November 2013 Russian “Hotspot” – Alarmists Are Overlooking Something

UPDATE: Corrected the typo in Figure 3. 1988 now correctly reads 1989.

# # #

There’s lots of blogosphere chatter about the warm temperatures in Russia in November 2013. In their global State of the Climate Report this month, NOAA stated:

According to Roshydromet, Russia observed its warmest November since national records began in 1891. Some areas of the Urals, Siberia, south of the Far East region, and on the Arctic islands in the Kara Sea had temperatures that were more than 8°C (14°F) higher than the monthly average.

NOAA even discussed the record warm temperatures on their global map here.

It might be true that Russian land surface air temperatures were at record levels for the month of November, but NOAA failed to present something that’s blatantly obvious in the data. In 1988, surface air temperature anomalies for much of Russia shifted upwards by more than 1 deg C.

The Russian “hotspot” stands out very clearly in the NOAA map presented in Figure 1. Based on it, I’ve used the coordinates of 50N-70N, 30E-140E for the NOAA NCDC data, and the climate model outputs, presented in the following graphs. That region covers a major portion of Russia.

Figure 1

Figure 1

Figure 2 presents the NCDC land surface air temperature anomalies for the Russian “hotspot”, for the period of January 1920 to November 2013. I’ve highlighted about when the shift occurred. Before that shift, surface temperatures there warmed very little, if at all. And after it, surface temperatures appear to have warmed, but not at an excessing rate. We’ll confirm that later.

Figure 2

Figure 2

The shift is much easier to see if we smooth the data with a 13-month filter, minimizing the visual impact of the monthly variations. In fact, with the aid of period average temperatures (the horizontal lines) and with some color-coding, the shift in 1988 becomes obvious. See Figure 3. Based on the period-average temperatures before and after 1988, that climate shift raised Russian “hotspot” surface temperatures by about 1.1 deg C.

Figure 3c

Figure 3

MODEL-DATA COMPARISON BEFORE AND AFTER THE 1988 SHIFT

Figure 4 is a model-data comparison graph for the surface air temperature anomalies of the Russian “hotspot” for the period of January 1920 through December 1987. Both the NCDC surface temperature data and the climate model outputs have been smoothed w/ 13-month running average filters. The climate models are the multi-model ensemble mean of the models stored in the CMIP5 archive, using the historic and RCP6.0 scenarios. The CMIP5 archive, as you’ll recall, was used by the IPCC for their 5th Assessment Report. And we discussed why we use the model mean in the post here.

Figure 4

Figure 4

NOTE: The trends in Figures 4 and 5 are based on the “raw” data and model outputs, not the smoothed versions.

The models did a reasonable job of simulating the warming rate from 1920 to 1987. In more than 65 years, they only overestimated the warming by about 0.23 Deg C. But the models perform quite poorly for the period from January 1989 to November 2013. See Figure 5. During this much-shorter 25-year period, the models overestimated the warming by more than 1.1 deg C.

Figure 5

Figure 5

Let’s state that again: the models overestimated the warming by more than 1.1 deg C over the most recent 25-year period.

Climate model failings at the regional levels are not unusual. We discussed those failings in numerous posts over the past year and in my book Climate Models Fail.

WHAT CAUSED THE SHIFT?

The timing of the shift in the Russian surface temperatures is similar to the shift in Scandinavian surface air temperatures. See the post here. There we discussed that the shift in surface temperature was possibly a response to a shift in the sea level pressure and interrelated wind patterns associated with the Arctic Oscillation.

Additionally, see de Laat and Crok (2013) A Late 20th Century European Climate Shift: Fingerprint of Regional Brightening? The authors argue that a shift in the North Atlantic Oscillation (similar to the Arctic Oscillation) in the late 1980s caused more sunlight to warm European surface temperatures in an apparent shift. I would suspect that something similar occurred over Russia at that time as well.

CLOSING

Like other regions, a climate shift, not the long-term effects of manmade greenhouse gases, is responsible for a major portion of the warming that occurred over much of Russia.

And, of course, climate models performed poorly when attempting to simulate the warming that occurred there since the 1988 shift, overestimating the warming by a large amount. So what else is new?

SOURCE

The NCDC surface temperature data and the CMIP5-archived climate model outputs are available through the KNMI Climate Explorer.

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Robert Brooke

Isn’t 1988/89 also when the ‘great dying of thermometers’ took place? Large numbers of Soviet military bases with weather stations, many in cold remote locations, closing post Glasnost.

