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binntho

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Re: "Stupid" Questions :o
« Reply #2600 on: September 10, 2019, 06:31:38 PM »
If the Lake Agassiz outburst cooled that area down rapidly, then the temps from the ice cores will show that.

How does that theory explain the rapidly rising (proxy) temperatures referred to in the video?

The ice core from the Greenland glacier collects snow that fell in the past. The snow, as with all precipitation, comes from evaporation some where else.

In the case of the Greenland glacier, the evaporates that form the snow are mostly from the surface of the North Atlantic Ocean south of Greenland, more or less where the cold blob has been hovering these last years.

The surface temperature of the ocean affects the proportion of oxygen isotopes in the evaporated water. So the proportion of oxygen isotopes is a proxy for the surface temperature where the water evaporated. The ice core is measured for the oxygen isotope ratio as a proxy for the surface temperatures of the Northen Atlantic.

Any and all proper publications that I have read that discuss the Greenland Ice Cores make this very clear - so I'm surprised every time somebody starts claiming that the Greenland ice cores somehow show global temperatures - they can be a good proxy, but one that is very vulnerable to anomalies in the Northern Atlantic.

Generally this can be taken as a fairly good proxy for global temperatures, with the caveat that the Northern hemisphere can and does fluctuate more than the middle latitudes.

But the lake Agassiz burstthrough (and similar events) have the potential to stop the AMOC and dump lots of cold water into the North Atlantic, causing a "mega cold blob".

So now the evaporates are from an anomalously cold area of ocean, and what's more, an area that has cooled down extremely rapidly. The oxygen isotope ratio reflects this.

In short: Greenland ice cores tell us what the surface temperatures were in the North Atlantic. As the recent cold blob in exactly that area shows, future palaeometeorologists(!) might well be fooled into thinking that the earth had not really been warming that much in the 2010's, since the proxy they are using is primarily sensitive to surface temperatures in the North Atlantic which is having a mega cold blob causing the oxygen isotope ratio in the evaporates that eventually snow down on the glacier to reflect the temperatures of the ocean surface where they evaporated ... don't know how to make this any clearer.

It has nothing to do with Greenland temperatures!!!!!!!!
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Jim Hunt

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Re: "Stupid" Questions :o
« Reply #2601 on: September 10, 2019, 08:51:53 PM »
Don't know how to make this any clearer.

You don't need to. I got the "cold blob" message the first time around. Thanks for the expanded explanation, but you still haven't answered my very own "stupid question".

What in your view caused the extremely rapid rise in (proxy) temperature mentioned by Jim White? The AMOC suddenly started up again?
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binntho

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Re: "Stupid" Questions :o
« Reply #2602 on: September 10, 2019, 09:00:31 PM »
Don't know how to make this any clearer.

You don't need to. I got the "cold blob" message the first time around. Thanks for the expanded explanation, but you still haven't answered my very own "stupid question".

What in your view caused the extremely rapid rise in (proxy) temperature mentioned by Jim White? The AMOC suddenly started up again?

I didn't watch the video (haven't got the attention span) but I'm familiar with the thing and I can see that I've been talking about sudden drops rather than spikes. A total brain fart as we used to say in my salad days.

Perhaps it was the AMOC starting up, perhaps it was something else that drew in proportionally more humidity from elsewhere and thus skewing the proxies for a while. I don't know.

The ice cores do not measure Greenland temperatures at all (increadibly common mistake) and are a somewhat unreliable proxy for global temperatures. If the ice core says that temps rise by 1 degree per year for 5 years (and does it twice?) then it's obviously showing something other than global temperatures.

