not to make this into a religious debate, but i studied religion as a minor in college, and while i've heard the ice/water barrier theory it doesn't hold up.
it's a nice idea, but the sun is referred to several times in the bible as appearing as it does now, and if there were some sort of barrier, that would not be the case.
there are a few theories as to the long lifespans that are commonly accepted.
1) it is merely symbolic, to show that these men (not everyone lived to be 900, just a select few) were larger than life figures and their influence lasted for a long time, and/or to set them apart from the rest of the people mentioned. this is widely considered to be the actual reasoning behind the high ages
2) some believe that somewhere in translation, someone mixed up a lunar cycle and a solar cycle. thus each year would = 1 month... so 969 would be about 78. highly unlikely, but idiots are everywhere, and since there is no urtext edition of the bible available, it's hard to disprove this. and for those of you wondering why bibles in hebrew wouldn't be correct... perhaps it was translated into say, greek or latin, then translated BACK into hebrew with the mistake, and that was the edition that got spread around.
3) creationists believe that there was some environmental factor (similar to the water/ice barrier) that caused all people to live longer... but not everyone in the bible at the time is depicted as living to even 500... just the select few that were very influential figures in the biblical history. so this doesn't make sense. if i remember correctly, this is part of the Christian Scientist doctrine.
i find all this very interesting.
yay for keeping me entertained at work.
