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crandles

I agree the it would be nice to get more than the "single-scalar volume number we get now". Maps are nice but that hindcast forecast is as much as I want in map format. What I would like are their G1, G1.5 and G2 numbers for each year from 1980 for 1 May, 1 Jun and 1 Jul.

(IIRC G1 represent area with thickness over 1 meter.)

9 columns, 32 rows - not a huge amount of information. Maybe G0.5 for 1 Jul and 1 Aug.


>"That seems to suggest that PIOMAS heavily overestimates ice-melting rate (in the middle of the pack) and underestimates melting at the ice margin."

Not sure I see that effect much - maybe I am expecting melting to slow down in rate after early Aug too much. It would be interesting to get some sort of estimate of relative melting rate for say within 100km of edge versus rest of pack. I would have thought they would have tested for whether the ratio the model arrives at is reproducing reality well or not and adjusted parameters until it is reproduced well.

Chris Reynolds

Rob,

"If it came from above, then thin ice would be mostly affected. After all, ice thickness increases heat insulation and thus any heat-from-above would not directly affect thick ice."

Whoa! Hold on there! ;)

You're forgetting Bitz and Roe.
http://www.atmos.washington.edu/~bitz/Bitz_and_Roe_2004.pdf

They argue(show) that because thick ice recovers more slowly from perturbations but thin ice recovers more quickly, that thinning concentrates in the thick ice category. So with a continual forcing as at present, thin ice is more quickly able to regrow every season whereas thick ice takes years to grow, and exhibits the greatest thinning.

My suggestion - don't think in terms of heat going down from atmosphere through the ice. Certainly during the Winter, and well into the melt season for the central Arctic net heat flux is firmly from ocean to atmosphere. During the Summer it's a zero sum game - the ocean is near 0degC under the ice, an the surface is at a similar temperature - melt pegs it at zero. What warming of the atmosphere from say -20 to -18 degC can do is reduce the temperature gradient across the ice, and hence reduce heat flux through the ice.

The Kwok/Untersteiner paper is the source of the a graphic I use quite a lot, see fig 3. I think this is why PIOMAS (and possibly NPS) is so important. Finding a 1W/m2 forcing is impossible with observations, so a model is the best option for figuring out what's gone on.

The Mercator model must be forced using atmospheric data, like PIOMAS uses NCEP/NCAR and NPS uses ECWMF. Any free running model would be useless in the context Mercator use their model - free running models rapidly develope their own realisation of climate, which deviates from the one we're in - reality. It's only by assimilation that fidelity with our 'realisation' can be maintained.

Chris Reynolds

Rob,

With regards PIOMAS melting behaviour.

The thinning in that model may be more severe in forecast than the model would have shown using observed atmosphere. IIRC they predicted a greater loss than happened.

Anyway I think I see a way of cutting through the complexity. Your suggestion is that the ice should continue to melt right down to nothing even where the patches of ice thin enough to continue melting are within the central pack. But we don't see a pack with an ice edge surrounding it, and large areas of open water within that ice edge. We do see large areas of broken ice, but have we any examples of large areas of open water opening within the pack due to this 'rotten ice'? Correct me if I'm wrong, but I don't recall seeing this.

I suspect something else is going on here. I'll think about it while watching the Big Bang Theory.

Rob Dekker

Chris, Your suggestion is that the ice should continue to melt right down to nothing even where the patches of ice thin enough to continue melting are within the central pack.

Actually, that's not really what I suggest.
I suggest that PIOMAS seems to overestimate melting of ice in the middle of the pack, and underestimate melting on the ice margin.
I don't think that I suggested that there should be "examples of large areas of open water opening within the pack".

On the contrary, I would say. Melting on the ice edge is typically much more intense than melting inside the icepack (due to ocean water 'eating away' the ice edge, and albedo effects being most pronounced on the ice edge. Is that observation in dispute
at all ?

Rob Dekker

BTW Thanks Chris for the Bitz and Roe paper. I'm in the process of reading the details.
It may very well be that atmospheric forcing of 1 W/m^2 could explain the loss of thick sea ice in the Arctic. If that is true, then we would not need a 1 W/m^2 ocean heat flux to explain it. Let me just note that IF atmospheric forcing of 1 W/m^2 would explain ice loss in the past few decades, that attribution to AGW would be rather simple.
Let me get back to you after I read Bitz and Roe.