The difference between the LGM at it's coldest and the Holocene maximum is somewhere around 8 degrees. The forcings needed to go from one to the other over a few thousand years are massive (and surprisingly badly understood), but there is simply no forcing that can cause a 1 degree global temperature rise, let alone do it several years in a row.
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Archimid

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Re: "Stupid" Questions :o
« Reply #2603 on: September 11, 2019, 12:06:37 AM »
Variability of Arctic sea-ice cover at decadal to millennial scales: the proxy records

http://www.pastglobalchanges.org/products/latest/11511-variability-of-arctic-sea-ice-cover-at-decadal-to-millennial-scales-the-proxy-records

Quote
Despite limitations, the marine data provide clues on sea-ice cover variations with time windows ranging from decades to centuries, thus yielding smoothed records. At the scale of the Holocene, proxy-data suggest limited variations in general, with likely resilient perennial sea ice in the central Arctic Ocean, but greater variations in the seasonal sea ice as expressed in terms of spring sea-ice concentration (Sha et al. 2014) or number of months of sea ice (de Vernal et al. 2013b) in the Arctic and subarctic seas which are within the limits of winter sea ice. In other words, the variability of sea-ice cover as reconstructed from marine proxies illustrates more the seasonality of its extent and concentration than the actual changes in the Arctic-wide extent of sea ice. The amplitude of local changes during the mid- and late Holocene seems to be mostly comprised within the range of interannual variations as recorded during the last decades.

From the link:




Edit: Missed the link, thanks for pointing it out.
« Last Edit: September 11, 2019, 12:15:58 PM by Archimid »
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binntho

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Re: "Stupid" Questions :o
« Reply #2604 on: September 11, 2019, 06:31:22 AM »
Variability of Arctic sea-ice cover at decadal to millennial scales: the proxy records

Quote
Despite limitations, the marine data provide clues on sea-ice cover variations with time windows ranging from decades to centuries, thus yielding smoothed records. At the scale of the Holocene, proxy-data suggest limited variations in general, with likely resilient perennial sea ice in the central Arctic Ocean, but greater variations in the seasonal sea ice as expressed in terms of spring sea-ice concentration (Sha et al. 2014) or number of months of sea ice (de Vernal et al. 2013b) in the Arctic and subarctic seas which are within the limits of winter sea ice. In other words, the variability of sea-ice cover as reconstructed from marine proxies illustrates more the seasonality of its extent and concentration than the actual changes in the Arctic-wide extent of sea ice. The amplitude of local changes during the mid- and late Holocene seems to be mostly comprised within the range of interannual variations as recorded during the last decades.

From the link:

You forgot the link Archimid, it looks like an interesting publication. But the quoted text, although in favour of perennial ice, is not conclusive.

because a thing is eloquently expressed it should not be taken to be as necessarily true
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KiwiGriff

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Re: "Stupid" Questions :o
« Reply #2605 on: September 11, 2019, 07:39:06 AM »
Google is your friend. ;)

link for Variability of Arctic sea-ice cover at decadal to millennial scales: the proxy records.

http://www.pastglobalchanges.org/products/latest/11511-variability-of-arctic-sea-ice-cover-at-decadal-to-millennial-scales-the-proxy-records





binntho

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Re: "Stupid" Questions :o
« Reply #2606 on: September 11, 2019, 08:58:41 AM »
Google is your friend. ;)

link for Variability of Arctic sea-ice cover at decadal to millennial scales: the proxy records.

http://www.pastglobalchanges.org/products/latest/11511-variability-of-arctic-sea-ice-cover-at-decadal-to-millennial-scales-the-proxy-records

Thans KiwiGriff. The article only adds to the uncertainty of the proxies they use. To my mind, the presence of wave ridges along the north shores of Greenland and the CAA are conclusive evidence of an Arctic Ocean sea ice cover that would be well under the 1MKm2 needed to qualify as a BOE (i.e. slightly less than 1/3 of what was left in 2012).

And per my personal conviction, I don't believe that a 1Mkm2 piece of ice floating at the North Pole surrounded by water has any realistic life expectancy.
because a thing is eloquently expressed it should not be taken to be as necessarily true
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Hopen Times

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Re: "Stupid" Questions :o
« Reply #2607 on: September 11, 2019, 11:37:31 AM »
It has nothing to do with Greenland temperatures!!!!!!!!

