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Jim Hunt

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Basic questions about melting physics
« on: May 28, 2019, 10:02:37 AM »
Further to one or two recent discussions, here is a place to discuss the basic physics of freezing and melting sea ice.

Here are my own helpful hints on that thorny topic (amongst other things):

http://GreatWhiteCon.info/resources/arctic-sea-ice-explanations/
« Last Edit: May 28, 2019, 10:35:21 AM by Jim Hunt »
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Jim Hunt

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Re: Basic questions about melting physics
« Reply #1 on: May 28, 2019, 10:05:21 AM »
With dramatic loss of old thick sea ice since 2010, I would expect that PIOMAS Sept. minimum would show stronger downward trend for 2010-2019, but while the long term trend is obvious, the last 10 years have been fairly flat.  Why doesn't loss of old thick ice show up more in PIOMAS Sept. minimum volume?
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Jim Hunt

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Re: Basic questions about melting physics
« Reply #2 on: May 28, 2019, 10:06:44 AM »
Why doesn't loss of old thick ice show up more in PIOMAS Sept. minimum volume?

Because thermodynamics means the new ice grows to 2 meters plus thick across the Arctic Basin over every winter?

See the "Slow Transition" thread for more details.
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Jim Hunt

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Re: Basic questions about melting physics
« Reply #3 on: May 28, 2019, 10:07:25 AM »
RE #1866 "Because thermodynamics means the new ice grows to 2 meters plus thick across the Arctic Basin over every winter?"

   But that 2 meter new ice growth happens with or without old thick ice.  So it seems that a year with less returning old thick ice from previous year + summer freezing/thickening would result in less volume than an earlier year that had more returning old thick ice and gets the same amount of  summer freezing/thickening.

   The only way I can figure it is that with lower portion of old thick ice, the young ice that replaces it allows faster thickening.  Perhaps the thinner ice cover over water allows more heat loss and thus more thickening, whereas old thick ice is a better insulator and is less dynamic.
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Jim Hunt

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Re: Basic questions about melting physics
« Reply #4 on: May 28, 2019, 10:08:00 AM »
   But that 2 meter new ice growth happens with or without old thick ice.  So it seems that a year with less returning old thick ice from previous year + summer freezing/thickening would result in less volume than an earlier year that had more returning old thick ice and gets the same amount of  summer freezing/thickening.
And indeed that has been the behavior. When old ice is lost, it is replaced with FYI and the result is a lower winter maximum volume trend. However the winter maximum also depends on the preceding summer minimum and on the effectiveness of autumn freezing. A look at the attached graph (day 120) shows how winter maximum volume has continued on a downward trend, with winter 2017 having an extreme record low. It also shows the correlation between summer minimum (day 260) and the following winter volume. The summer minimum result depends more heavily on weather and is more volatile.

Note: the data includes CAB, Beaufort, Chukchi, ESS, Laptev, Kara, CAA, Greenland Sea - the regions where MYI actually exists.

Note 2: do read the "slow transition" thread, very interesting.

« Last Edit: May 28, 2019, 10:15:29 AM by Jim Hunt »
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Jim Hunt

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Re: Basic questions about melting physics
« Reply #5 on: May 28, 2019, 10:08:32 AM »
Thick ice (of any age - rafting, etc. can make thick year old ice) will not grow 2 meters thicker during the winter when nearby thin ice will.  Attached chart shows an example for lake ice - how it grows less as thickness increases.  Ice, basically, is an insulator.

Decades ago, with much of the Arctic covered in MYI (multiyear ice), there was less volume increase in the central Arctic than during recent winters.

« Last Edit: May 28, 2019, 10:15:57 AM by Jim Hunt »
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b_lumenkraft

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Re: Basic questions about melting physics
« Reply #6 on: May 28, 2019, 10:30:29 AM »
Good move Jim!

Flocke

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Re: Basic questions about melting physics
« Reply #7 on: May 28, 2019, 11:31:40 AM »
Ice, basically, is an insulator.

Maybe depending on how you define insulator. Ice has a thermal conductivity of around 2.25 W/mK, water only 0.56 W/mK (both temperature dependant). The difference seems to be the missing convection in ice.

Archimid

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Re: Basic questions about melting physics
« Reply #8 on: May 28, 2019, 01:32:01 PM »
Quote
With dramatic loss of old thick sea ice since 2010, I would expect that PIOMAS Sept. minimum would show stronger downward trend for 2010-2019, but while the long term trend is obvious, the last 10 years have been fairly flat.  Why doesn't loss of old thick ice show up more in PIOMAS Sept. minimum volume?


The attached graph shows yearly volume gain and losses calculated from PIOMAS. After 2007 there is a significant increase in losses, but there is also a significant increase in gains. The increase in losses have many causes,  thinner ice being one of them. That's one way that the loss of thick ice shows.

I think the gains in volume are mostly explained by the loss of thick multiyear ice. That's where it really shows. Like Jim said and you correctly explained, thin ice grows to 2 meters very fast relative to ice growth beyond 2 meters.

See Lebedev formula:



The loss of thick ice implies there is much more thin ice, and when there is thin ice volume increases much faster.

Before I finish, I would like to echo Jim and Oren. Read the slow transition thread.
I am an energy reservoir seemingly intent on lowering entropy for self preservation.

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Re: Basic questions about melting physics
« Reply #9 on: May 29, 2019, 11:23:32 AM »
I am not certain I have calculations right so if I don’t let me know.


