Single Year Global Sea Ice Shows Minimal Trend
Posted by Jeff Id on February 28, 2012
As regular readers know, we have been using daily gridded sea ice data from satellite microwave sounders published by the NSIDC to discover what we can learn both from and about the data. With Layman Lurker’s help we recently discovered that nearly 100% of the sea ice to the South of 72 degrees North latitude and the equator has melted every year since satellite records started. Since the Antarctic sea ice is known to melt annually as well, we can say that nearly all of the sea ice south outside of 18 degrees from the North pole vanishes every single year. Figure 1 below takes a moment to interpret, but is interesting in that it shows the loss and recovery of Arctic sea ice over a typical year (2003 was chosen at random). This post has a video of all years for your confirmation.
The near complete melt of Arctic ice can be seen at Day of the Year ~250 and latitude 72. The red dot below indicates the visually approximated time and latitude of maximum melt.
So I masked off and ignored all of the data north of 72 degrees representing basically the only multi-annual sea ice on Earth. This assumption corresponds to all of the ice outside of the bright green circle in Figure 3 below. Notice that there is a lot of Northern hemisphere sea ice to the South of this circle.
Figure 4 depicts the approximate latitude for maximum and minimum sea ice formation in our comfortable interglacial period. Since the Antarctic ice also melts nearly completely every year, the red line at the North and upward represents the entire area of multi-year sea ice on Earth. The lines in the southern hemisphere give an interesting reference but you can confirm from video that the Antarctic sea ice melts almost completely every year.
So with all of this groundwork in place, we can now look at the behavior of single year sea ice trends from a reasonably knowledgeable position. The Northern hemisphere trend of single-year ice (Figure 5) is strongly significant – meaning we can measure it easily outside of its normal variance. A linear least squares fit reveals a downward trend of about 800,000 km^2 over the length of the satellite record but remember, the ice in this entire post basically reaches zero area every summer. For reference, there is an average of about 4 million km^2 of this ice in the Northern hemisphere averaged over the entire 34 year record. In a future post, we will look at how this ice trend can be compared to the perennial ice further to the north but from past work, this decline represents a bit under half of the total northern hemisphere trend.
Below is the Southern hemisphere sea ice. I did not mask the Antarctic data because Antarctic sea ice melts almost completely in the summer. The Antarctic has shown a widely un-publicized, statistically significant increase in ice over the years of satellite data. One critique I have heard in past analyses (typically from the extremist types) is that the Arctic ice is most important because it is multi-year ice. We will look at that in the future but that criticism is what has driven this investigation. I would expect that in an extreme global warming event, the multi-year ice would show a heavy downward trend because it wouldn’t form as completely in the winter. The Antarctic has other ideas though.
Below, I have combined what amounts to nearly all of the annually formed sea ice into a single global ice anomaly (Figure 7). We have a negative trend which to my surprise, fell just short of the 95% significance estimate. We know the planet is warming so this was a little puzzling.
Now I’m sure that people will call this exercise “cherry picking” but I don’t think that is a fair critique having picked all of the annual sea ice. Perhaps I could spend even more time to mask the North pole by something other than latitude but I doubt it would change anything. The post shows the trend is nearly significant so other statistical tests would undoubtedly cross the 95% threshold and still other tests would expand the confidence interval even further from the trend. My intent though was to see just how much of the ice melt, was the result of reduced annual refreeze. So with that said, isn’t it interesting that global single-year sea ice has not shown a trend which is easily differentiable from noise in the past 34 years?