Obviously, all such information is proxy but very strong.  Changing tree-line history, when determinable, is one but one of the best is pollen in northern lake bottom core samples.  Species come and go depending on climate.  If, over the next hundred years or so, climate continues to get warmer, it may well be warmer but, to date, it appears to be no warmer now than 1000 years ago.  It would certainly be convenient if Arctic sea ice was deep enough and permanent to core back 1000-2000 years but, at 2-3 meters deep, it turns over completely every few years so data comparable to ice sheet data is simply not available:
Duplessy, J.C., et al., 2001. Holocene paleooceanography of the northern Barents Sea and variations of the northward heat transport of the Atlantic Ocean. Boreas, vol 30, 2-16.
Johnsen et al., 2001. Oxygen isotope and paleotemperature records from six Greenland ice-core stations. J. Quat. Sci. vol 16, 299-307
Davis et al., 2003. The temperature of Europe during the Holocene reconstructed from pollen data. Quat. Sci. Rev. Vol 22, 1710-1716.
MacDonald G.M., et al., 2000. Holocene treeline history and climate change across northern Eurasia. Quat. Res., vol 53, 302-311.
Webb, T., et al, 1998. Late quaternary climate change in eastern North America. Quat. Sci. Rev. vol 17, 587-606.
Kaufman, D.S, et al. Holocene thermal maximum in the western Arctic. Quat. Sci. Rev. vol 23, 529-560.

At 11:26 PM 9/11/2012, Paul Tanner wrote:
Where did you get this "at least half the time over the last 10,000
years the Arctic sea ice has melted off completely over the summer"
and why do you believe it?

Anthony Watts? Do you know that he has a college degree in nothing?
That's right: He's a dropout after spending several years at Purdue.
Didn't finish even just an undergraduate degree. (For many years now
he utterly refuses to answer this. Of course he does not. Admitting
this truth would destroy his credibility among many of his followers.)

This is yet another example of how conservatism just makes up "facts"
in its fact-denial and science-denial in the name of political

The truth:

First, this intro from Wikipedia:


Quote: "There are differing scientific opinions about how long
perennial sea ice has existed in the Arctic. Estimates range from
700,000 to 4 million years."

If we stick to only the refereed papers in the reputable peer-reviewed
journals, this is what we get:

Just some of what anyone can find:



"Arctic sea-ice extent and volume are declining rapidly. Several
studies project that the Arctic Ocean may become seasonally ice-free
by the year 2040 or even earlier. Putting this into perspective
requires information on the history of Arctic sea-ice conditions
through the geologic past. This information can be provided by proxy
records from the Arctic Ocean floor and from the surrounding coasts.
Although existing records are far from complete, they indicate that
sea ice became a feature of the Arctic by 47 Ma, following a
pronounced decline in atmospheric pCO2 after the Paleocene­Eocene
Thermal Optimum, and consistently covered at least part of the Arctic
Ocean for no less than the last 13­14 million years. Ice was
apparently most widespread during the last 2­3 million years, in
accordance with Earth’s overall cooler climate. Nevertheless, episodes
of considerably reduced sea ice or even seasonally ice-free conditions
occurred during warmer periods linked to orbital variations. The last
low-ice event related to orbital forcing (high insolation) was in the
early Holocene, after which the northern high latitudes cooled
overall, with some superimposed shorterterm (multidecadal to
millennial-scale) and lower-magnitude variability. The current
reduction in Arctic ice cover started in the late 19th century,
consistent with the rapidly warming climate, and became very
pronounced over the last three decades. This ice loss appears to be
unmatched over at least the last few thousand years and unexplainable
by any of the known natural variabilities."

This is what the lead author said in an interview when asked when was
the last time the Arctic was ice-free in the summer:


"The paleo data we have so far is very scant, so we can’t know for
sure when the Arctic was ice free in the summer last time. To be
conservative, the closest candidate is the early Holocene (roughly ~10
kyr ago), when the insolation in the Arctic was high due to the
beneficial orbital configuration; however, the more data I see, the
stronger is my impression that there was not that little ice at that
time. The next best (actually, better) candidate is the Last
Interglacial, about 125kyr ago, again due to orbitally-driven high
insolation: the ice was likely very low, but we can’t say whether it
was completely ice free in summer or not. There are also a few other
major interglacials, which may have had a similar picture, in
particular Marine Isotopic Stage 11, about 450 kyr ago. In any case we
are talking about very rare events controlled by a forcing very
different from today. If none of those intervals was really ice free,
then a million year assessment would be correct."

Not only all that, but consider again this animated gif graph:


and see again that in the last 30 years not only are we on the verge
of no ice in the summer, but we are 1/3 of the way to having no ice in
the Arctic not just in the summers but all year round including in the
winters. This has not happened for millions of years.

And we're seeing all this happen in just decades. It has never
happened so fast naturally.

Let's see whether you find even a college dropout like Watts who would
say, "Sure, we're had no ice year round in the Arctic plenty of times

Finally, note this: The science denying argument is this: We've had
no-ice conditions before humans could have caused no-ice conditions,
therefore what is happening now is natural. That's like saying that
the mass killings of humans before the 20th century was natural
(starvation or disease), therefore the mass killing in the 20th
century was natural.

On Wed, Sep 12, 2012 at 1:06 AM, Wayne Bishop <wbishop@calstatela.edu> wrote:
> The real problem comes with the "anthropogenic" cause and, if so (which is
> less than certain), appreciably slowing down the process without millions of
> lives on the line, the great majority of whom would be very poor, nonwhite
> people who currently are barely eking out a living.  A very liberal,
> progressive concept.
> More science is that at least half the time over the last 10,000 years the
> Arctic sea ice has melted off completely over the summer.  That's why the
> polar bears and Inuits all died out millennia ago.
> Wayne
> At 09:31 PM 9/11/2012, Paul Tanner wrote:
> On Tue, Sep 11, 2012 at 8:22 PM, Robert Hansen <bob@rsccore.com> wrote:
>> On Sep 11, 2012, at 6:51 PM, Paul Tanner <upprho@gmail.com> wrote:
>> Regardless, to meet your challenge above that scaling cannot be a
>> model for the multiplication of arbitrary real numbers - given that I
>> utterly reject your assumption above that a model must be computable -
>> and I note that your assumption above denies all of geometry based on
>> real numbers because you deny almost all real numbers:
>> Joe must be a conservative real number denier.
>> Bob Hansen
> Since you've given me permission:
> You can all you want make light of the fact that there is much
> crankery out there by a very big lot of cranks in terms of denial of
> science and/or mathematics, and that very unfortunately for
> conservatism, it almost always comes from within conservatism. Do we
> need to wonder why?
> For fun, consider this: The most recent denial of science by so many
> conservatives on the Internet is their mind-blowing attempts at trying
> to continue to deny that the globe is warming never mind that we now
> have new satellite data that confirms that starting less than 30 years
> ago, all of the Arctic ice is collapsing before our very eyes, more
> than 3/4 of the way to zero volume during summer, 1/3 of way to zero
> volume all year round.  And since just 2006, more than half of
> whatever summer ice existed then is now gone - we are more than
> halfway to zero volume starting from just six years ago.
> See these for the jaw-dropping truth:
> http://tamino.files.wordpress.com/2012/09/piomas.gif
> http://img.photobucket.com/albums/v224/Chiloe/12_Climate/sea_ice_PIOMAS_min_to_date.png
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