On the molecular scale freezing is an ordered stacking of water molecules so it makes sense that putting ions in the way makes that harder. On the macro scale we see evidence of that when the freezing point decreases.


It is common to see -1.8 C as freezing point of sea water but it is a bit more complicated. -1.8C corresponds to 29.52 g of salts/kg seawater. Molar ratios of  ions are listed below for 35g solute/kg seawater


0.546 moles Cl-
0.469 moles Na+
0.053 moles Mg2+
0.028 moles SO42-
0.0103 moles Ca2+
0.0102 moles K+


Anyway salinity in the artic near the surface varies from about 26 g of salts/kg seawater with a freezing point of -1.58C to 36 g of salts/kg seawater with a freezing point of -2.21C. For most purposes -1.8C is good enough.


But that is not really how sea ice freezes it starts at the temperature associated with the salinity of the water but the ice is nearly pure water. It rejects the salt when it freezes this increases the salinity of the water. So the water has to get a little bit colder to freeze the next bit. This process continues until the salty water gets trapped in ice. Eventually the last of the water freezes at about -21C.  This is first year Ice. It has pockets with high salt concentrations in it. Some of the pockets even most may not of frozen solid.


When the temperature climbs above -21C the pockets of high salt concentration melt first. The temperature is still too low to melt the pure ice.  Since Ice melts at the solid to liquid (or solid to gas interface but that is not relevant here) interface and as the salty water is in contact with pure ice and gets a little bit warmer it can melt a little bit more ice. This lowers the salinity and the ice can’t continue to melt until it gets warmer still.


The high concentration salt water can often burrow out of the pure ice before most of the pure ice melts. When the temperature drops again lower concentration salt water freezes inside. This is how multiyear ice is formed each freeze and thaw cycle of salty water can more and more salts out until it is pure ice with no salts. That makes the multiyear ice fresher and more melt resistant.

The melting temperature of the pure multiyear ice is dependent on the salinity of the surrounding liquid. When the pure multiyear ice melts the local salinity drops and the temperature must increase to melt more. That’s another reason it is more resistant to melt than first year ice.



So to succinctly answer the original question the purified ice melts at the temperature determined by the salinity of the surrounding water. That may be higher than -1.8C but it is not 0C.


Freezing point depression
https://en.wikipedia.org/wiki/Freezing-point_depression
 presentation on chemical composition in sea water
https://www.soest.hawaii.edu/oceanography/courses/OCN623/Spring%202015/Salinity2015web.pdf

Rich

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Re: Basic questions about melting physics
« Reply #10 on: May 29, 2019, 12:56:38 PM »
I am not certain I have calculations right so if I don’t let me know.


On the molecular scale freezing is an ordered stacking of water molecules so it makes sense that putting ions in the way makes that harder. On the macro scale we see evidence of that when the freezing point decreases.


It is common to see -1.8 C as freezing point of sea water but it is a bit more complicated. -1.8C corresponds to 29.52 g of salts/kg seawater. Molar ratios of  ions are listed below for 35g solute/kg seawater


0.546 moles Cl-
0.469 moles Na+
0.053 moles Mg2+
0.028 moles SO42-
0.0103 moles Ca2+
0.0102 moles K+


Anyway salinity in the artic near the surface varies from about 26 g of salts/kg seawater with a freezing point of -1.58C to 36 g of salts/kg seawater with a freezing point of -2.21C. For most purposes -1.8C is good enough.


But that is not really how sea ice freezes it starts at the temperature associated with the salinity of the water but the ice is nearly pure water. It rejects the salt when it freezes this increases the salinity of the water. So the water has to get a little bit colder to freeze the next bit. This process continues until the salty water gets trapped in ice. Eventually the last of the water freezes at about -21C.  This is first year Ice. It has pockets with high salt concentrations in it. Some of the pockets even most may not of frozen solid.


When the temperature climbs above -21C the pockets of high salt concentration melt first. The temperature is still too low to melt the pure ice.  Since Ice melts at the solid to liquid (or solid to gas interface but that is not relevant here) interface and as the salty water is in contact with pure ice and gets a little bit warmer it can melt a little bit more ice. This lowers the salinity and the ice can’t continue to melt until it gets warmer still.


The high concentration salt water can often burrow out of the pure ice before most of the pure ice melts. When the temperature drops again lower concentration salt water freezes inside. This is how multiyear ice is formed each freeze and thaw cycle of salty water can more and more salts out until it is pure ice with no salts. That makes the multiyear ice fresher and more melt resistant.

The melting temperature of the pure multiyear ice is dependent on the salinity of the surrounding liquid. When the pure multiyear ice melts the local salinity drops and the temperature must increase to melt more. That’s another reason it is more resistant to melt than first year ice.



So to succinctly answer the original question the purified ice melts at the temperature determined by the salinity of the surrounding water. That may be higher than -1.8C but it is not 0C.


Freezing point depression
https://en.wikipedia.org/wiki/Freezing-point_depression
 presentation on chemical composition in sea water
https://www.soest.hawaii.edu/oceanography/courses/OCN623/Spring%202015/Salinity2015web.pdf

This doesn't make sense.

Sea water contains H2O molecules and NaCl dissolved in a solution.

When sea water freezes, the hydrogen bonds between the water molecules spread out into a fixed lattice structure and the NaCl falls out of the solution into the enlarged spaces between the hydrogen bonds.