Chris Reynolds

Rob,

I'd misunderstood what you were getting at, no we don't have the disagreement I saw.

I'm loathe to simply reject PIOMAS in this respect, favouring a flaw in my understanding. The thickness based on area doesn't support the end of season thicknessses seen in the PIOMAS projection Crandles linked to. As I keep going on - the startling thing about the last two years is the implied thickness loss, from 1.5m previously to 1m in the last two years.

I think there are three broad factors at work on the ice, atmospheric heat, ocean heat and mechanical effects. Kwok & Untersteiner's 1W/m2 is over the whole period since 1979 IIRC. Much of that period had fairly small losses, it's in the last ten years things have really speeded up. So it may seem small against the larger figures for AW, but AW doesn't stick to the surface (warming the deeps as well). So in a qualitative sense the picture may not be so confusing - no more confusing than anything else about the Arctic.

Rob Dekker

Chris,
I read Bitz and Roe, and I really like the paper.

Since I have been looking at the physics of finding an equilibrium ice thickness based on surface and bottom heat fluxes during growth in winter and melt in summer, their analysis captured my interest.

The issue at hand here was if a change in atmospheric flux (such as by AGW forcing) would have the same effect as a change in ocean heat flux on the final equilibrium ice thickness.

In Blitz and Roe, they use the Thorndike equations to calculate influence of AGW atmospheric forcing.

However, they are making quite a number of assumptions which may be hard to justify.
For example, they do not distinguish between top- and bottom melt in their equation 2 (summer melt). In other words, they assume that all summer heat flux causing melting is the same, ragardless of if that is a 10cm melt pond or a 10cm bottom melt caused by ocean heat flux, even though a melting pond freezes over in no time in fall, while 10cm bottom melt in 3 meter ice will require a significantly cold winter to grow back (especially in the presence of a little bit of ocean heat flux).

That way, ice thickness indeed becomes very sensitive to atmospheric flux (almost as sensitive as it is to ocean heat flux itself.

Interpolated over years and over the entire Arctic, it may turn out to be true that AGW atmospheric forcing caused the reduction in ice thickness that PIOMAS presented, but I don't see that their simple model provides enough physical evidence to conclude that.

Chris Reynolds

Rob,

All the Bitz & Roe paper is trying to do is to explain the puzzling fact that in both models and observations thicker ice thins more than thin. I see no problem in it and think they explain the difference - put simply - thin ice can grow faster than thick. Each winter the thin ice grows back, but this doesn't happen with slower thick ice - so the thick ice thins more. In a sense the thick ice has a memory of the forcings it is subjected to, it is an integrator of those forcings, the memory of thin ice is wiped regularly. The findings of Maslanik's drift age model suggest that the bulk of the ice pack is now FY ice, that which survives to become MY ice has a lifetime of only 3 to 4 years - a short memory indeed.

It matters not whether the forcing comes from top or bottom. In your explanation about the difference I think you merely underscore the mechanism involved, yes the thick ice can't recover as fast - that is their point.

The Polyakov paper suggests (fig 4) that while the Arctic ocean has warmed, most of this warming is below 100m so is not directly participating in melting the thick ice. Furthermore Bitz & Roe use CCSM models, in the recently discussed Malowski paper (see recent PIOMAS post's comments) it is shown that CCSM3 underestimates ocean heat fluxes. Does this mean the models used by Bitz & Roe also underestimated ocean heat flux? For my argument this is not important, because we can expect that the free-running models of CCSM used by Bitz & Roe had different heat fluxes, even for the same model under different realisations.

My point here is that with a large variation of ocean heat fluxes between observations and models we still see the same behaviour - thin ice less than thick ice. This indicates that we are not dealing with some issue around specific fluxes, but we are dealing with a more basic process. And the differing time constants of thin and thick ice is basic enough to be able apply despite the various differences - between models and reality and in changes of flux over the period of thinning.

William Crump

Chris and Rob:

In addition to the observation that it is easier to create new ice where none exists at the start of the freeze-up season and to add thickness to younger ice than it is too add thickness to old ice, I was wondering if you can quantify how much of the disparate volume loss in 2007 and 2010 can be explained by the advection of thick multi-year ice out of the Arctic through the Fram Strait rather than just melting in place factors?


Surface melt should be relatively the same regardless of whether the ice is thick or thin (please let me know if this is an incorrect assumption); therefore, the difference in the amount of decline in thickness between different types of ice must be coming from bottom melting. Since the warmer arctic waters are too deep at 100 meters to be in direct contact with Arctic ice the difference in bottom melt must be coming from a different mechanism.