Gary

But let’s not forget the governmental change at that time. How many stations were reporting pre- and post-1988? Does it make a difference? Given the well-documented fudging of other data by the former USSR, people have speculated that temperatures were under-reported for reasons of convenience (do you want to trudge out to the thermometer station in the depth of winter?) and to support pleas for more heating fuel from central control. This timing coincidence is precise enough to warrant verifying that the measurements are accurate before saying the step increase is real.

izen

The climate shift seen in Russia coincides with the rapid rate of ice loss in the Arctic that started in the late eighties as well as warming in Scandinavia.
It is most likely a response to the polar amplification of the AGW which was predicted, with warmer air moving further North. It is not possible to find a previous episode of similar magnitude of warming that correlates with changes in the Atlantic Osscilation. The fact that models do not duplicate this local detail is a red herring. Models do not simulate the local climate and regional shifts with this level of detail.
Ascribing the observed warming to an unforced variation when there is no historical precedent, but a forced climate change does account for the observations might be mistaken.

mogamboguru

Well, frankly, I rather suspect a sensing issue or data-processing issue, combined with the collapse of the soviet heavy industry after the collapse of the Soviet Union during the late 1980ies to be the cause of the shift in Russia.
The shift described here has taken place exactly during the times of the collapse of the Soviet Union. After the fall of the Soviet Union, thousands of meteorological sites had been abandoned, due to further lack of funding. Therefore, a failure of a station still in use would weigh in much heavier to the combined result of all stations, because the size of the sample was substantially reduced.
Also, after the fall of the Soviet Union, the Soviet Union’s heavy industry had taken a serious blow, which MUST have had a substancial effect on the cleanliness of the air over Russia and, hence, on the resulting land temperatures.

herkimer

Yes , Bob, you are right about the AO index being a factor . There was a major shift of the AO index from negative to mostly positive in 1989, allowing more warm air north . The AO index has has again been strongly positive the last 2 months.
http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/JFM_season_ao_index.shtml

izen

@- Gary
The claim it might be connected with the change of political control and the closure of observation sites is refuted by the observations being confirmed by satellite data.

Green Sand

“….and on the Arctic islands in the Kara Sea had temperatures that were more than 8°C (14°F) higher than the monthly average.”
The Kara Sea has approx 50% more ice area than this time last year. I wonder how quickly it would have frozen if it hadn’t been subjected the Nov “heat wave”?
http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/recent365.anom.region.7.jpg

jai mitchell

The temperatures went up in Siberia when Perestroika led to the collapse of the soviet union. This caused a collapse in their production of sulfur dioxide emissions which, up to that point, had provided a significant and localized cooling event.
http://www.manicore.com/documentation/petrole/picp_charbon_graph10.jpg

beng

Let’s ask the Russians in west Siberia — are you suffering from the November “heat”?

Nothing new, happen before.
http://www.vukcevic.talktalk.net/69-71.htm
Need to ‘persuade’ citizenry that working in Siberia ain’t that bad, a convenient temperature chart helps a bit

We have family in Russia and it was one of the coldest Autumns in recent history. As usual with the warm-tards, eco-fascists, the devil is in the data details. I would not bet Sheryl Crow’s square piece of toilet paper, that the Russian data has anything to do with reality or is reliable.

There was a large drop in the number of weather stations worldwide following the collapse of the USSR. These stations were mostly lost from Siberia. At the same time global average temperatures were reported to be increasing. Clearly the problem was introduced sample bias due to loss of stations from a colder than average area of the globe.
http://www.uoguelph.ca/~rmckitri/research/nvst.html
http://www.uoguelph.ca/~rmckitri/research/nvst.jpg

Rob

I question whether new remote electronic equipment started being used. Electronics providing heat inside the temperature sensor box.

herkimer

As fast as it came , the hot spot is gone and North Asia is back to the cold weather ahttp://www.findlocalweather.com/weather_maps/temperature_north_asia.html

jhborn

Forgive the tangent, but here’s something that seems to be apparent to everyone but me: Why the 13-month filter? I know that’s what everyone does, but if a 12-month filter were used instead it would essentially eliminate the 1/ year component, and the resultant values could be placed at the month boundaries.
I’m sure I’m going to be embarrassed by how obvious the answer is, but I’m too curious to hold back. Can someone help me out?

