28 August 2014

Exploring Arctic Ice Minima

Every year 2007-2013 had a lower Arctic sea ice extent than every year before 2007.  2014 seems likely to continue this record.  I'll also suggest below that maybe the Arctic has entered a 'new normal', with September ice extents bouncing around 4.7 million km^2. 

For some data to work with further, I pulled the NSIDC September figures.  It's a small, simple text file, so you can check yourself what follows.  First up, let's draw a figure of what we're looking at -- but don't connect the observation dots.  Our eyes tend to be led to conclusions by the superposed lines.
You can check some of the sources for ice before 1979 and see that figures below 5.5 million km^2 are unprecedented in the longer records as well.  To have data precision and consistency, though, I'll stay with the 1979-present.

What else can we say from eyeballing the data?  Since the 1979 starting point:
  • There have been 2 record highs (1980 and 1996)
  • There have been 8 record lows (1984, 1985, 1990, 1995, 2002, 2005, 2007, 2012)
  • There have been more record lows in the last 10 years (3) than record highs in the full 35 year record
  • 1996 is about the last year one could say there was no trend in the data
  • Versus eyeball curve fitting, 1996 is the most exceptionally high year (not just an absolute record, but even higher above smooth curves we'd try to fit to the data than any other year).
  • More recent years look like they have more scatter than the earlier years
  • It looks like we might want to divide the period in to 3 intervals -- 1979-1996 (the longest arguably trendless span), 1997-2006 (an intermediate with at least some overlap on the earlier figures) and 2007-present (entirely outside the range of the previous years)
But maybe your eyeballs disagree with mine, and perhaps the appearances are deceiving.  So on to working with numbers, which will also lead us to some additional ideas.
First, I'll put in the lines between data points and ask you to reconsider what leaps out to your eyes.  One change for me is that 1997-2006 look less disconnected from 1979-1996 than with just the data points.
Now a bit of spreadsheet computation.  I'll make up 4 periods -- the 3 I mentioned before, plus adding in 1979-2006.
Span stdev intercept slope
1979-1996 0.515 7.467 -0.036
1997-2006 0.379 8.484 -0.100
2007-2013 0.614 4.498 +0.007
1979-2006 0.651 7.642 -0.060
For doing this, I just used the spreadsheet functions stdev, intercept, slope.  For research purposes, I wouldn't, as there are some subtleties in how the computations should be done that spreadsheets don't necessarily respect.  My eyeball notion of increased variability in the recent span doesn't seem to hold up.  Oh well, onwards.

The intercept and slope show 2007-2013 to be wildly, drastically, spectacularly different from the prior periods.  We already pretty much knew that, but this gives another way of seeing it.  Intercept and slope give us straight lines.  The intercept is the sea ice extent for 1979 (it was actually 7.20, with 1980 being 7.85), and then the slope says how much less ice (if negative) there is year by year after that.  I'll plot all three of these lines on the data:

 The line for 2007-2013 is far below all prior observations.  It shows a very small upwards trend, so small that it's almost certainly meaningless statistically.  (It would take 300+ years to get ice cover back to the range of the first span of data; nevertheless, start your stopwatches for someone to claim Arctic ice is 'recovering').  It intersects with the line from the middle period in 2016, at about 4.78 million km^2.  The scattering of observations above and below the line, and essentially zero slope, in this period suggest the hypothesis "Arctic ice entered a new normal with the year 2007, where the September minimum averages 4.71 million km^2"

The two years which stand out most from the fitted lines are 1996, which is indeed the most drastically above the lines, and 2012, which is the most drastically below.  So, not exactly a hypothesis, but data-driven research questions -- What happened in 1995-1996, 2011-2012 to make for such extraordinary changes in the sea ice pack?

Since 2007 is the first year of the 'new normal', what happened in 2006-2007 to make that change of normals?

Looking at the line for the middle period (1997-2006), we see that as you look backwards in time, it is rising spectacularly fast.  By the early 1970s, it is far above anything previously seen.  One thing this says is that whatever was causing the decline in this period, it hasn't operated for long before.  If it had for more than a decade or two (1-2 million km^2 changes), the prior observations wouldn't look anything like they do (around the 7-8 million km^2 range, give or take).  This line is a little below 2013's figure, which suggests it isn't entirely out to lunch for present and future considerations.  If we run it straight forward, it hits zero in 2064.  You could apply my earlier idea on how to estimate the disappearance time of sea ice to this straight line instead of my more complex curve from then to get a statistical range and confidence levels for when the Arctic ice pack might go away. 

