am not too bothered about issues of CO2 fertilisation skewing the record for the recent past.

 

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How is past climate inferred from tree-rings and corals?
« Reply #6 on: 11/01/2010 11:26:17 »

 

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