Clearly it takes more heat input to melt more of the thicker ice than is being applied to the reduced melting of thin ice.

So what are the possible mechanisms that would allow greater exposure to heat for thick multi-year ice compared to thinner new and first year ice?

Does the thicker ice present a more irregular shape and greater profile that presents greater surface areas for bottom melting than the more uniform depth of thin ice?

Are the mechanics of maintaining a thin insulating layer of cold water better for thinner ice than it is for thicker ice which has irregular depths?

Are there other possibilities for why greater heat is reaching multi-year ice than new and first year ice?

Chris Reynolds

William,

"Clearly it takes more heat input to melt more of the thicker ice than is being applied to the reduced melting of thin ice."

More heat because it's integrated over a longer period of time. Not more heat per se, IMO.

Rob Dekker

Chris,
Thanks for your response, and insights. I truely appreciate this discussion.

It matters not whether the forcing comes from top or bottom.

But Chris, basic physics do NOT support that statement outright.

For example, imagine some VERY thick Arctic sea ice. Something like 10-100m ice or more, something like what was probably the norm during the Last Glacial Period. An ice pack this size would only top-melt a bit (form some melting ponds) in summer, which would freeze over quickly in fall, and probably little if any heat would make it to the bottom. So essentially it does not thin out in summer at all. What does this ice do in winter ? Well, to grows a tiny bit each winter. Actually basic physics dictate that without ocean heat flux it will grow thicker forever, albeit following a SQRT function over time. Even if 'atmospheric forcing' would increase, this would only slow down growth in winter, but if no heat makes it to the bottom of the ice, there will be nothing to stop it from growing thicker and thicker over the decades, centuries and millennia.

Now bring in ocean heat. We know that even minor ocean heat of some 1 W/m^2 will limit the ice to a couple of meters.

Thus, from basic physics point of view, it DOES matter where the heat comes from. And thus it DOES matter (big time) what the mechanism is.

More specifically, simple physical models need to calculate heat flux at the BOTTOM of ice, and cannot a-priory assume that top heat (atmospheric forcing) will dictate thinning of very thick ice.

As I mentioned in my last line in my last post, "it may turn out to be true that AGW atmospheric forcing caused the reduction in ice thickness that PIOMAS presented, but I don't see that their simple model provides enough physical evidence to conclude that."

Rob Dekker

William Crump said :

I was wondering if you can quantify how much of the disparate volume loss in 2007 and 2010 can be explained by the advection of thick multi-year ice out of the Arctic through the Fram Strait rather than just melting in place factors?

Bill, dozens of papers have been written about the 2007 anomaly. You hang around there quarters long enough. Why don't YOU present the paper that you found presented the most convincing argument. Then we can discuss that one.

Chris Reynolds

Rob,

To clarify my statement that it doesn't matter where the forcing comes from was part of the whole paragraph. Perhaps it would be better phrased as - regardless of whether the forcing comes from ocean or atmosphere, thick ice can't recover as fast as thin ice. So from the point of view of what they are seeking to demonstrate it really doesn't matter where the forcing comes from.

We can drop the issue of whether Bitz & Roe's model is adequate by cutting to the heart of the matter - thin ice can respond faster than thick, but can also recover faster.

I think you're arguing that ocean heat fluxes better are responsible for the loss of MY ice. Earlier you said:

> Clearly it takes more heat input to melt
> more of the thicker ice than is being a
> applied to the reduced melting of thin
> ice.

I disagree. The point is that because of its longer 'history' thick ice integrates the heat flux applied to it over its lifetime. Take two patches of ice - one FY and one MY ice. Yes the FY will be subjected to less heat input to melt it, than the (presumably) much thicker MY ice.

But change perspective to two geographically fixed areas, one containing MY ice one FY ice. The FY ice comes and goes, yet the MY ice persists. However if the ocean heat fluxes are the same then the net input over the period taken to melt the MY ice is the same. Its just that one year's FY ice doesn't stick around for long enough to experience the full flux that the MY ice does.

This however is merely another way of restating the findings of Bitz & Roe.

I don't think anyone is seriously saying that ocean heat fluxes aren't significant. But I really don't think they answer the question answered by Bitz & Roe.

