Chuck Gould writes:

Undoubtedly you entirely missed the comment in my original post where
I theorized that the NW Passage must have been commonly open during
some periods of time prior to the 16 & 17 century expeditions that
were sent to look for it. (Most rumors have at least some basis in
fact, and stories of passage across the top of North America had to be
common enough and convincing enough that huge sums of money and
resources would be engaged to try to find and confirm it).

If you missed that, you also missed my comment that a period warm
enough to remove ice from the NW Passage 500 years or more ago could
not have been caused by the internal combustion engine.

500 years ago it was pretty cold. The last Viking colonies
on Greenland gave up in the 1400s due to cold climate. Surviving
winter in Europe was a big struggle for most Europeans.
17 century saw the Baltic ice covered on a regular basis and very
bad times in Chine due to low crop yields.

If you look for a warm era look at the 18th century (Carl von
Linné moved out from down town Uppsala because he was worried
about malaria. Malaria was brought to what would later be the
USA by English settlers.) or a thousand years ago when harbours
had to be re-built time again due to rising sea levels---even
here where the land is rising by 5 mm/year as a result of
missing ice of the latest ice-age.

--
Martin Schöön "Problems worthy of attack
show their worth by hitting back."
Piet Hein