When satellite observations of Arctic ice extent began in 1979 it was obvious that a long-term decline was already underway. That decline appeared to be monotonic until the mid-2000s when, for a while at least, it seemed to have accelerated. The ice extent in the summer of 2007 was a record low, and was accompanied by cries from some quarters of imminent collapse.
The same was said in 2012 when another low was observed. However 2012 was an unusual year as an intense storm occurred in August and its effects on concentrating the ice cover can be clearly seen in the data. Likewise 2007 was an exceptional year.
We now know that year had what was later called an “unusual atmospheric pattern,” that is clear skies under high pressure that promoted a strong melt and at the same time winds brought warm air into the region.
These exceptional years became statistically important as using them to guide a straight line through the Arctic ice decline made its gradient even steeper.
A New “Pause?”
Examining the sea ice extent data for the past eight years it is obvious that there has not been any statistically significant downward trend, even though there is more noise (interannual variability) in the data. There are interannual variations but they do not form a trend. For the 2002 – 2006 period the annual differences are mostly in the extent of maximum and not minimum ice cover. The period 1990 – 1996 displays much more interannual variability. The main difference between the ice-curves is that in recent years there has been an increase in the gradient around the beginning of June.
Of the general decline and the interannual variability how much is due to external forcing and how much to internal variability? Estimate from climate models give about equal measure to forcing and internal variability, Kay et al (2011), Stroeve et al (2012). That 50% internal variability is almost never illustrated graphically when presenting Arctic ice data.
That the minimal extent of Arctic ice has “paused” is admitted by Swart et al (2015)
“…from 2007–2013 there was a near-zero trend in observed Arctic September sea-ice extent, in large part due to a strong uptick of the ice-pack in 2013, which has continued into 2014.”
Swart et al (2015) maintain that “cherry-picking” such short periods can be “misleading about longer-term changes, when such trends show either rapid or slow ice loss.”