Finally, let's look at the first line.  It has a relatively shallow slope, enough so that if we run it back in time, it doesn't get outrageously high (I'll define that as 9 million km^2) for a longish time (1936, it turns out).  So it has at least some plausibility for telling us about earlier times.  Whenever we fit a line to data, the data should sometimes be above, and sometimes below the line.  Further, the points should be on both sides of the line in both the early and late period of the line.  For the first line, the last time data goes above the line is 2001.  (we aren't happy about the fact that all 4 years before 2001 are below the line, and 2001 is only marginally above, but it's a start.)  This is more recent than the 1996 I had stopped with earlier.  By eye, it also looks like we might be able to put a line through the data 2002-2013.  One benefit to this is that then we only have two lines instead of three.  If we're arguing that something changed in the Arctic system (which I'm not yet, but might want to later), it's easier to explain 1 change than 2 (or 3, or ...).
Ok, I can't say I'm excited by this version.  The early line, with its additional 5 years, is a little different, but still entirely out to lunch past its last data point (2001).  And it's now dropping more steeply, so becomes unreasonable a decade earlier in the past (per above definition of 'unreasonable').  The line for the more recent (2002-2013) period doesn't look too bad, though.  At least it remains plausible back to 1996, maybe 1992, 6-10 years before its starting date, and observations in the more recent period bounce to either side of the line.  Sending the line forward, it passes zero in about 2042, which is in the range of other people's estimates (as is the 2064).

What have I proven?  Nothing, this is science after all.  It's mathematicians who get to prove things.  But even in the more casual usage of 'prove', nothing except that with only 35 data points I can write quite a lot of words.  Not news to anyone who's been here before or otherwise knows me :-)

This is, as advertised, an exploration.  The purpose of exploration is to see what's going on and get ideas for future research.  I'll take this up in additional posts.  You've noticed a number of question marks and hypotheses above.  These are all items to look in to, yourself or me.  There are several years which seem to be particularly interesting, in that they don't fit nicely on the curves (lines for now), and the transition times between lines.  Was anything unusual happening meteorologically then?  Oceanographically?  Somewhere else?  Could they have caused what we saw in the sea ice?  Was it instead that something happened in the data source, not in nature?  Have we really hit a new normal, or is the ice still declining?  How would we tell the difference?

Applying my 'new normal' hypothesis gives a guess for 2014 of 4.71 million km^2.  It looks likely that this won't be very close.  But it shouldn't be too bad compared to other methods.  Again, more later.


Anonymous said...

I don't know… 2007-14 is such a short time period, calling it a "new normal" just because there's no trend, rather than assuming it is a continuation of a declining trend where 2007 jumped ahead of the curve… seems unlikely. Not impossible, but I'd want a physical reasoning rather than just eyeballing and random trend fitting…


Robert Grumbine said...

I agree with you -- nothing proven, and we definitely want some physical mechanisms.

The eyeballing and playing with lines isn't for making conclusions, just for raising questions to explore. If there really is a new normal for ice, how can we distinguish between that and a continuing trend? Is it reflected in new normals for the atmosphere or ocean? And others.

Probably this will go the way of most hypotheses -- be tossed. But it struck me as possibly educational to pursue matters out loud.

Evan H said...

The general idea that I had pieced together from assorted sources was:

- We started (as of the beginning of the 20th century) with a big thick sheet of multi-year ice covering a large area of the Arctic.

- As we warmed the climate, the multi-year ice began to thin, but since we were only melting a little more in summer than was forming each winter, it took a long time to melt all the way through.

- Measurements of extent don't pick up thinning ice sheets very well until they start getting holes in them, so over most of the 20th century it didn't look like the Arctic was doing much.

- By the late '90s, the ice sheets had thinned enough that they started getting the aforementioned big holes and shrinking around the edges. This showed up on the satellite data as a dramatic decline in ice cover even though it was really just a continuation of ongoing volume loss.

If the above is correct, then I don't think there's any reason to believe the recent slowing of the decline is 'real'. It could be that:

1. We're running out of 'easy' ice. It could be that what remains of the multi-year ice started out much thicker than the portions which have already melted. If so, we could still be losing volume at an alarming rate without much loss in area.

