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Rob Dekker

September historic reconstructions are more difficult than August, because of the sparse observations available.

But since this thread started with Diablo's reconstruction for September 1935 - present, I owed a September reconstruction using my Match-and-Merge spatial/temporal fill-in algorithm over that period.

Note that I started with the Walsh reconstruction, then removed Walsh's spatial/temporal fill-in source, and I removed the Walsh&Johnson source, but let the other observations (such as the Navo yearbooks, the Dehn collection, AARI, DMI etc) stand. Then I let my match-and-merge algorithm fill it the rest. Here is the result :

Click for a larger image.

First thing to note is that the September reconstruction more closely follows Walsh reconstruction than for August. That's probably since September does not use Kelly fields, and this suggests that the August Kelly fields in Walsh reconstruction are really no good.

Second, I wanted to get some sort of "uncertainty" metric for these reconstructions, and the best I could come up with is to plot the difference between August and September. Remember that both months are based on independent observations, so if there is a wild difference between the months, then there is probably a large uncertainty in the reconstruction.

But as you can see, the difference between August and September is rather stable (hovers around 900 k km^2) and this gives confidence that the uncertainty in the August and September reconstruction is rather small (maybe in the order of a few hundred k km^2 SD).

Finally, let's compare the reconstructions.
Diablo, I do not have the numbers for Meier et al and your reconstruction, so could you please provide a 'final' overview plot of how our reconstructions of Arctic sea ice in September from 1935 to present day differ ? If you need my exact numbers for 1935-1978, please let me know and I can send them to you (if I know your email) or post them here.

Rob Dekker

Second plot here is for the entire 1850-present period, for the month of September :

Notice that with my fill-in algorithm, there appears a small uptrend from 1850 to about 1900. I'm not sure if that is real, but I suspect it is not. After all, with very sparse observations my algorithm tends to amplify the 'open water' area ('open-water bias' as explained earlier) and with the very sparse observations of the pre-1900 era, that effect may amplify.

Another indication that that 'up-trend' in the 19th century is not real is the August-minus-September graph plotted in blue : before 1900 the difference between August and September appears to go up, which is very unlikely to be realistic.

In all, for the full 1850-present period, I prefer my August reconstruction (presented earlier). There are more observations in August, and it results is a robust and realistic result, especially when you analyze individual years as I did in this thread.

Rob Dekker

Final note : Because of the "open water" bias of my fill-in algorithm, I consider my September reconstruction a lower bound for ice extent since 1850. Maybe the difference with Walsh' original which is (for pre-1900) more based on a fixed 'climatology', the difference between the graphs is an indication of the uncertainty in observation.

Diablobanquisa

Thank you for your work, Rob.


Diablo, I do not have the numbers for Meier et al and your reconstruction, so could you please provide a 'final' overview plot of how our reconstructions of Arctic sea ice in September from 1935 to present day differ ? If you need my exact numbers for 1935-1978, please let me know and I can send them to you (if I know your email) or post them here.

I have extracted your extent numbers from the graph you posted above. It's not exact, but I think it should be accurate enough for these comparison purposes. Just to check I did it right: your lowest pre-satellite value is 1941 around 6.42 M km^2? And the highest is 1963 with 8.78 M km^2?
(Alternatively, you can download our numbers from: https://doi.org/10.5281/zenodo.44758 and plot them yourself).

The graph below compares Walsh et al. 2016 (blue), my results (red) and yours (black), for September 1935-2013 (click for a larger version):


I think that the three time series look pretty consistent.


Regarding some of your comments:

First thing to note is that the September reconstruction more closely follows Walsh reconstruction than for August. That's probably since September does not use Kelly fields, and this suggests that the August Kelly fields in Walsh reconstruction are really no good.

I agree.



Another indication that that 'up-trend' in the 19th century is not real is the August-minus-September graph plotted in blue : before 1900 the difference between August and September appears to go up, which is very unlikely to be realistic.

I consider my September reconstruction a lower bound for ice extent since 1850. Maybe the difference with Walsh' original which is (for pre-1900) more based on a fixed 'climatology', the difference between the graphs is an indication of the uncertainty in observation.

I think that Walsh et al. and you have done a great work reconstructing the 1850-1934 period. However, I think that even August data before 1935 must be taken with a grain of salt. And I wouldn't put too much faith in any results before 1900: there are simply no enough direct observations available. I think the uncertainties and error margins before 1900 are huge.


Diablobanquisa

And this is the graph comparing with Meier et al. (click for a larger version):

Diablobanquisa

I've also extracted your August numbers and plotted them against your and our results for September (click for a larger version):

Rob Dekker

Thanks Diablo,
It is remarkable how closely the post-1935 September reconstructions match. Especially the difference between your and my reconstruction is surprising, since we used different methods, and had only AARI observations in common. Apologies for being so skeptical at the start of this thread.

Regarding the full 1850-present reconstruction, Walsh and his team deserve all the credit. All I did was take out the suspect Kelly fields and filled them in with a regional match-and-merge spatial/temporal fill-in algorithm. In that regard, I feel that my best contribution to this work is my August reconstruction :

My August reconstruction simply matches much better with the September numbers from Meiers, you and Walsh own numbers, so
if anything came out of this analysis, it is the hope that Walsh et al takes the Kelly fields source out of the next version of their historical Arctic sea ice extent reconstruction and replace it by their spatial/temporal fill-in algorithm.

I'm still disappointed that I have not been able to come up with a statistically sound method to determine "uncertainty" in the reconstruction, but if I find a method, I'll surely post it here.

All in all, it has been marvelous thread, and I appreciate your clarity of insight and attention to detail from you, Diablo. If you ever think of publishing another paper, please let me know and I'd be happy to contribute. Neven has my contact info.

Diablobanquisa

Thanks for everything, Rob, I have really enjoyed this thread too.

Rob Dekker

When you look at the images I posted, it will tell to upgrade the photobucket account to one that supports 3rd party hosting. Photobucket charges $400/year for that and I'm not going to pay that amount of money.

So, to look at the images I posted, right-click the image and open in a new window. Thanks !

Jim Hunt

I tried that Rob, but as a 3rd party I still only saw the "Please update your account" image.

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