Ice is less dense than liquid water. Fewer molecules in a given volume means more space between molecules for the salt / brine to exit the the lattice structure.

There is no frozen salt water in the Arctic. The ice is like a building made out of water molecules. Just as you would not consider a human inside of a building to be part of the building, a salt molecule inside of an ice lattice is not part of the ice.

The "screws" that hold the ice building together are hydrogen bonds. Weaker than covalent bonds inside the water molecules itself, these are intermolecular attractions between positively charged H and negatively charged O atoms.

To unscrew the connections in the building (melt the ice), it is necessary to apply heat to those H-O-H intermolecular bonds. The presence or absence of salt in the vicinity is irrelevant to the properties of the screws holding the H2O house together.

At 1 atm. of pressure, the house is going to fall apart at 0C.

oren

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Re: Basic questions about melting physics
« Reply #11 on: May 29, 2019, 04:28:45 PM »
Rich, at the risk of exposing my ignorance, I think you got it wrong there. At the bottom, the ice is floating is salt water, and will melt below 0C. At the top you may be correct.

Rich

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Re: Basic questions about melting physics
« Reply #12 on: May 29, 2019, 05:12:02 PM »
Rich, at the risk of exposing my ignorance, I think you got it wrong there. At the bottom, the ice is floating is salt water, and will melt below 0C. At the top you may be correct.

I said the ice will melt at 0C at 1 atm of pressure. This describes surface conditions.

Under water,  pressure increases and changes the melting point.

Ice at depth in a fresh water lake will also melt at temps below 0C due to increased pressure.

Tor Bejnar

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Re: Basic questions about melting physics
« Reply #13 on: May 29, 2019, 06:39:39 PM »
Sorry for the aside, but there can be a problem with
Quote
you would not consider a human inside of a building to be part of the building
   When calculating the energy needs of a building (e.g., heating and A/C), the humans can be a significant component.  (Years ago I did public building energy audits.)

I have no clue, however, about salt water freezing and 'fresh' ice thawing issues.
« Last Edit: May 29, 2019, 08:03:50 PM by Tor Bejnar »
Arctic ice is healthy for children and other living things.

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Re: Basic questions about melting physics
« Reply #14 on: May 29, 2019, 07:04:00 PM »
Thank you for starting this thread. As someone with less knowledge of the natural sciences than of political (unnatural?) science I have many questions about the behavior of water and ice. I have been reading some threads, mostly concerning extent and current season, for a few years. As my curiosity about the physics of water and aqueous solutions has grown, I joined with the hope of asking questions such as those addressed above. This thread and the information have provided above is much appreciated.

oren

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Re: Basic questions about melting physics
« Reply #15 on: May 29, 2019, 07:15:29 PM »
Sorry for the back and forth despite my being a layman on this subject. But: I am talking about ice touching salt water, not at depth but nearly at the surface where pressure change is negligible. It is "known" that salt hastens the melting of ice. In other words lowering the melting point, even if the ice is pure freshwater.
Can some expert step in?  ???

johnm33

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Re: Basic questions about melting physics
« Reply #16 on: May 29, 2019, 11:54:53 PM »
From W.Ds. blog "New sea ice starts from 3 important concurring factors: -1.8 C water, little or no sea waves and colder than -11 C surface temperatures "
http://eh2r.blogspot.com/2016/10/new-sea-ice-starts-from-3-important.html
So away from the shore much colder than-11C probably important.
What's the rate of sublimation from ice at various air temps.? since there can't be an energetic 'free lunch' how much impact does this have on ice formation/cooling.
« Last Edit: May 30, 2019, 01:09:37 AM by johnm33 »

petm

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Re: Basic questions about melting physics
« Reply #17 on: May 29, 2019, 11:57:06 PM »
I'm no expert but I know that it's common practice to throw salt on e.g. roads and sidewalks to melt the ice. Presumably sea ice in contact with sea water at a different salinity would have a melting temperature somewhere between the two. https://phys.org/news/2019-02-salt-doesnt-ice-winter-streets.html

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Re: Basic questions about melting physics
« Reply #18 on: May 30, 2019, 03:15:25 AM »
Temperature is a measure of kinetic energy of the system. Kinetic energy includes translational rotational and vibrational modes.
Temperature does not consider potential energy of the system.  Potential energy includes the energy of bonding. This include strong bonding like ionic and covalent as well as weaker interactions such as hydrogen bonding, vander wahls interactions and others.
Pure water has a higher potential energy than pure ice. If the system is at equilibrium the water and ice have the same kinetic energy namely temperature. The difference in energy is the potential energy of hydrogen bonding you mentioned.
Salt water has a lower potential energy than pure water because of the interactions between ions and water molecules. This interaction is weaker than h bonding so salt water has a higher potential energy then pure ice.

Potential energy of each system from high to low is pure water, salt water and then pure ice. I am not talking about temperature here just bonding energies. 
The potential energy of the saltwater changes with concentration higher concentrations of ions gives lower potential energy of the system.
the difference in potential energy of a saltwater pure ice system is lower than the potential energy difference between pure water and pure ice. Due to conservation of energy potential energy can be converted from kinetic energy. So the kinetic energy required to melt the saltwater pure ice system is lower than for the pure water pure ice system. In other words saltwater pure ice system melts at a lower temperature than pure water pure ice system.

binntho

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Re: Basic questions about melting physics
« Reply #19 on: May 30, 2019, 07:11:36 AM »
I should think that Oren is right - salt at the boundary between ice (even if made from pure water) will lower the melting point of that ice. The salt in the sea water causes the ice to melt at a lower temperature.

binntho

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Re: Basic questions about melting physics
« Reply #20 on: May 30, 2019, 07:16:36 AM »
Another thing that I've been thinking about lately, given the discussions re. insolation and melt ponds:

For those familiar with snow and ice, seeing ice melt in direct sunlight even when the air temperature is below zero is not unknown. Ice (and snow) will slowly sublimate directly into vapor in dry air, and sunlight helps this process along.