Chris Reynolds

William,

See this page:
http://dosbat.blogspot.com/2012/04/musings-on-models.html
On it there's a quick summary of what happened in 2007 - I think you'll also find links to the source papers so you can check to see if you disagree with me. For some reason my blog's down right now. :(

I don't think Fram Strait was relevant to 2010 - the high pressure NCEP/NCAR shows is a far more likely candidate.

Rob Dekker

Chris,
I'm not sure if we are talking about the same thing any more. I agree with you that thick ice has a slower response time than thin ice, so it takes more time to recover from an anomalous year (like 2007).

But I thought we were discussion the difference between atmospheric forcing and ocean heat flux.

Basic physics suggest that (even a few W/m^2 increase in) ocean heat flux could very well explain the loss in volume (and thinning of thick MYI) over the past 3 decades.

Basic physics also suggest that and increase in atmospheric forcing (say, in the form of warmer winters) should affect thin and thick ice equally. For example, the 1 C warmer winters from the past decade or so represent something like a 5% decrease in heat flux through the ice in winter (ERA 40 winter is average -20 C), which ice can compensate for (obtain a new equilibrium) that is approximately 5% thinner. No matter the thickness !

An increase of Ocean heat flux of some fixed amount (say 2-5 W/m^2) will seriously eat away any ice more than a few meters until it reaches a new equilibrium.

So if history suggests that thick ice thinned a lot more than thin ice, the most plausible cause of that would be heat from below.

In general I think that ocean heat flux has been sort of the ignored child in the modeling world, with some models not even including ocean heat flux. This may be because the data (a few W/m^2 change over several decades, underneith Arctic sea ice) is incredably hard to obtain. So we seriously lack accurate data.

The common thinking goes as you report in your (by the way excellent!) blog post "musings-on-models", that Atlantic heat increase ends up more that 100m deep and 'probably' does not make it to the surface due to stratification. However, if we assume that previously the stratification layer was in thermal balance, then an increase in deep water temperature without a change in salinity gradient MOST cause an increase (possibly significant) in heat flux going up...

I think that Maslowski's high-resolution ocean current models capture that increased heat flux pretty well, and incidentally are also the most 'alarming' w.r.t. further collapse of MYI and an ice-free Arctic...

Rob Dekker

In summary, if we stick to simple physics of ice melt and freeze, then the observed atmospheric heat flux can be responsible for 5-10, maybe 20 % of ice volume loss over the past decades.

But to explain the 50-75 % volume loss that PIOMAS reports (and confirmed by hundreds of in-situ measurements) something else must be at work. Short of resorting to secondary effects (increased ice dynamics due to thinning of ice) the only effect I can think of that can even theoretically have that much of an effect on thick ice is under-ice ocean heat flux.

Chris Reynolds

Rob,

Actually when I asked if you wanted change threads I hadn't seen this reply - we'll continue here, although I suspect we may have reached an impasse.

"So if history suggests that thick ice thinned a lot more than thin ice, the most plausible cause of that would be heat from below. "

I made a point earlier about models and what Maslowski has said (their ocean heat flux is too small) - the preferred thinning of thicker ice in both different models and different runs (realisations) seems to me to suggest it's not ocean heat flux but a more fundamental process as Bitz & Roe find.

"In general I think that ocean heat flux has been sort of the ignored child in the modeling world, with some models not even including ocean heat flux"

Yet the models show similar preferential thinning of thick ice to that observed....

"I think that Maslowski's high-resolution ocean current models capture that increased heat flux pretty well, and incidentally are also the most 'alarming' w.r.t. further collapse of MYI and an ice-free Arctic..."

As I've posted on the May PIOMAS thread - I don't think careful reading of Schweiger's paper on PIOMAS uncertainty supports this w.r.t. PIOMAS - which shows volume decline every bit as severe as NPS.

Rob Dekker

Chris,

You mention we may have reached an impasse., but I disagree. I think we are actually getting to the heart of the matter.

I don't think careful reading of Schweiger's paper on PIOMAS uncertainty supports this w.r.t. PIOMAS - which shows volume decline every bit as severe as NPS.

True, but PIOMAS does not know (much) about ocean heat flux. So may it be that the decline in volume that they report is mostly because of the in-situ measurements used to calibrate the model ?
After all, the credibility of PIOMAS hangs on the calibration points from hundreds of submarine measurements and other (ICEsat) observations, does it not ?

Other models that do not know about changes in ocean heat flux are the IPCC GCMs. And how well are these faring in projecting ice extent and volume decline ?