Nylo

I can’t remember where I read it, but at some point in the past local russian authorities had an interest in showing their temperatures as low as possible, because that meant that they were entitled to a bigger ammount of gas for heating. I don’t recall the dates, though. But it would not surprise me if this practice of fabricating lower temperatures had ended around 1988. Anyone here remembers about it?

Steve from Rockwood

Seems hard to believe you could have a one-time shift in almost 100 years of temperature data caused by an effect that is defined as an “oscillation”. A shift up should be followed by and preceded by a shift down. Where is the shift down?

Björn

Gary says: “But let’s not forget the governmental change at that time. How many stations were reporting pre-and post-1988?”.
Under the old order when almost everyones paycheck was issued by the state , the people who lived in the northern Siberia usually got some extra cold weather incentives inversly proportional to the locally recorded temperatures added to the standard fare, and I have been told that this practice did have some effect on the low temperatures recorded there, and that when this custom was abandoned or changed under the new political system , it may have had something to do with the big step change in the temperture history that occured after the demise of the USSR. I have never personally seen any concrete proofs that it is true though, so it could just be an urban myth.

Steve from Rockwood

From Wikipedia…”In June 1988, at the CPSU’s Nineteenth Party Conference, Gorbachev launched radical reforms meant to reduce party control of the government apparatus. On 1 December 1988, the Supreme Soviet amended the Soviet constitution to allow for the establishment of a Congress of People’s Deputies as the Soviet Union’s new supreme legislative body.”
Was there a loss of some weather station reporting during the collapse of the Soviet Union? I visited Russia several times a decade ago and recall the stories of rapid infrastructure change. One day the geologists are working in their offices, the next day the offices are condominiums with a new owner and the geologists are out on the streets with their maps and rock samples.

herkimer

My previous post did not show the current weather in NORTH ASIA properly
http://www.findlocalweather.com/weather_maps/temperature_north_asia.html

Steve from Rockwood

Nice catch on the shift Bob Tisdale. From your Figure 3 it looks “obvious”. The minimums after 1988 barely reach the average prior to the shift. Has anyone else ever reported on this and if not, did you just recently find the shift? If the latter it makes me wonder what to think of world temperature data.

Gerard

Yes it was indeed very warm this autumn in Russia. Let’s compare snow cover with 2008 another year of rapid refreeze in the arctic:
http://tinyurl.com/nmhdp6a
Anyway the relative warmth in Russia is more then compensated by the cool in the US, north Africa, Middle East, Southern Asia, Caraïbs and Northern South America.

MikeN

Regarding the model temperature graph, are you saying they overestimated global temperatures by 1C, or temperatures in Russia?

Bob or Others:
SHORT VERSION: I have seen this 13 month averaging a number of times now and cannot see the justification for that time period. It would retain a sinusoidal element in the calculated data as periods are over-represented by the first month.
EXTENDED VERSION: For instance, a 13 month average, centred on January 1, 2012 will have July 2011 and July 2012 included. In my neck of the woods, July is the hottest month. This average would, all other things being equal, be hotter than the average centred on August 1, 2012 which will include February 2012 to February 2013. I realize that an odd number is preferred for centred moving averages as this allows the averaged data point to be located and associated with a raw data point. This would seem to be outweighed by the implications of retaining a sinusoidal element that I assume we are trying to smooth out. This element would be inverted relative to the original data, with the maximum during the coldest month and the minimum during the hottest month. Try it out in excell.
Data and method follow.
Use this data:
10 01-Jul-11
9 01-Aug-11
8 01-Sep-11
7 01-Oct-11
6 01-Nov-11 December, January and February
5 01-Dec-11 are the same temp. to effect
5 01-Jan-12 a mirror like data set with January
5 01-Feb-12 as the mirror line. Note that this
6 01-Mar-12 is necessary with an even number of
7 01-Apr-12 months in a year.
8 01-May-12
9 01-Jun-12
Copy and paste the numbers on the left as the “temperature”. Increase the months for a few years (3 will do)
The centred 13 month average for 01-Jan-12 will include the data from 01-Jul-11 to 01-Jul-12 (a 10 not shown). This gives a value of 7.308 (for all Januaries).
The centred 13 month average for 1-Jun-12 (and July and August) is 6.923.
Note that the correct point for a centred moving average is at the centre of the data used, hence the preference for an odd number of data points.
We have indeed smoothed the data, somewhat, but we now have a new sinusoid that reflects the periodic change in data, but is shifted 6 months. If we use a 12 month moving average each average is 7.08 (the true average of the periodic data) and we have smoothed the variance associated with the 1 year period. I won’t get into the implications of months with fewer days, etc.
Just curious.
Cheers and Merry Christmas
JE