I've also noticed myself how snow that has lying for some time without melt will still have different surface characteristics depending on whether it has been exposed to direct sunlight. In the shadows, the snow is loose while in direct sunlight it seems to gain a brittle crust.

So my point is: Direct sunlight will cause surface melt and surface changes, even if air temperatures are below zero.

wdmn

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Re: Basic questions about melting physics
« Reply #21 on: May 30, 2019, 07:25:57 AM »
This is a question/comment about enthalpy of fusion and albedo. It could probably go elsewhere but fits here too, I think.

Ice at 0 Celsius takes a lot of energy to melt from latent heat.

According to wikipedia, it takes 333.55 kJ of energy to melt 1 kg of ice, but only 83.6 kJ of energy to increase the temperature of 1kg of water by 20 degrees celsius. That means that 333.55 kJ of energy raises the temperature of water ~80 degrees C.

Discussion around decreasing arctic sea ice tends to be about albedo. But doesn't decreasing sea ice also mean an increasing amount of available latent heat to warm the oceans/adjacent land? And what might that mean as we approach <1mk2 of summer sea ice? It seems like it would be simple math to quantify the excess heat going into the exposed ocean, and I wonder if anyone has done that already?

Thanks

binntho

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Re: Basic questions about melting physics
« Reply #22 on: May 30, 2019, 07:50:57 AM »
Definitely during summer the latent heat uptake of melting ice keeps the temperature hovering just over the 0 mark as can be seen on the DMI graphs. There are also temperature spikes in autumn when the freezing gets going, with release of latent heat that presumably is lost out through the atmosphere.

On the other hand, the ice insulates the underlying sea from losing too much heat to the atmosphere, so I´m not at all sure what the net result of a more-or-less ice free arctic would be.

wdmn

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Re: Basic questions about melting physics
« Reply #23 on: May 30, 2019, 08:35:50 AM »
Thank you. Always something to make things more complex...

But there's got to be some way to think about this numerically. I wonder if anyone's done it? Anyway, I'll give it some more thought.

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Re: Basic questions about melting physics
« Reply #24 on: May 30, 2019, 10:18:29 AM »
This is a question/comment about enthalpy of fusion and albedo. It could probably go elsewhere but fits here too, I think.

Ice at 0 Celsius takes a lot of energy to melt from latent heat.

According to wikipedia, it takes 333.55 kJ of energy to melt 1 kg of ice, but only 83.6 kJ of energy to increase the temperature of 1kg of water by 20 degrees celsius. That means that 333.55 kJ of energy raises the temperature of water ~80 degrees C.

Discussion around decreasing arctic sea ice tends to be about albedo. But doesn't decreasing sea ice also mean an increasing amount of available latent heat to warm the oceans/adjacent land? And what might that mean as we approach <1mk2 of summer sea ice? It seems like it would be simple math to quantify the excess heat going into the exposed ocean, and I wonder if anyone has done that already?

Thanks


the temperature is kinetic energy and the latent heat of fusion is potential this energy can and readily does converts from one to the other but that doesn't change the total energy of the system.


if you think about the water and ice of the ocean as a system with thorough mixing and no change in energy to or from the system the temperature will be at the freezing point and there will be a fixed amount of ice. Of course the ocean isn't perfectly mixed but it is easier to think about.



Adding energy to the ocean system can decrease the amount of ice in the ocean or increase the temperature. If it is not thouroghly mixed it can do both at the same time at different locations.


So whats really important is the energy imbalance of the system overtime. Solar energy from space can add energy to the system and warm objects emit energy in the infrared to space. Solar energy entering the atmosphere is mostly predictable based on the season. their is some variation of in solar cycles but that is not as significant as the season in the arctic. The amount of energy that an object radiates is a function of material and temperature. The albedo of the atmosphere and ice are harder to predict and control how much energy makes it in and out of the system.   


Yes in general someone has calculated how much energy is tied up in melting ice but it is not very useful as it is a simple calculation of volume of ice melted times the latent heat of fusion. If you can predict energy flow over time it would be relatively straight forward to predict the volume.


On a related note volume numbers are mostly based on models and little on direct measurement. Models can only get better with lots of direct measurement. Only the thinnest of ice can be measured by satellite. Occasionally people go to measure ice thickness but that isn't nearly enough and only produces one data point at a time. Because of this their is large variability in the thickness data from different models. They are working on a system measure thickness from below I believe using sonar but I am not sure.

petm

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Re: Basic questions about melting physics
« Reply #25 on: May 30, 2019, 02:36:54 PM »
According to wikipedia, it takes 333.55 kJ of energy to melt 1 kg of ice, but only 83.6 kJ of energy to increase the temperature of 1kg of water by 20 degrees celsius. That means that 333.55 kJ of energy raises the temperature of water ~80 degrees C.