I am not sure if the University of Copenhagen agrees with you:
Quote
Under which conditions did the water evaporate, and how much cooling did the vapour experience on its way to the ice cap before forming the precipitation that ended up as the layers in the ice cap?
http://www.iceandclimate.nbi.ku.dk/research/past_atmos/past_temperature_moisture/

Quote
Every time precipitation forms, the air mass will become more depleted in heavy isotopes. In the language of physics, fractionation takes place. During cold conditions (e.g. during winter or in a cold climatic period), the air masses arriving in Greenland have cooled more on the way, thereby having formed more precipitation and the remaining vapor is therefore more depleted in heavy isotopes (corresponding to lower δ18O vales).
http://www.iceandclimate.nbi.ku.dk/research/past_atmos/past_temperature_moisture/fractionation_and_temperature/

Quote
From analyses of the isotopic composition of Greenland ice cores it is possible to obtain a record of past Greenland climate reaching more than 100,000 years back in time. ... But the isotopes also tell about the climatic conditions in the areas from which the moisture source originates.
http://www.iceandclimate.nbi.ku.dk/research/past_atmos/past_temperature_moisture/

As I understand University of Copenhagen, Greenland ice cores reflects both the temperature in the the areas from which the moisture source originates and the temperature the vapour experienced on its way travelling, over both sea and ice. 

This is not my field, so I am very open for that there is something I am missing here.

Edit: More on the same topic.
Quote
So while the individual δ18O and δD records tell about local temperatures, small differences between the two records tell about moisture source temperatures.
http://www.iceandclimate.nbi.ku.dk/research/past_atmos/past_temperature_moisture/isotopes_reveal/
« Last Edit: September 11, 2019, 01:09:09 PM by Hopen Times »

binntho

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Re: "Stupid" Questions :o
« Reply #2608 on: September 11, 2019, 01:47:32 PM »
It has nothing to do with Greenland temperatures!!!!!!!!

I am not sure if the University of Copenhagen agrees with you:

As I understand University of Copenhagen, Greenland ice cores reflects both the temperature in the the areas from which the moisture source originates and the temperature the vapour experienced on its way travelling, over both sea and ice. 

This is not my field, so I am very open for that there is something I am missing here.

You are right, I was way to categorical in my denials above. This is what happens when you think you remember things correctly ("Never trust memory" I was taught in High School, and I usually try not to ...)

And the plot deepens, I found this:

Quote from: https://www.clim-past.net/9/2525/2013/cp-9-2525-2013.pdf
To sum up, all the results derived either from borehole paleothermometry or from isotopic anomalies significantly underestimate temperature changes in central Greenland, thus seriously challenging the conventional isotopic approach.

where the point was that the traditional oxygen isotope approach showed what seemed to be way too large fluctuations, but a closer look showed that they were in fact bigger than they seemed! And amazingly enough, the difference between LGM and present is a full 25 degrees centigrades (but where? At the top of the ice shield it seems)

But I think I can definitely claim the following: This is not what happens to global temperatures, and thus the value of the Greenland cores as a proxy for global changes becomes somewhat doubtful.

So Jim Hunt : I didn't know the answer when I tried the first time, I know even less now!

But back to the original point: Everyone can go and look at graphs of estimated global temperatures between the last glacial and the Holocene and see that the difference on a global scale is around 6 - 8 degrees Centigrade. And according to Greenland ice cores there is a very fast rise at the end of the Younger Dryas / beginning of the Holocene, near vertical at 10 degrees.

But ice cores from Vostok do not show the Younger Dryas event! They show a change in temperatures of perhaps 10 degrees from the LGM at ~26,0000 ky to the the Holocene proper at ~14,000, in two distinct steps, the later of which was pretty steep at 4 degrees in perhaps 1200 years, or a surprisingly rapid 0.3 degrees / century.



Deep sea sediments from the Pacific and Indian oceans (i.e. mid latitude) show a much smaller difference between LMG and Holocene, perhaps as little as 2 or 3 degrees, and no Younger Dryas event.



After some searching I managed to find a paper on a subject that I find quite interesting, i.e. the Holocene temperature conundrum which has a very good graph of the warming at the end of the last ice age:



This graph shows a max difference of 5 degrees and even if it misses out on the LGM, the real warming only began at the ~17,000 ka so no harm done. And from this graph I am unable to maintain that temps might have risen at anywhere close to our current rate of 1.2 degrees per century. The fastest seems to be just over 1 degree in just under 1000 years, or at least 10 times slower than today.
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Jim Hunt

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Re: "Stupid" Questions :o
« Reply #2609 on: September 11, 2019, 03:23:03 PM »
But ice cores from Vostok do not show the Younger Dryas event!