Gail Combs

Robert Brooke says: @ December 20, 2013 at 5:29 am
Isn’t 1988/89 also when the ‘great dying of thermometers’ took place? Large numbers of Soviet military bases with weather stations, many in cold remote locations, closing post Glasnost.
>>>>>>>>>>>>>>>>>>>>
EM Smith looked into that a while ago.
Thermometer Years by Latitude Warm Globe: As the Thermometers March South, We Find Warmth
http://chiefio.wordpress.com/2009/08/17/thermometer-years-by-latitude-warm-globe/
Also: http://chiefio.wordpress.com/2009/08/05/agw-is-a-thermometer-count-artifact/
And: http://chiefio.wordpress.com/2010/03/08/assume-a-spherical-cow-therefore-all-steaks-are-round/
Then there is Verity Jones (Digging in the clay) who has also done a lot of work and a series of posts on The ‘Station drop out’ problem
http://diggingintheclay.wordpress.com/2010/01/21/the-station-drop-out-problem/
She says of her graphs of Asia

The key obsevation from Figures 7 and 8 is that Chinese stations are the dominant contribution in Asia. It is also clear that are the primary cause of the sudden increase in the number of reporting stations after 1950. Note that the increase in the numbers for Japan and the Russain Federations is much more gradual. All three countries show the ‘precipitous drop out’ of reporting stations around 1989/1990. Why? The Chinese stations in particular drop from a high number of 361 in 1990 to only 14 in 1991. Very odd? In addition to upsetting Environment Canada, has NOAA also broken off diplomatic relations with the Chinese? It looks like diplomatic relations with Mongolia were broken off a little earlier than they were with China as the Mongolian station ‘drop out’ occurs after 1982/83. Meanwhile, having enjoyed good relations with South Korea from 1973 when the number of reporting stations increased to over 60, sadly after 1993, the South Koreans appear to have also fallen out with NOAA with the numbers dropping to only 10 in the subsequent years.
http://diggingintheclay.wordpress.com/2010/01/21/the-station-drop-out-problem/

Blow up of the graphs:
http://www.climateapplications.com/GHCN/images/Raw_Asia_Stacked.png
http://www.climateapplications.com/GHCN/images/Raw_Asia_Unstacked.png
….
Frank Lansner over at Jo Nova’s also addressed the Russian station issue.

Oh there is many more than 5-8 temperature stations in Siberia, and they hold really many long series, so its really good to work with.
This article, “RUTI: Russia” is nof finished, but for each square i show I use around 5-8 long series from unadjusted GHCN:
http://hidethedecline.eu/pages/ruti/russia.php
Its even possible to get hands on more series.
But at the time I was making this article Appinsys was down, so I used Crutem3 via KNMI.
Is the area I show just 5-8 stations in CRUTEM3, is that what you say?
Anyways, “Russia” has complained to CRU about their warm trended choice of stations in Russia, but since i work with detrended data, this issue is not present too much. The specific peaks and dives show a remarkable connection with nino3,4 and unless this is a “coincidence” i think that Crutem3 is ok for this use. But perhaps GHCN would be slightly better.

“Russia” has complained to CRU refers to this 2009 report:

Russia affected by Climategate
….Climategate has already affected Russia. On Tuesday, the Moscow-based Institute of Economic Analysis (IEA) issued a report claiming that the Hadley Center for Climate Change based at the headquarters of the British Meteorological Office in Exeter (Devon, England) had probably tampered with Russian-climate data.
The IEA believes that Russian meteorological-station data did not substantiate the anthropogenic global-warming theory.
Analysts say Russian meteorological stations cover most of the country’s territory, and that the Hadley Center had used data submitted by only 25% of such stations in its reports.
Over 40% of Russian territory was not included in global-temperature calculations for some other reasons, rather than the lack of meteorological stations and observations.
The data of stations located in areas not listed in the Hadley Climate Research Unit Temperature UK (HadCRUT) survey often does not show any substantial warming in the late 20th century and the early 21st century.
The HadCRUT database includes specific stations providing incomplete data and highlighting the global-warming process, rather than stations facilitating uninterrupted observations.
On the whole, climatologists use the incomplete findings of meteorological stations far more often than those providing complete observations

http://en.ria.ru/papers/20091216/157260660.html

And finally WUWT: http://wattsupwiththat.com/2010/03/08/on-the-march-of-the-thermometers/

