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.
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)
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.
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 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 ...).
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.