However the original question we are endeavouring to answer was about the Arctic, not the Antarctic? Paraphrasing somewhat:

Quote
Was there, or was there not, a BOE [at the Holocene maximum]?

Jim White is from the University of Colorado's Institute of Arctic and Alpine Research

https://www.researchgate.net/profile/James_White16

Here's a 1989 paper of his with Willi Dansgaard and Sigfus Johnsen:

https://www.tandfonline.com/doi/pdf/10.3402/tellusb.v41i4.15100

which concludes that:

Quote
We present evidence that, in contrast to low elevation stations, the high elevation areas of the Greenland ice sheet receive precipitation, which mainly originates from subtropical moisture sources, under present as well as under glacial conditions.

which I guess we're all agreed on now? In the video Jim states that:

Quote
We (i.e. including Dansgaard and Johnsen) also noted that deuterium excess, which is an indication of ocean conditions, probably sea ice out in the North Atlantic, changed even faster than that.

Their 1989 paper also states that:

Quote
In the search for an explanatory mechanism behind the abrupt climatic shifts, Broecker (1987) points out that in the present ocean circulation system, cold and salty water sinks in the North Atlantic Ocean, from where it is transported southward by deep Ocean currents, via the southern hemisphere oceans, and then northward to the North Pacific Ocean, where some of it upwells. This deep water transport is compensated by surface currents, e.g., the warm North
Atlantic Current, a shut-down of which would cool the adjacent lands by 6-8°C. Conversely, reestablishing the present circulation would result in 6-8°C warming. If this were the immediate cause of the abrupt 𝛿 shifts under glacial conditions, it might also "explain" the drastic, temporary 10‰ drop in 𝛿 at the end of the Eemian interglacial (Dansgaard et al., 1972)

However thus far my limited trawl of the literature hasn't revealed any further insight into a possible "Holocene [Arctic] blue ocean event".

« Last Edit: September 11, 2019, 03:34:42 PM by Jim Hunt »
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wallen

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Re: "Stupid" Questions :o
« Reply #2610 on: September 14, 2019, 09:00:50 AM »
Could some please explain why recent images of the North pole on Worldview, are very bright, with an almost yellowish tinge.
                                     Thanks.

blumenkraft

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Re: "Stupid" Questions :o
« Reply #2611 on: September 14, 2019, 09:23:18 AM »
I would say this is due to the low solar altitude, Wallen.

Happy to be corrected on this.
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Jim Hunt

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Re: "Stupid" Questions :o
« Reply #2612 on: September 14, 2019, 11:26:04 AM »
I would say this is due to the low solar altitude, Wallen.

Spot on I would say. The North Pole on Worldview from September 16th 2018

https://go.nasa.gov/301mtDT

Switch to this year and you can see the winter "pole hole" appearing as we speak.
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slow wing

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Re: "Stupid" Questions :o
« Reply #2613 on: September 15, 2019, 08:35:18 AM »
"The 2019 melting season", Reply #6567
... with melt potentially continuing through this next week given the heat anomalies ... last years min was on 21 Sep!


Three questions I have been wondering about that relate to this melt season:

1) Are the yearly extent minima getting later?

2) Has the Arctic basin been carrying more moisture in the atmosphere at this time of year than in past decades? (Iirc there was an identified step change relating to an extreme weather event in December 2016, was it the 27th? Also, there is more open water in recent years. So this question could include a part 2b: why?)

3) If 'yes' to both 1 & 2, could 2 be at least partially the reason for 1 due to increased moisture in the atmosphere over Arctic basin retarding the loss of heat by long wave radiation?


Have any scientists been watching this and can answer one or more of those questions? Thanks.

oren

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Re: "Stupid" Questions :o
« Reply #2614 on: September 15, 2019, 09:01:07 AM »
1) The date of minima is not showing a drift either earlier or later.
2) I don't know. But I think the event you refer to was in Dec 2015.