Gail Combs
izen

@- Steve from Rockwood
“Seems hard to believe you could have a one-time shift in almost 100 years of temperature data caused by an effect that is defined as an “oscillation”.”
Good point.
There is no historical data that shows any similar temperature shifts from this “oscillation”.
@-” A shift up should be followed by and preceded by a shift down. Where is the shift down?”
The AO shifted back down to the pre-eighties level some years ago, which makes attributing this November’s warmth to the same cause as the shift in 1988 very unconvincing.

MarkB

Joe Born says:
December 20, 2013 at 6:45 am
Forgive the tangent, but here’s something that seems to be apparent to everyone but me: Why the 13-month filter?

An odd numbered filter is used because it includes the “current month” and an integer number of months on both sides so one winds up with smoothed data centered around a point common to the unfiltered data. If an even numbered filter is applied one would have smoothed data referenced to midway between two of the original data points. If one were plotting such data in Excel or some such they’d have to adjust the indices appropriately which is slightly less convenient.

JB

Off topic but interesting news … especially, if you know where Ellesmere Island is!
A biologist has discovered 400-year-old moss in Nunavut. The moss was buried under a glacier on Ellesmere Island where it survived under the ice. Catherine Lafarge (sp) is a biologist with the University of Alberta. She was able to grow the moss in the lab. Lafarge says this discovery could help in space travel.
“Looking at is there any life on Mars or whatever. I do think something like Arctic organisms would be one of the first group of organisms that you would try to see whether they could survive in extraterrestrial systems.”
Lafarge plans to look at ice patches for ancient plant species on Baffin Island and in the Yukon next year. Some of the ice is up to 120,000 years old.

mpaul

Well lets not forget Climategate:

Climategate has already affected Russia. On Tuesday, the Moscow-based Institute of Economic Analysis (IEA) issued a report claiming that the Hadley Center for Climate Change based at the headquarters of the British Meteorological Office in Exeter (Devon, England) had probably tampered with Russian-climate data.

http://blogs.telegraph.co.uk/news/jamesdelingpole/100020126/

Curt

Joe Born: On the 13-month filter, you need an odd number of samples to make the filter symmetrical about the month in question. 13 months gives you six months before and six months after the month.
Joe and John: The 13-month smoothing averaging is done on the anomalies, effectively the residuals that remain after averages for each month over many years are taken. So the idea that, say a January 13-month average is raised by including two July readings is not valid. It is common analytic practice to look for patterns in residuals of fits.

Pippen Kool

If you go to Fig 3 (Monthly November ice extent for 1978 to 2013) at http://nsidc.org/arcticseaicenews/ you will see an “obvious” break in the curve at 1988 that would correspond to your 1 degree jump. The reason no one has commented on it before is that it looks like noise.

Curt

Bob: What do the satellite measurements for the Siberia area report? Do they see a similar step up at the end of the 1980s?

Steve from Rockwood

A 13-month filter is the shortest possible filter for eliminating seasonal effects while remaining symmetric about a given month. The next available filter would be 25 points long. A 3 point filter would show the average seasonal effect, which should reveal itself as a sinusoid (all this assumes continuous monthly data – unshifted).

Thanks Bob, this is very interesting. I can see a scientific paper here.

herkimer

The a strongly positive ARCTIC OSCILLATION does seem to correlate with warm tempertaures in Russia . For example , the AO INDEX was 3.106 and 3.279 in January and Februray 1989. . The winter temperature in Moscow during January and Februray of 1989 was only -2.3C and -.0.7C when the tyicals are vastly colder [ january/february average in the 1980’s was closer to -8C and as high as -17.7 in January 1987 and -14.1 in February 1987]. So like in November 2013 , there was a spike in the warm AO and the temperatures went up.[ daily high for AO in NOVEMBER 2013 was over 4]. What caused this short term spike in the positive AO? . During November 2013, there was a major spike also in solar activity when the solar flux shot up to 174 and the sunspot number close to 290. I don’t know if there is a connection but the extra solar activity could account for the sudden higher positive AO].

