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27 September 2010

Science Cafe

This Thursday (September 30th) I'll be talking about ice, and, better, yet, answering questions about ice at the Annapolis Cafe Scientifique.  The time will be 6 PM instead of the usual 6:30.  Same location as usual -- Cafe 49 West.  Local folks are invited, and non-local are welcome to pose questions here.
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3 comments:

  1. jg29 September, 2010 11:53

    I hope this is an ice question: A recent Nature article showed temperatures preceding the holocene warming inferred from Greenland ice cores. The graphs had adjustments. E.g., a solid lines showing the actual data, but where the trend dropped noticably, there were adjustments continuing the trend in dotted lines (I see this a lot). I understand that these adjustments are made to compensate for periods when greenland snowfall is thought to be more influenced by summer precipitation whereas most of the trend reflects winter precipitation. I wish I had a better understanding of how these data are corrected, e.g., how they know the data need adjustment and how much to adjust for. I'll hunt down the article, but wanted to ask before your Thurs deadline.

    thanks,
    jg

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  2. Robert Grumbine29 September, 2010 13:20

    For more recent snows (last 17,000 years or so) one can just look at the snow/ice itself. Summer snow/ice is cloudier than winter's. That's what makes for the fairly easy counting of layers to get ages for this span in Greenland.

    I forget what is used farther back. Will have to look it up to be sure. As I recall it, the correction is based on things which you observe in the ice core itself. Something like (apply due grains of salt) this: Two different stable isotopes reflect temperature of the snow formation -- oxygen-18 and deuterium (heavy hydrogen). Both are also affected, but differently, in terms of where the water came from (was evaporated from) originally. So you measure both isotopes (this is definitely true) and then use the mismatch in the temperatures to show you how much of the snow came from summer versus winter (the water sources being farther away in winter). This latter part is where I need to recheck.

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  3. jg01 October, 2010 15:45

    Thank you, for your answer. Having a ~17 kyr demarcation in mind will make examing ice core data a little more meaningful and memorable.

    I regret that I can't locate the paper that prompted my original question. It or another suitable example will probably turn up when I'm not looking for it.
    jg

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