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Author Topic: SLIDING GLACIERS  (Read 12899 times)

Offline crandles

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Re: SLIDING GLACIERS
« Reply #25 on: 29/07/2006 23:48:22 »
I just don't know about whether dust and dirt at the bottom could cause weakness. However I would question the relevance:

Dust and dirt from the last 150 years would be on top of the glacier. This will make it easier to melt the top by reducing the albedo and also the melting point. However, I am not sure how much effect this has.

Since we are seeing glaciers retreat in large numbers of diverse locations (~98% of glaciers, I think), it would be very odd if it could be put down to local factors eg volcanoes.

I am not sure why you would expect a difference at the bottom eg comparing ice that formed 10,000 years ago with ice from 10,100 years ago.

 

another_someone

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Re: SLIDING GLACIERS
« Reply #26 on: 31/07/2006 03:52:30 »
quote:
Originally posted by crandles
I just don't know about whether dust and dirt at the bottom could cause weakness. However I would question the relevance:

Dust and dirt from the last 150 years would be on top of the glacier. This will make it easier to melt the top by reducing the albedo and also the melting point. However, I am not sure how much effect this has.

Since we are seeing glaciers retreat in large numbers of diverse locations (~98% of glaciers, I think), it would be very odd if it could be put down to local factors eg volcanoes.

I am not sure why you would expect a difference at the bottom eg comparing ice that formed 10,000 years ago with ice from 10,100 years ago.



I agree that dirt at the bottom of glaciers would scarcely make much different not least because the enormous weight of the glacier would tend to crush whatever is at the bottom into the underlying soil.

On the other hand, I would scarcely regard volcanic activity as a local phenomenon a single large volcano can easily send dust circumnavigating the Earth.

I am not at all sure that anyone knows what percentage of ice is melting, since like so much else regarding climate, there are so many different ways of measuring it, and so much of it that still is not measured.




George
 

Offline ukmicky

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Re: SLIDING GLACIERS
« Reply #27 on: 31/07/2006 04:30:25 »
I mean a dusty layer of ice near the bottom of a glacier.  would  such a layer of dusty ice  be weak compared to uncontaminated layers allowing it to fracture through things like glacial tremors and then melt due to the energy stored up in the now free to move glacier.

Michael
 

Offline crandles

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Re: SLIDING GLACIERS
« Reply #28 on: 05/08/2006 22:08:28 »
quote:
Originally posted by another_someone
I am not at all sure that anyone knows what percentage of ice is melting, since like so much else regarding climate, there are so many different ways of measuring it, and so much of it that still is not measured.





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

quote:

Impact on glaciers
 
Global glacial mass balance in the last fifty years, reported to the WGMS and the NSIDC. The increased downward trend in the late 1980s is symptomatic of the increased rate and number of retreating glaciers.Global warming has led to negative glacier mass balance, causing glacier retreat around the world. Oerlemans (2005) showed a net decline in 142 of the 144 mountain glaciers with records from 1900 to 1980. Since 1980 global glacier retreat has increased significantly. Similarly, Dyurgerov and Meier (2005) averaged glacier data across large scale regions (e.g. Europe) and found that every region had a net decline from 1960 to 2002, though a few local regions (e.g. Scandinavia) have shown increases. Some glaciers that are in disequilibrium with present climate have already disappeared [30] and increasing temperatures are expected to cause continued retreat in the majority of alpine glaciers around the world. Upwards of 90% of glaciers reported to the World Glacier Monitoring Service have retreated since 1995 [31].



I guess that 98% of glaciers retreating that I quoted was from Oerlemans (2005)'s 142 of 144.

The percentage of ice is a rather different thing to percentage of glaciers retreating. However, why do you think a sample selection (if done using an appropriate random selection technique) is not going to give a good representation of the population?
 

another_someone

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Re: SLIDING GLACIERS
« Reply #29 on: 05/08/2006 23:14:12 »
quote:
Originally posted by crandles
I guess that 98% of glaciers retreating that I quoted was from Oerlemans (2005)'s 142 of 144.

The percentage of ice is a rather different thing to percentage of glaciers retreating. However, why do you think a sample selection (if done using an appropriate random selection technique) is not going to give a good representation of the population?



Because I don't believe it is random a lot of that depends upon where there are long term records, which itself depends upon how accessible the ice is, thus possibly precluding measurements from more inaccessible areas, which may possibly have a very different pattern of behaviour.

In general, one would expect better records to be kept for glaciers that are more often frequented by people and/or are closer to human habitation.  This might well put particular pressures on these glaciers (such as land use pressures, use of the mountains for recreation, etc) that are simply not there for more remote glaciers.  It may also be that human habitation is generally greater in regions with milder climates, and this itself could distort the microclimates around those glaciers that are more easily accessible to humans.  Ofcourse, even the converse may be true, it could be that since the more accessible glaciers are already in warmer climes, their retreat is less significant (i.e. they were smaller to start with, so they actually contribute less to the global average).

None of this is to say that glaciers are not retreating very probably they are only that I am somewhat dubious about any attempt at quantifying those figures, particularly when relying on long term observations.



George
 

Offline crandles

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Re: SLIDING GLACIERS
« Reply #30 on: 06/08/2006 15:29:33 »
Sure those variation are possible and if your random sample was just randomly selected from all the sites with adequately long records that could be an issue. Such an issue would be difficult to solve if your sample size was just 50. If the sample size is much larger though you can split it in two according to how easily accessable the different sites are. If the 2 subsamples show differing trends then a more sophisticated regionalised sample technique is needed. The remoter sample items are given appropriate weight to represent all the remote glaciers in the region. A similar splitting of the sample can be done based on length of record available.

If the real figure for retreating glaciers was less than 90% then it would be odd for an appropriate random selection technique using an appropriatly large sample size arrived at an over 98% figure. A larger discrepancy may be possible if an inappropriate sample technique had been used. However, I don't see that you have made any reference to the techniques used and why they are inappropriate yet. Until you do that, I won't think the 98% figure is perfect but I won't think it is far out either.
 

another_someone

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Re: SLIDING GLACIERS
« Reply #31 on: 06/08/2006 21:19:08 »
quote:
Originally posted by crandles

Sure those variation are possible and if your random sample was just randomly selected from all the sites with adequately long records that could be an issue. Such an issue would be difficult to solve if your sample size was just 50. If the sample size is much larger though you can split it in two according to how easily accessable the different sites are. If the 2 subsamples show differing trends then a more sophisticated regionalised sample technique is needed. The remoter sample items are given appropriate weight to represent all the remote glaciers in the region. A similar splitting of the sample can be done based on length of record available.

If the real figure for retreating glaciers was less than 90% then it would be odd for an appropriate random selection technique using an appropriatly large sample size arrived at an over 98% figure. A larger discrepancy may be possible if an inappropriate sample technique had been used. However, I don't see that you have made any reference to the techniques used and why they are inappropriate yet. Until you do that, I won't think the 98% figure is perfect but I won't think it is far out either.



As you say, neither of us know the sampling mechanisms used.

I have never said the 98% is wrong, I have merely said there is great scope for it to be wrong, and thus must be suspect.

'Lies, damned lies, and statistics' Disreali.



George
 

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Re: SLIDING GLACIERS
« Reply #31 on: 06/08/2006 21:19:08 »

 

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