Yes, and once that energy becomes available, what happens to the weather? In vulnerable regions, could some storms mix up to the surface warm Atlantic waters from below the halocline? If so, could this become a positive feedback, further increasing storm intensity and subsequent mixing? And what could be the result of such a feedback on global atmospheric circulation patterns, winter re-freeze, etc.?

At some point, the Arctic will become ice-free year-round. This will very probably require centuries more warming, but no one really knows how long or the path that will take us there.

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Re: Basic questions about melting physics
« Reply #26 on: May 30, 2019, 03:28:47 PM »
     Simpler framing of the original question makes the answer easier to see.
Original mystery (to me at least) was why wasn't PIOMAS volume decreasing in sync  with loss of old think ice?

    My new dope-slap-forehead observation - winter maximums have declined, but not as much as summer minimums (maxima, minima for those of you who remember 7th grade Latin).  Thus it is clear that winter refreeze has increased along with declining summer minima.  Not enough to fully compensate for increased summer losses, but enough to reduce their impact on winter maxima.
 
   As helpfully pointed out by Oren and others, that is indeed the case.  As to why winter ice gain increases with declining summer minima, two mechanisms are
1) thinner ice is able to increase thickness faster, and
2) ice cover acts as an insulator, thus less insulation means faster cooling in the following fall/winter. 
     There may be other mechanisms in addition to 1 and 2 (changes in cloud cover or wind patterns?).  And within 1 and 2 there are more detailed explanations for how/why they work.

   So nothing new here in this post!  I just thought anybody who was also puzzled by the original question, lack of 1:1 correlation between dramatic loss of older, thick ice, and the more subtle (but consistent trend) of PIOMAS ice volumes would find a bit of catch up and summary useful. 

   Two particularly interesting and useful points that arose -
    a) thick ice does not necessarily mean old ice.  Thinner fractured ice floes are more susceptible to being transported by wind or currents into thicker piles.
   b) salinity differences account for why older ice is more resistant to melt; and younger, saltier ice is less resistant to melt (though not everybody seems to agree about the chemistry at the molecular level).
   

gerontocrat

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Re: Basic questions about melting physics
« Reply #27 on: May 30, 2019, 03:30:08 PM »

But there's got to be some way to think about this numerically. I wonder if anyone's done it?
I like this example from the Polar Science Center, which brings in both the overall vast energy quantities involved and a dim light bulb at the micro level.

http://psc.apl.uw.edu/research/projects/arctic-sea-ice-volume-anomaly/
Quote
Perspective: Ice Loss and Energy
It takes energy to melt sea ice. How much energy? The energy required to melt the 16,400 Km3 of ice that are lost every year (1979-2010 average) from April to September as part of the natural annual cycle is about 5 x 1021 Joules. For comparison, the U.S. Energy consumption for 2009 (www.eia.gov/totalenergy) was about 1 x 1020 J. So it takes about the 50 times the annual U.S. energy consumption to melt this much ice every year. This energy comes from the change in the distribution of solar radiation as the earth rotates around the sun.

To melt the additional 280 km3 of sea ice, the amount we have have been losing on an annual basis based on PIOMAS calculations, it takes roughly 8.6 x 1019 J or 86% of U.S. energy consumption.

However, when spread over the area  covered by Arctic sea ice, the additional energy required to melt this much sea ice is actually quite small. It corresponds to about 0.4 Wm-2 . That’s like leaving a very small and dim flashlight bulb continuously burning on every square meter of ice. Tracking down such a small difference in energy is very difficult, and underscores why we need to look at longer time series and consider the uncertainties in our measurements and calculations.
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binntho

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Re: Basic questions about melting physics
« Reply #28 on: May 30, 2019, 04:16:06 PM »
It's not a simple matter!

       Two particularly interesting and useful points that arose -
    a) thick ice does not necessarily mean old ice.  Thinner fractured ice floes are more susceptible to being transported by wind or currents into thicker piles.
   b) salinity differences account for why older ice is more resistant to melt; and younger, saltier ice is less resistant to melt (though not everybody seems to agree about the chemistry at the molecular level).

I think that a) is a very useful insight, but b) is perhaps not wholly correct. Younger ice will tend to be more porous and even contain pockets of brine, both of which help with faster melting.

And I don't think there is any real disagreement abut the molecular-level chemistry - it's fairly basic after all. But applying molecular-level chemistry to floating sea ice is perhaps a bit more tricky.

LRC1962

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Re: Basic questions about melting physics
« Reply #29 on: May 30, 2019, 09:57:31 PM »
https://nsidc.org/cryosphere/seaice/index.html
Quote
Can you drink melted sea ice?
New ice is usually very salty because it contains concentrated droplets called brine that are trapped in pockets between the ice crystals, and so it would not make good drinking water. As ice ages, the brine eventually drains through the ice, and by the time it becomes multiyear ice, nearly all of the brine is gone. Most multiyear ice is fresh enough that someone could drink its melted water. In fact, multiyear ice often supplies the fresh water needed for polar expeditions. See Salinity and Brine in the Characteristics section for more information.
https://nsidc.org/cryosphere/seaice/characteristics/brine_salinity.html
Quote
Fresh water freezes at 0 degrees Celsius (32 degrees Fahrenheit), but the freezing point of sea water varies. For every 5 psu increase in salinity, the freezing point decreases by 0.28 degrees Celsius (0.5 degrees Fahrenheit); thus, in polar regions with an ocean salinity of about 32 psu, the water begins to freeze at -1.8 degrees Celsius (28.8 degrees Fahrenheit). The Arctic Ocean is generally fresher than other oceans, somewhere between 30 and 34 psu, but salinity levels vary by region, and areas with strong river inflow may have even lower salinity.
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Glen Koehler

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Re: Basic questions about melting physics
« Reply #30 on: May 31, 2019, 01:59:15 AM »
Doing some homework by reading the Slow Transition forum. https://forum.arctic-sea-ice.net/index.php/topic,933
    To expedite access to the take home points for others still getting their arms around the original question, here are some selected items from that discussion that provide a plausible explanation.  The quotes are from 2014 and I think they hold up well 5 years later.

