On Thu, 18 Sep 2008 20:50:13 +0100, Andy Champ 
wrote:

>dennis@home wrote:
>> 
>> Lets see, how accurate were maps 400 years ago?
>> 
>> How much of the ice did they actually map each year?
>> 
>> How did they measure the thickness?
>
>They didn't measure the thickness.
>
>But they know when the harbours iced up, and when they freed, and that's 
>probably as good a dataset as we need.
>
>Andy

Any measurement of 'first year' ice is likely to mean very little.
First year ice forms and decays on an annual basis, if it doesn't
decay then it goes to create multi-year ice. The meaningful data is of
the creation or decay of multi year ice.  It appears that multi year
iceat both poles is decaying at a significant rate and that is what
the predictions are based upon.

Increase of multi year ice=cooling
Decay of multi year ice=warming