Tuesday, 17 April 2012

That mid month miscellanea post again...

Firstly here are two excellent videos from Greenman AKA Peter Sinclair of Climate Crocks. Part 1 and Part 2. As usual I give links to videos as I've still not figured out how to make the Blogger player window large enough for details. The videos are about the recent weird weather and links to the Arctic, there are interviews with a stack of scientists, and clips from the recent Horizon programme on Global Weirding. Total time is only 20 minutes, they really are a must see. As an aside, it's interesting that the clips from news and TV are from the BBC, yet most of the scientists and meteorologists interviewed are American. I suspect the media in the US aren't serving their audience a diet of reality.

Over at RealClimate there's a post by Axel Schweiger, Ron Lindsay, and Cecilia Bitz on the use of extrapolations of PIOMAS volume, here. The basic message - such extrapolations aren't of much use. I'm of a similar view, I think the comment that sums up my view on extrapolation is "You might as well draw the Arctic’s horoscope."

The comments at RC have generally consisted of disagreement with the main post, but in amongst them some points kicked off my neurons. Hank Roberts, in his usual role as librarian to RC's peanut gallery linked to two theses from the US Navy Postgraduate School, here, One of these contained some details about NPS, the model Maslowski has used for his volume extrapolations. It seems that it's trend extrapolation of volume from both NPS and PIOMAS that's behind talk of the Arctic being ice free within a few years, so is there anything common between those models that might help identify what's going on, and why volume loss seems to be so fast? Here's my handwaving blue sky thought:
Both NPS and PIOMAS retrospective runs use ‘observed’ data for their atmospheric components. PIOMAS seems to use NCEP/NCAR, NPS uses ECWMF. Maslowski has stated resolution is a major factor in the ‘early ice free’ implications of volume loss. PIOMAS “has a horizontal resolution of 40 km X 40 km, 21 vertical ocean levels, and 12 thickness categories each for undeformed ice, ridged ice, ice enthalpy, and snow.” NPS “is configured using a horizontal, rotated spherical grid covering 1280×720 cells at a 1/12 degree (approximately 9km) resolution. It has 45 vertical layers” in the ocean. So whilst Maslwoski has asserted that resolution is a key factor it doesn’t seem to be the case when PIOMAS is considered, NPS is 9km square, PIOMAS 40km square.  
Going back to data assimilation. Zhang et al 2010 reveals inflection points when going from historic NCEP/NCAR to a combination of GCM temperature spine and randomly shuffled NCEP/NCAR weather to represent a warming trend with weather variability. The authors note that randomly shuffling the ‘weather’ in this way will neglect contribution of trends in weather that are reinforcing the loss of ice. Part of the inflection may well be due to the GCM projection. However what both NPS and PIOMAS have in common is the assimilation, and their volume outputs have both been argued to imply an early loss of sea-ice.  
So how much do changes in the Arctic atmosphere play a role in the loss of sea-ice volume and the apparent failure of the GCMs to reflect the current volume loss? Am I in a rut when I point the finger at the Arctic Dipole?
Which brings me right back to the two videos linked to at the top of this post - just how massive and comprehensive are the changes we're seeing? I don't know, but other matters are starting to seem a bit more clear to me. I've mentioned before that NSIDC have stated that the Arctic dipole has played a role in the Arctic summer weather after 2007. In my most popular post (huh?), Summer Daze, I've pointed out a robust change to the Arctic Summer atmosphere after 2007, a change that seems to have mid-latitude impacts. I'm beginning to suspect that what I've shown in that post is simply the Arctic Dipole (AD).

There was a recent AD emergence as pointed out by Chris Biscan at Neven's Sea Ice blog, I initially said I couldn't see it, but corrected myself in a subsequent reply. Give the following links up to a minute to work - they're direct to NCEP/NCAR not postings of images. This is visible in NCEP/NCAR. This has got me thinking, because I've always looked at NCEP/NCAR in retrospect and recently have begun to wonder if the anomaly from climatological mean would show an AD pattern. However here an AD seen in ECWMF is then visible once the reanalysis data is updated. That said I usually use NCEP/NCAR on a monthly or seasonal basis, April's data aren't in yet (obviously) but running from 13 Mar to 13 April as an ersatz month, the AD of early April has no impact on the average over that period. So it's quite possible I'm missing significant and prolonged outbreaks of the AD by using monthly averages. Only an exceptional AD like 2007 will stand out, e.g. NCEP/NCAR July 2007 average SLP, here.

I'm also interested in whether the Spring volume reductions in PIOMAS are real, due to their size it seems that the implied thickness losses at the September minimas are so large they are the fingerprint to look for. In this graph, the September minima are marked, the large loss of thickness is clear. In searching for evidence of the Spring volume losses in PIOMAS I found this site. From that the most likely sources seem to be North Pole, and the Canadian Arctic Archipelago (CAA). The former ends before Spring 2010, the latter looked more promising: I've examined the thickness data from the CAA but of the few sites that cover post 2010 there is no substantial thickness reduction, this however doesn't say much about the bulk of the Arctic.