1. Chris Reynolds #6  July 2014
"...2007 and 2012 saw massive gains in volume despite delays to the onset of melt. This is because the rate of growth of ice for open water and thin ice is extremely fast. The following graphic is from Thorndike 1975."



2. Chris Reynolds #49  July 2014
"…the contention that future April Arctic Ocean volume will be set by ice growing to thermodynamic equilibrium thickness (TET) from September to April, then further volume loss events leading to net thinning of the pack and enabling further increases of melt season losses of volume look unlikely.   Because with a mainly first year ice pack further drops in volume in years like 2012 will be followed by rapid recoveries to the volume implied by the TET around the time of those drops within a few years at most (Tietsche et al)."

3. From Chris Reynolds  Dosbat Blog.  http://dosbat.blogspot.com/2014/07/the-slow-transition.html
    "I am becoming convinced that the approximate levelling of PIOMAS volume over the last few winters is telling us that the pack is becoming dominated by FYI, whose thermodynamic equilibrium thickness is largely setting the peak volume in April."

    "I was persuaded that the loss of MYI represented energy that would then have to go into melting FYI after the MYI had declined. However because FYI regrows in the winter it vents this notional energy. The energy that once went into melting MYI is thus vented into the atmosphere and radiated to space in autumn/winter."

Tor Bejnar

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Re: Basic questions about melting physics
« Reply #31 on: May 31, 2019, 02:35:06 AM »
I 'found' this 1991 paper Physical and Dynamic Properties of Sea Ice in the Polar Oceans linked in a 2017 Barneo post.  I've been reading about ice salinity and the various mechanisms that desalinate ice. (40+ pages)
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Archimid

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Re: Basic questions about melting physics
« Reply #32 on: May 31, 2019, 02:54:31 AM »
Quote
"The energy that once went into melting MYI is thus vented into the atmosphere and radiated to space in autumn/winter."

And that shows as much warmer fall/winter temperatures, and in some cases a delay in freezing until the extra atmospheric and ocean heat is "vented".

See the fall/winter of 2016/17

https://www.climate.gov/news-features/featured-images/2017-arctic-report-card-extreme-fall-warmth-drove-near-record-annual



This is where I believe the slow transition theory has the highest chance for failing early.
I am an energy reservoir seemingly intent on lowering entropy for self preservation.

oren

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Re: Basic questions about melting physics
« Reply #33 on: May 31, 2019, 03:09:42 AM »
Doing some homework by reading the Slow Transition forum. https://forum.arctic-sea-ice.net/index.php/topic,933
    To expedite access to the take home points for others still getting their arms around the original question, here are some selected items from that discussion that provide a plausible explanation.  The quotes are from 2014 and I think they hold up well 5 years later.
While Chris's theory was brilliant and I think very innovative for its time, the winter of 2016-2017 showed that it was not foolproof. Refreeze was so delayed and autumn temps so high (as noted by Archimid above) that April saw a record low volume in the Arctic Ocean. I cannot manage to recreate Chris's numbers but here's a chart showing PIOMAS volume for the inner basin plus CAA and Greenland Sea for day 120, with 2010-2014's flatline highlighted. The break below 18k km3 clearly shows that the transition may not be as slow as postulated/hoped for - because the FYI may not thicken as much as it used to.

Dharma Rupa

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Re: Basic questions about melting physics
« Reply #34 on: June 02, 2019, 07:27:47 PM »
My memory of the slow transition thread is that the main questionmark was the future behavior of water vapor.  My contention at the time, and to this day, is that the Arctic is in transition from a desert climate to a maritime climate.  The Arctic nights are going to get cloudier and lose less heat to space.  I think the transition from desert to maritime has already happened rather abruptly in late December 2015, and we are now simply waiting for enough ice to melt in Summer that it cannot reach thermal equilibrium the following Winter.


Dharma Rupa

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Re: Basic questions about melting physics
« Reply #35 on: June 02, 2019, 07:29:18 PM »
Sorry for the back and forth despite my being a layman on this subject. But: I am talking about ice touching salt water, not at depth but nearly at the surface where pressure change is negligible. It is "known" that salt hastens the melting of ice. In other words lowering the melting point, even if the ice is pure freshwater.
Can some expert step in?  ???

How about someone who has made ice cream?

johnm33

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Re: Basic questions about melting physics
« Reply #36 on: June 02, 2019, 09:47:03 PM »
" Salty water freezes at a lower temperature than plain water. But the ice is made of plain water, so it melts at 0 degrees Celsius. Since the ice keeps melting, but the water no longer freezes (because there is only salt water, which doesn't freeze at 0 degrees), the temperature goes down.

The heat gained by the ice as it melts is no longer offset by the heat given up by freezing water (since the water is no longer freezing back onto the ice). The heat gain has to come from somewhere else. It comes from the ice cream and your hands.