Chris, the issue that I am putting forward is that basic physics dictate that ocean heat flux increase of even a few W/m^2 has a profound impact on ice thickness and can explain volume losses from the past three decades, that such increase is very plausible due to increased heat input from Atlantic/Bering waters, even through stratified layers, and that FLUXbouy data is unable to determine changes like that with statistical significance, and that most GCMs and even PIOMAS do not account for that increased heat flux.

And secondly that GCMs overestimate ice extent by more than 2 standard deviations, and that PIOMAS is constrained by in-situ ice thickness observations.

Could you possibly consider that maybe ocean heat flux DOES comply with basic physics, and thus MAY actually have a more significant influence than Bitz and Roe suggest ?

Rob Dekker

Actually, Bitz and Roe don't deserve much critisism, since they essentially ignored ocean heat flux. Instead, their simple models, in my opinion, overestimated ice melt due to atmospheric forcing (assuming top-melt equals bottom-melt) as do many of the more complex models that do not consider physical effects of melting ponds.

Chris Reynolds

Rob,

I think we're at an impasse because I'm just repeating myself here. I'm not denying that ocean heat flux has a role. But you're taking the matter further than I think the evidence allows - that OHF "...can explain volume losses from the past three decades..."

"So may it be that the decline in volume that they report is mostly because of the in-situ measurements used to calibrate the model ?"

If that's the case then PIOMAS is little better than statistical extrapolation! I doubt that is the case.

"Other models that do not know about changes in ocean heat flux are the IPCC GCMs. And how well are these faring in projecting ice extent and volume decline ?"

Then can you explain why the same models also find that thicker ice thins more? I am repeating myself here, but this is an issue you seem to be ignoring.

It might interest you to see this graphic:
http://nsidc.org/images/arcticseaicenews/20110816_Figure4.png
From here:
http://nsidc.org/arcticseaicenews/2011/08/arctic-sea-ice-at-the-crossroads/

It is notable from there that: Most of the melt is top melt. The areas of substantial bottom melt are near the regions of open water. The areas in the central Arctic (where the MY ice is) show little bottom melt.

This implies that most of the ocean heat flux causing bottom melt is actually from ic-albedo driven warming of the ocean, with a possible input from the Bering Strait. In the Central Arctic it implies that the contribution of ocean influx from the Atlantic is small - otherwise bottom melt in the central Arctic should be greater. This last implication tallies with the findings of Polyakov regarding the depth of Atlantic influx which suggests a lack of interaction with the ice at the surface.

Werther

I don’t care much for the data coming out of buoys on just three locations. As I did my CAD count on day 160 on a 90000 km2 area within MODIS r03c03, buoy 2011C wasn’t far north of there, close to the pole. I found just 43% unscathed floes, the rest was rubble in a dense pattern of leads. It looks perfectly logic to me that bottom melt under a lot of the floes there would be insignificant compared to surface melt. Some of these floes might actually still keel tens of meters deep (under old pressure ridges).
But what about the situation in the rubble-leads? It wouldn’t surprise me that that’s where an intricate pattern of mixing, eddying takes place. Of course, this is guess-work. We probably won’t be able to measure it all before an ice free state has arrived.
In fact, it doesn’t really matter which medium is carrying the heat. It’s happening. I’d like very much to know actual thickness in r04c04, FI. Because it looks bad out there. But there isn’t going to be a dataset. There’s just extent and area, and these still count let’s say 100 and 97 percent. But the leads are exposing strength and indirectly volume. This isn’t a region where thinning is the issue. It’s a lot of FYI that hasn’t thickened much during winter. Mean temps haven’t been real cold (surface). And the water underneath is warming. Whether it’s due to built-up ice-albedo effect in the summer before or upwelling from the conveyor that is redistributing heat.
But I'll read in on Polyakov..