Perhaps more useful is the Ice Mass Balance Buoy data, here. Most of the candidate buoys don't have thickness data available, so I've resorted to examining the initial thickness data, as these give a widespread set in both time and area. The results are inconclusive, for most the scatter is so great no trend either way is discernible. North Pole in April suggests no change in thickness apart from a suggestion of a drop after 2007, but this is too early in the season as the implied thickness drop is greatest in September at the minima. Beaufort in August suggests a large drop in thickness, but I'm not convinced - the data are too scattered. One explanation for these mass balance buoy results could be that these buoys are deliberately sited on thicker floes of ice, so there is the risk of a sampling bias.

I've also been looking at Operation Ice Bridge, here, that site doesn't support IE, I use Chrome as well and it seems OK with that. I've yet to get properly to grips with IceBridge, but that may not matter. From what I've seen the data seems to be in raw form, I've read enough about airborne EM surveys to know I don't trust myself to properly analyse raw data from those surveys and get usable thickness data. One snippet from EM surveys was the Alfred Wegner Institute flights that found average thicknesses in 2009 1.7m, 2010 1.6m, and 2011 1.4m, these were flights off the CAA. But that's just a snapshot of one area and doesn't go far enough back to say if these drops were continuation of a trend, or evidence of larger drops in the 2010/2011 period.

So I can't confirm or refute the results from PIOMAS regarding the Spring volume losses and the implications for September thickness. I've decided to email a few scientists if these losses repeat this year, perhaps someone out there is sitting on data that can answer this question. But if anyone has more evidence of changes of thickness at the September minima, or at other times over the period of 2010, 2011, please let me know.

And finally on a completely different subject. The news here in the UK has been running with the trial of some pointless nobody in Norway. This reminded me of a pertinent little video clip from Charlie Brooker's Newswipe: Video here.


Peter said...

I'm afraid I didn't find Zhang et al 2010 very convincing - I know this is an extraordinarily arrogant statement from a molecular biologist, but there you go.

The problem is that each year in the subsequent forward projection effectively "resets" the warming trend to whatever the value was during the year the magic 8-ball picked during in the resampling process that created the forcing pool. Physically, what that does is assume that The process of creating that forcing pool inherently assumes that all warming occurring during the 1948-2009 record is random, and there is no AGW trend in this time period. For periods where there is a significant trend you can't simply use an ensemble over this period to estimate CV, since part of the change over the period is due to the (genuine) trend rather than to random variation.

Scenarios A1 and B1 (i.e. forcings permuted randomly from the entire 1948–2009 record) are thus blatantly unrealistic and should be simply dismissed without further consideration. Just look the second panel of figure 1 - these scenarios effectively assume an immediate 2 degree drop in Arctic SAT!

A2 and B2 have the same problem, but to a lesser extent since the pool is drawn from the generally warmer 1989-2009 period. Of these two, A2 assumes that Arctic warming will continue at around the same rate as the historical trend from 1977-2010. B2 assumes that the rate of Arctic warming halves. Although this range is based on IPCC GCMs, I thin B2 is hard to justify, leaving A2 as the only realistic projection.

They do however have apparently encouraging conclusions for the A2 run - September ice extent stays above ~5 million through 2020, and above ~2 million through to 2050. This apparently is repeated in two further runs A2′ and A2″ - which I assume they ran in the knowledge that A2 is the only realistic scenario.

However, look at the volume figures. As they themselves acknowledge, these plummet.

"The September ice volumes of A2 and B2 drop to very low
levels as early as around 2025 and remain almost flat in
the following years when the projected September ice
extents continue to fluctuate significantly annually (Figures 1d
and 1e). This suggests that summer ice volume is more sensitive
to AW than summer ice extent"

At face value these suggest the Arctic will be left with an extensive cover (~4 million km^2) of ice less than half a metre thick.

Rather than saying ice area is robust to AGW, I think this just shows the model doesn't adequately represent ice breakup processes. Under a given thickness it just fragments, gets mixed with surface waters and melts out. PIOMAS is known to overstate the thickness of thin ice anyway.

Short version: Even if the Arctic ice follows their predictions, by 2025 it's not going to be recognisable - it'll be an eggshell-thin layer than poses no barrier to shipping.

Chris Reynolds said...


I think your points are quite reasonable.

Actually I've just checked the Zhang paper and I've misremembered their method. The method they use actually does include the temperature field from the NCEP/NCAR climate variability (weather) forcing. So the behaviour I note with regards PIOMAS projections could simmply be due to temperature.

So is the volume loss of assimilation models purely due to warming ahead of what the GCMs show? And what role does the atmosphere play in this?

There's a bit of circularity here as Screen and Simmonds show that a large part of the recent warming is due to the loss of sea ice. The assimilation models (NPS/PIOMAS) shouldn't underestimate this because PIOMAS (and I presume) NPS assimilate observed sea-ice concentration. So I'm still pondering the emergence to dominance of the Arctic Dipole after 2003.

It's also worth pointing out that PIOMAS volume is now lower than the plots in figure 1e. September month averages are:
2008 7.25
2009 6.98
2010 4.59
2011 4.21
figures 1000km^3.

Somehow the science around the Arctic leaves me feeling more confused and more concerned the more I look at it.