2. We're running out of ice, period. Since the flux of energy into the ice is a function of surface area, we can't expect to keep losing volume at the same absolute rate when the surface area is reduced dramatically.

Am I on the right track here?

Robert Grumbine said...

Yes, that's all a fair summary.

Fernando Leanme said...

Read my "predicting the future" in which I go over the pitfalls of extrapolatin' without having a sense of the clockwork machinery. I used the USA's oil production forecasts but it applies to lots of stuff.

Arctic ice seems to be decreasing. I don't think it will allow us to use the northern sea route safely, but it may allow extreme tourism to try to reach the North Pole in an ice breaker to be a fairly routine trip.

I do wonder, why do you fret so much over this issue? The Arctic has always been very erratic and we don't know that much about how things work in such nasty weather zones.

Robert Grumbine said...

Behind the scenes, I do have some understanding of the clockwork machinery behind sea ice. In my follow up note(s) ('any day now') I'll say a bit more about that.

I think we agree that one cannot _just_ play games with data -- run curves through it that we decided beforehand (one straight line, three straight lines, a parabola, ...) and declare that this is the future. As will develop, that's not what my plan is for this either. Start, though, with descriptive looks at the data we do have, and then examine some more.

The Arctic sea ice pack, actually, is historically (up to the 1990s) pretty stable on the whole. At a professional meeting in 1993, Norbert Untersteiner, one of the grand old men in the field, observed that although one part or another of the Arctic would have more or less ice than usual (sometimes quite a bit more or less), it was always pretty nearly compensated by a deviation of the same size and opposite direction elsewhere in the Arctic. You might have exceptionally little ice in the Beaufort/Chukchi seas, for instance, but there'd be exceptionally extensive ice in the Barents/Kara seas. Arctic total held pretty steady. He based that not only on the satellites (1979-present) but on his (and others') field work in the Arctic (1950s-1990s).

Ironically, or as an object lesson for me about being too quick to make generalizations, he made that speech in almost the last year that it could be said to be true.

Robert Grumbine said...

... p.s. I looked for your article and didn't come up with anything. Just some comments to blogs that don't include the data and analysis. Please do provide a link. My link policy is pretty broad -- http://moregrumbinescience.blogspot.com/2008/08/linking.html
If you have substantive, relevant, comments to add that don't fit easily in a blog comment box, please do provide the link. (No ads for miracle cures, etc., but doesn't sound like that's a problem here.)

also, sorry about my delay in approving the comment. I'm back from vacation and it's taking particularly long to catch up.

Philip H said...

Sorry I missed your talk on the 24th - somehow I managed to ignore the cute flyer. Seems like this line of work on minima has a real potential to be a game changer in both the sea ice realm, as well as resource management. That would have been an interesting sidebar over coffee after. Oh well.

Robert Grumbine said...

(For other folks: I gave a talk in NOAA/National Ocean Service on the 24th, titled 'sea ice question time' or some such. The pretty poster was largely someone else's photo of a banded (er, ribbon) seal, which is an Arctic animal I find cute. Had some nice discussion with folks in the talk itself and more afterward the talk proper.)

The coffee chat, or other discussion would have been fun for me too.

My take is that the 'new normal' I suggest in this post is probably going to be short-lived. And I might want to up the baseline figure some from that 4.7 million km^2. Mostly here I just wanted to toss up several hypotheses for a later ('real soon now') post on the method of multiple working hypotheses and our duty in science to go around slaying hypotheses.

The 'short lived' aspect to this new normal is because I expect that at some point (homework to do before naming a time) I think we'll lurch to yet another new normal. Partly this involves things that EvanH mentioned -- the sea ice thickness changing even while area and extent aren't showing much action.

Fernando Leanme said...

Sorry, here´s the link. I really didn´t mean to have people go from here to my blog, because it´s mostly about politics and a bit of comedy.


The post is extremely simple, but I do happen to have expertise in the subject, and I find forecasting in that particular field to be quite haphazard.

Regarding the Arctic ice extent, I happened to work on an Arctic project in the early 1990´s. The gentleman you mentioned may not have been keeping track of long term trends. At the time we could see the Barents to Kara sector was definitely losing ice cover. We also saw a significant decrease in multi year ice.

When I see people prepare Arctic wide figures and plots I remind them it gets much more interesting if they plot the Barents to the Kara gate, and up to just beyond Spitsbergen. Don´t you think the Barents is the key? Or am I missing something?