Werther

I’ve spent some time on Recent Changes of Arctic Multiyear Sea Ice
Coverage and the Likely Causes, feb 2012, Polyakov, Walsh and Kwok.
Good information, for sure. I’m not a scholar. I don’t pretend to know what it’s all about. But I can look around and think for myself. I noticed the cascading graphs for five flux portals. They show pretty well the 100-200 m overlaying cold water. In the text, indications of heat flux up to the ice bottom are given and qualified. It is there, but not clear how much or how relevant. New and specialised research should be done for an answer, the report mentions.
But I also noticed that the article is based on data and research up to 2009. The last two years are not comprised. It’s interesting that 2010 and 2011 were La Nina years. 2010 had this anomalous ocean warming around Greenland and the Labrador Sea. 2011 showed the same, in the Barentsz-Kara region. I have a strong feeling that it’s not just Polyakov’s graphs that cascade…

Rob Dekker

Chris,

you're taking the matter further than I think the evidence allows

You may very well be right, Chris. But part of the problem is that (as also Werther pointed out) we really don't have enough accurate data. So in a sense, with my 'basic physics' analysis of thin versus thick ice, and significance of attribution of Ocean Heat Flux as a cause of volume loss over the decades, I am still arguing within the uncertainties that the observational evidence can disprove.

Remember that we are only talking about 1 W/m^2 (Kwok and Understeiner) over the decades. And only 10cm/year bottom melt.
The CRREL that you present I actually use to make my point sometimes. Bottom melt is reported only over a short period in summer, but in winter very small Ocean Heat Flux would be very hard to detect. Maybe only if some instrument would record that bottom-melt occurs while atmospheric temps are still below freezing.

Then can you explain why the same models also find that thicker ice thins more?

I thought about that for a while, and I think you are right. Without ocean heat flux in the model, thick ice should NOT thin more (relative to it's thickness) than thin ice, even in the presence of melting ponds and minor leads...

Do you have a reference to where you found that (thicker ice thins more) in the IPCC models ?

Rob Dekker

Unless of course if these models make the same mistake (of assuming that top-melt is physically the same as bottom-melt) as the simple models in Bitz and Roe. If they do make that mistake, then it's easy to explain that they find that thick ice thins more over the decades.

Rob Dekker

Werther, I hear you. With all the ice-crunching going on, it is hard to imagine that any theory or model would even be able to get close to the real thing. However, ice crunching was always happening, and still the decline is consistent and clear and faster than model projections.

It's like we are living in a house with highly variant weather outside, but after a couple of decades we find that nat.gas. usage for heating went up steadily...

Chris Reynolds

Rob,

"Do you have a reference to where you found that (thicker ice thins more) in the IPCC models ?"

Bitz & Roe as discussed here (fig 2 and section 2), and Bitz 2008 "Some aspects of uncertainty in predicting sea ice thinning."
http://www.atmos.washington.edu/~bitz/Bitz_2008.pdf

Rob Dekker

I'm sorry Chris. I realize that I did not explain my points clearly enough. Thank you for hanging in there with me and continuing this discussion.

"Do you have a reference to where you found that (thicker ice thins more) in the IPCC models ?"

Bitz & Roe as discussed here (fig 2 and section 2),..

This point I wanted to make is that basic physics dictates that atmospheric forcing alone should cause a 'linear' response in ice thickness reduction. And that we see very clearly in Bitz and Roe fig 2, for all three models presented. ECHAM3 shows something like 25% reduction of all ice thickness, HadCM3 some 40% and CCSM2 is more variable, but on still linear reduction of something like 50% reduction in ice thickness, for all initial ice thicknesses. This is a clear signature of atmospheric forcing, and does not seem to suffer from the mistakes in their 'simple' model.

But the issue is this : These model runs show 2xCO2 (3.7 W/m^2 TOA forcing) experiments, and they show nicely linear ice thickness reduction, so volume loss is between 25% and 50%.

In reality, we are only at 30% of 2xCO2 (since pre-industrial era). So from these models, one would expect less than only between 1/3rd or between 8% and 16% or so in volume reduction due to atmospheric forcing.

As I pointed out before : the observed atmospheric heat flux can be responsible for 5-10, maybe 20 % of ice volume loss over the past decades

PIOMAS, as calibrated by numerous ice thickness measurements, suggests ice volume reduction of some 75% in September. What caused that reduction ?

I suggest that that is due to ocean heat flux, which would show a quadratic curve over the ice thickness melt : Ice of X meters initially will reduce twice as fast as 1/2 X initial thick ice.

Another way of looking at this, is to take the 8-16% linear line (of atmospheric forcing-caused GCM thickness reduction) and plot it fig 1 from Bitz and Roe. You will see that up till 2 meter ice, the submarine ice measurements could fit, but beyond that (thicker ice) there has certainly been more melting than the atmospheric forcing as modeled in IPCC GCMs could explain.

I learned a lot from this discussion and if I ever do a blog post on this subject, I would certainly thank you for providing a skeptical analysis to my thoughts.

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