The sodium and chlorine in the salt split apart into charged ions, and these ions attract water molecules to form weak chemical bonds.

The resulting compound has a freezing point of -21.1 degrees Celsius (-5.98 degrees Fahrenheit). This is 21.1 degrees colder than ice (37.98 degrees Fahrenheit colder than ice). " from https://sci-toys.com/scitoys/scitoys/thermo/ice_cream/ice_cream.html
So it seems that any windborne salt could 'suck' heat out of the wet surface layer of ice creating I presume a drop of brine at>-0C but cooling the remainder down to a possible -21C but no further effect on saltwater.

SteveMDFP

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Re: Basic questions about melting physics
« Reply #37 on: June 02, 2019, 11:43:56 PM »

The resulting compound has a freezing point of -21.1 degrees Celsius (-5.98 degrees Fahrenheit). This is 21.1 degrees colder than ice (37.98 degrees Fahrenheit colder than ice). " from https://sci-toys.com/scitoys/scitoys/thermo/ice_cream/ice_cream.html
So it seems that any windborne salt could 'suck' heat out of the wet surface layer of ice creating I presume a drop of brine at>-0C but cooling the remainder down to a possible -21C but no further effect on saltwater.

Windborne salt?  On the forum somewhere recently it was stated that 2/3 of overall melt is bottom surface melt.  Only 1/3 is top surface.  Salty seawater will melt the bottoms of floes until an equilibrium temp is reached.  Although the freshening of the seawater by melt will slow the process, there's always some mixing of seawater going on, and also conduction of heat from another meter or so down.

Phil.

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Re: Basic questions about melting physics
« Reply #38 on: June 04, 2019, 02:02:19 PM »
Temperature is a measure of kinetic energy of the system. Kinetic energy includes translational rotational and vibrational modes.
Temperature does not consider potential energy of the system.  Potential energy includes the energy of bonding. This include strong bonding like ionic and covalent as well as weaker interactions such as hydrogen bonding, vander wahls interactions and others.
Pure water has a higher potential energy than pure ice. If the system is at equilibrium the water and ice have the same kinetic energy namely temperature. The difference in energy is the potential energy of hydrogen bonding you mentioned.
Salt water has a lower potential energy than pure water because of the interactions between ions and water molecules. This interaction is weaker than h bonding so salt water has a higher potential energy then pure ice.

Potential energy of each system from high to low is pure water, salt water and then pure ice. I am not talking about temperature here just bonding energies. 
The potential energy of the saltwater changes with concentration higher concentrations of ions gives lower potential energy of the system.
the difference in potential energy of a saltwater pure ice system is lower than the potential energy difference between pure water and pure ice. Due to conservation of energy potential energy can be converted from kinetic energy. So the kinetic energy required to melt the saltwater pure ice system is lower than for the pure water pure ice system. In other words saltwater pure ice system melts at a lower temperature than pure water pure ice system.

The other important factor in the solutions is entropy which can have a bigger contribution to Free Energy than enthalpy in such mixture.  The 'order' due to hydrogen bonding around the ions being a major contributor in some biological systems the 'order' effects can be the drivers rather than enthalpy differences:  DeltaG=DeltaH-T*DeltaS

Tor Bejnar

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Re: Basic questions about melting physics
« Reply #39 on: July 25, 2019, 05:41:45 PM »
There was some discussion on a 'main' thread abut 850 temperatures vs. 2m temperatures.

Towards our mutual understanding of whys and wherefores, I offer the following.  I know extremely little about atmospheric science, so cannot offer (really) anything, but hope those who do will share more, even extensively!

An internet search led me to WeatherPrediction.com
Quote
FORECASTING SURFACE HIGH USING 850-mb TEMP 

METEOROLOGIST JEFF HABY

LIMITATIONS

 Forecasting the surface high using the 850-mb temperature has been a popular forecasting technique. First Iwill go over the limitations of using this method:

 1. Method does not work on cloudy days or days with afternoon precipitation

 2. There is a tendency that temperature will be higher than predicted on days the wind is light and will be lower than predicted on days the wind is strong. This is because the low level wind effectsthe depth of mixing.

 3. Method assumes air is mixed only between the surface and 850 mb. If the air mixes to a height significantly above or below 850 mb the technique will not work accurately.

 4. Method does not account for elevation. High elevation regions have a greater chance of mixing air that is above 850 mb.

 5. Daylight hours effect accuracy. There is a significant difference in daylight hours between the warm and cool season.

 6. Method only works in a barotropic atmosphere. Fronts or differential advection will contaminate technique.

 7. Method does not work well in regions with complex topography or near mesoscale temperature gradients such as coastal areas, very hilly areas, and areas near large lakes.

 The method works best in locations near sea level, in the warm season, on barotropic days, with flat topography, on moderate windy cloud-free days. If any of these conditions are not met then take that into account on the temperature prediction.

THE METHOD

 The method itself is very easy. Method needs to be done on a Skew-T to ensure accuracy. From the morning sounding, note the 850-mb temperature. Take a parcel of air at 850 mb using the 850-mb temperature and bring it dry adiabatically to the surface. The temperature of this parcel after it is brought to the surface will estimate the high temperature for the day.
[minor editing - some words became 'run-on words' when copied]
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Rich

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Re: Basic questions about melting physics
« Reply #40 on: July 25, 2019, 06:39:57 PM »
Thank you Tor !!

