For the last 25 years or so there has been a decrease in the amount of ice that remains on the surface of the Arctic Ocean every summer. This is a trend that can be attributed to global warming, which in turn, can be attributed to the steady release of previously fossilized Carbon to the atmosphere by the burning of coal, oil, and natural gas. But over the last few years, this decrease in ice has been much more dramatic. The trend has steepened. The formation and melting of ice has to do with air and water temperatures. This in turn can be affected by how much ice there is, because ice reflects energy from the sun, while open water absorbs it. There are other factors as well having to do with the distribution of air masses. It is all very complicated, but the bottom line is that reduction in ice can lead to further reduction in ice. It is quite possible that the recent very steep declines in Arctic sea ice represents not just a trend that happens to be, for random reasons, a bit steeper than usual for a while, or that it is even a long term change in the rate of inter-annual loss. Rather, it could be that the phenomenon of summer sea ice in the Arctic is simply something that is no longer sustainable because of new conditions caused by climate change, and that the sea ice we see now is mainly the remnant of a previously existing, no longer operational, system. Time will tell. Probably not much time, in fact. If the recent dramatic reduction in summer ice is a random blip, then the amount of summer sea ice over the next five years or so may go up rather than down. If the recent reduction is a result of a different relationship between sun's energy, water temperature, air temperature, and ice formation, then over the next five years or so the trend of declining ice should continue and at some point settle on a new equilibrium with variation around a new mean.
There is probably a rule of thumb that could be applied here. One of the most important Quasi-cyclic climate variations, the El Niño–Southern Oscillation (ENSO), has an average period of about 5 years. Let's assume that a full cycle is a strong El Niño followed by opposite conditions for the same period of time. Conservatively, we might want to let a climate change in some part of the Earth's system to run for 10 years without changing direction before we can be pretty sure that it isn't just a perturbation that will later readjust. Consider a ten-year rule of thumb in relation to the following graphic showing Arctic sea ice from 1979-2012:
That's a nice graph, produced by Andy Lee Robinson. Robinson created this very nice looking animation from data he had earlier depicted in a simpler two dimensional form, which actually is in some ways more dramatic (It's a moving GIF, click to watch it move, then come back!):
The story behind these graphs, including data sources, is summarized in this post by Peter Sinclair.
In these graphics, the last seven years show sea ice surface area occurring several years in a row at a much lower average than previously, and with a steady decline continuing. But there is a strong declining trend that actually starts earlier, closer to 2002. It is a little hard to put an exact year on this. If there is a new equilibrium being reached, when we look back on these data in a decade or so, where would we start the trend? Late 1990s? 2000? 2002? 2007? Hard to say.
But there is a problem with this view of the Arctic: The sea ice that rebuilds every winter starts with a core of "old ice" which during cold trends is added to every year. This is thick ice. A huge amount of the volume of sea ice in a give year, say back in the 1970s, is actually stacked up vertically. This ice has been melting as well, and climate scientists who study the arctic regard the decline of this old ice as more important than the simple reduction in surface area. In other words, the important area here might be volume rather than surface area. Volume is harder to measure over the long period because the instrumentation needed to make good volume estimates is has not been applied to the Arctic for very long, but we do have an idea of what has happened.
Have a look at this demonstration of the importance of volume:
If we look at volume rather than surface area over time, in a graph from here, we get something like this:
Each of the colored ringoids on this graph is a decadal average of ice volume around the seasonal cycle. The drop in volume each year is a function of both decreased ice formation in the summer and decrease in volume, but the latter, volume, is driving the change you see here. Noticed that each decade has less ice volume than the previous decade. Also noice that the 2000-2009 decade is dramatically smaller in volume than the previous decades, and the last few years have at least the same amount of decrease. This shows that the dramatic drop in Arctic sea ice may be a thing that has been happening both very recently and for enough years that the 10-year rule of thumb may already apply. We'll see what happens over the next few years.
It may not be obvious to readers that the second, apparently very simple and cryptic, PIOMAS graph should also be clicked on and watched. Having done so, I would be inclined to say the trend begins no later than 1994. And it is terrifying. I remember lazily assuming, about that time, that if climate change was really a problem, it would be a problem for future generations - surely I would not live long enough to see any real difference in the climate. When we all see a satellite image of an ice-free Arctic on the news, five or ten summers from now, will the public finally be shocked into action? Or will they just say oh, well, that's the cost of doing business?