Thanks Bob, Very good post.
Again to show that ENSO causes permanent surface temperature shifts, not just oscillations.

MarkB

For what it’s worth, if one splits this temperature record into a pair of before/after linear trends without the (hokey IMHO) jump discontinuity, the natural break point is circa 1970 and the CMIP5 model comparison would look a lot better than what Tisdale has shown.
http://i42.tinypic.com/27dr8.png

izen

@- Curt
” What do the satellite measurements for the Siberia area report? Do they see a similar step up at the end of the 1980s?”
Yes, both the 1988 ‘shift’ and the present extreme are validated and confirmed by satellite data and other readings of temperature and ice extent from non-Russian sources.
The changes in weather monitoring stations in the USSR from the political collapse are a red herring.
Especially as the big collapse happened AFTER November 1988.

What Shift?
please note. The NCDC chart is based on Monthly data and of course when you use a subset of all the data you’ll increase the odds of finding odd stuff. A good analyst looks at all datasources. Plus, defining russia by lat lon is a really stupid approach when you can actually use the exact border to pull out the data.
http://berkeleyearth.lbl.gov/auto/Regional/TAVG/Figures/russia-TAVG-Trend.pdf
http://berkeleyearth.lbl.gov/auto/Regional/TMAX/Figures/russia-TMAX-Trend.pdf
http://berkeleyearth.lbl.gov/auto/Regional/TMIN/Figures/russia-TMIN-Trend.pdf
note also.
http://static.berkeleyearth.org/graphics/figure1.pdf
http://static.berkeleyearth.org/graphics/figure2.pdf
REPLY: I see the shift in your Tavg chart. If you had presented graphs that were at useful time scales to the discussion at hand, you’d like see it too. Remember from our conversation at AGU I see the “specks” that you do not. I have tool to test for this, I’ll run it tonight and do a new post. – Anthony

Bernie Hutchins

I’m not at all sure using a 13 month filter rather than 12 months is a good idea. True you can get linear phase (or zero phase with respect to the center) if you use 13, but at the expense of imperfect cancellation of the full-year periodicity. If you must use 13, re-weight the first and last months by 1/2.

talldave2

That’s right folks: global warming caused the Soviet Union to fall.

Jimbo

Back in the day I was told that co2 positive feedback crazy warming would be gradual not in big shifts.

lurker, passing through laughing

Whatever the source of the change or the motive for the change, it is once again evidence that the manmade part of AGW has a lot to do with how data is ‘handled’ and very little to do with actual temperatures.

Werner Brozek

herkimer says:
December 20, 2013 at 7:01 am
My previous post did not show the current weather in NORTH ASIA properly
http://www.findlocalweather.com/weather_maps/temperature_north_asia.html

What I find interesting is the two cold areas. If those two cold areas were warm in November, and no data was in the middle, GISS and HadCRUT4 would have very different anomalies. Unfortunately we are forced to have this discussion without knowing the HadCRUT4 November anomaly. However the satellite data, whatever their differences with GISS, do not give grounds for confidence here by ranking November 2013 as 9th for UAH and 16th for RSS.

crabalocker

I can`t recall when it was…but wasn’t there a year when the headlines all screamed record warm October and it was later realized that Russia used their September data for October?
Just a thought.

Duke C.

Slightly OT-
Wind patterns over Russia for Nov. 15, 2013. Cool app.
http://earth.nullschool.net/#2013/11/15/1500Z/wind/isobaric/1000hPa/orthographic=-267.35,35.26,279

Steve from Rockwood

John Eggert says:
December 20, 2013 at 7:25 am
———————————————-
John, It doesn’t make enough of a difference because you are talking about only one extra month divided by 13. While July is much warmer than January (where I live) it isn’t much different than adjacent months June and August. I tried your idea of weighting the first and last months by 0.5 (e.g. 0.5, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0.5 divided by 12) compared to equal weights and the one graph is hidden below the other virtually indistinguishable (I used the HadCrut 3 data from 1850 onward).

Matt G

The shift in 1988 doesn’t seem to be sea ice related around Russia, where from 1987 to 1988, in the North of the country there was very little sea ice change in just one year.
Most sea ice extent in March 1987
http://www.ospo.noaa.gov/data/sst/mean_mon/March.87.monmean.gif
Most sea ice extent in March 1988
http://www.ospo.noaa.gov/data/sst/mean_mon/March.88.monmean.gif