Tor Bejnar

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Re: Basic questions about melting physics
« Reply #41 on: July 25, 2019, 06:40:13 PM »
Here is an apparent conflict between physics and 'experience'.
Quote
You do know that there are no "sources" of cold?

Of course there are sources of cold. If I have a glass of warm water and I need a source of cold I just go get some ice and throw it in.
<Snippage>
OK, while creative, not really A Thing.

You don't really have "sources of cold" any more than you have "sources of vaccuum".  What "cold" indicates is a difference in enthalpy - net heat content components of a system, and thanks to the laws of thermodynamics heat will attempt to equilibrate across it - thus your ice cubes melting. 

There wasn't any "cold source" here, just the heat of varying levels being redistributed.

This does bring me to a point which I feel people have been overlooking.  It unfortunately is one for which we probably have the least instrumentation for - net enthalpy of the Arctic ocean and surrounding seas.

*This* will be the key factor in the tipping point.
<<a great deal of material removed - it is basically Arctic-specific.>>

In the northern hemisphere mid-latitudes when a 'cold North wind' blows, it gets colder.  A balloon floating in that cold air physically goes south, so I say the cold air came south too.  The source of that cold air was 'someplace' north of me.  Yes, it is a 'distribution' thing, but in the local sense, it sure seems like there is a 'source'.  The balloon 'came from the north', why can't the 'cold'?  OK: the 'air packet' came from the north, and it happened to be cold.  That air packet is cold because it lost its heat to the heavens [angels, I guess, lug the heat up when taking breaks from harp playing].  So, a source for the air, but not the cold.

(Those drink-cooling ice cubes were clearly made by a 'heat-redistribution machine'.)
How wrong did I go, physicists of the world?
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DrTskoul

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Re: Basic questions about melting physics
« Reply #42 on: July 25, 2019, 07:34:30 PM »
From a mathematical point of view, similar to a current of "holes" in semiconductors , a current and a source of "cold" for a higher level ( not deep) understanding are perfectly valid as long as one appreciates the simplifying assumptions. For me as a chemical engineer, I can solve the problem and give you the same result regardless the framework.

binntho

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Re: Basic questions about melting physics
« Reply #43 on: July 26, 2019, 07:15:30 AM »
Here is an apparent conflict between physics and 'experience'.
Quote
You do know that there are no "sources" of cold?

Of course there are sources of cold. If I have a glass of warm water and I need a source of cold I just go get some ice and throw it in.
<Snippage>
OK, while creative, not really A Thing.

You don't really have "sources of cold" any more than you have "sources of vaccuum".  What "cold" indicates is a difference in enthalpy - net heat content components of a system, and thanks to the laws of thermodynamics heat will attempt to equilibrate across it - thus your ice cubes melting. 

There wasn't any "cold source" here, just the heat of varying levels being redistributed.

This does bring me to a point which I feel people have been overlooking.  It unfortunately is one for which we probably have the least instrumentation for - net enthalpy of the Arctic ocean and surrounding seas.

*This* will be the key factor in the tipping point.
<<a great deal of material removed - it is basically Arctic-specific.>>

In the northern hemisphere mid-latitudes when a 'cold North wind' blows, it gets colder.  A balloon floating in that cold air physically goes south, so I say the cold air came south too.  The source of that cold air was 'someplace' north of me.  Yes, it is a 'distribution' thing, but in the local sense, it sure seems like there is a 'source'.  The balloon 'came from the north', why can't the 'cold'?  OK: the 'air packet' came from the north, and it happened to be cold.  That air packet is cold because it lost its heat to the heavens [angels, I guess, lug the heat up when taking breaks from harp playing].  So, a source for the air, but not the cold.

(Those drink-cooling ice cubes were clearly made by a 'heat-redistribution machine'.)
How wrong did I go, physicists of the world?
In daily parlance, having a "source" of cold is quite normal. Having just had my air-conditioner fixed has made me thankful for that particular source of cold when I'm trying to sleep.

But when talking about larger systems, hemispheric and global weather for example, one has to be clear that  there is never a source of cold, only sources of heat.

I started this whole "source of cold" discussion after reading a post by Archimid about how there were this and that source of cold that could have this and that effect on the Arctic. That was so obviously a wrong way to talk about weather and climate that I had to ask, and he has been strangely reticent in agreeing with the physics.

I also wanted to point out (and perhaps get a little bit closer to the topic of this thread? Edit: Didn't realize that this was a new and more appropriate thread for this sort of thing) that space is a great sucker of warmth, a massive sink of heat, and would be by very far the hugest "source of cold" ever.

And the Arctic, during the winter months, loses massive amounts of heat to space and will continue to do so. Losing summer ice is only going to increase this heat loss, and if there ever is an equitable climate, this heat loss is going to get even bigger, leading to a fall in temperatures elsewhere.

Which all counts as a huge negative feedback, reducing the speed of global warming and generally working to cool the planet down.

Of course, there is another, very potent positive feedback in losing Arctic ice, so an ice-free Arctic ocean will presumably have a significant net effect of warming rather than cooling the planet. But never forget that negative feedback that's going to have it's say as well!

And once all the ice is gone, the positive feedback of losing ice stops. But the negative feedback of losing heat just keeps growing with increasing temperatures. How this will all play out is a total mystery to me, but some people seem to think they've got it all worked out.
« Last Edit: July 26, 2019, 09:56:52 AM by binntho »