### Sea ice extent

I've run across several skeptic claims recently that Arctic sea ice extent is within the normal range for this time of year, as well as claims that Antarctic sea ice is growing  While both are true, the implication that we shouldn't be concerned with the Arctic sea ice melting because of those facts is pure nonsense.

First, here's a look at Arctic sea ice extent since 1979:

You can clearly see that there is a large yearly cycle, as well as a decline in both the maximum extent and the minimum each year.  The minimum represents the amount of multi-year sea ice left each year.  The difference between the minimum and maximum is the amount of short-lived winter ice that forms each winter.  Even in this view, you can clearly see that the yearly minimum has been dropping rapidly whereas the yearly maximum has been dropping far more slowly.  You can also see that the amount of short-lived ice has increased.  That should be the first clue as to why having Arctic sea ice extent within the normal range for this time of year is meaningless–we're at the start of the melting season when most of the ice is that short-lived winter ice.  Taking a 12-month running average to factor out the yearly cycle and reveal the overall trend provides the next clue:

I've added a loess trend line (in blue) to highlight the trend and a linear trend line (in red) showing the average loss of -54,435.38 square kilometers per year since 1979.  The trend is clear, even without the loess and linear regression.  And there's no sign of stopping.  If I took just the amount of permanent ice left each year, the decline is even more dramatic:

Antarctic sea ice looks like this:

It's clearly dominated by the yearly cycle, making any trend very difficult to see, unlike the Arctic plot.  However, it looks like there's a slight increase in the yearly minimum.  Again using a 12-month running average to remove the effects of the yearly cycle shows this:

The average gain is +16,860.51 square kilometers per year, which is far smaller than the -54,435.38 square kilometers the Arctic loses in an average year.

So what we have is a) the continued lose of Arctic sea ice, particularly the multi-year ice and b) on average the Arctic is losing -37,574.87 square kilometers more sea ice each year than the Antarctic is gaining.  Add in the fact that we know Antarctic sea ice is growing because of meltwater from the Antarctic continent (Bintanja et al. 2013) and the average yearly loss of -71 billion metric tons of ice from the Antarctic continent (Shepherd et al. 2012) and the overall story becomes one of the loss of ice.  And that's the story that those skeptics are ignoring when they make their claims.

Why should we be concerned about the loss of ice?  Beyond the potential loss of those species which depend on ice (i.e. polar bears), Arctic sea ice helps control the weather in the temperate zone, which is most of the US and Europe.  Losing that ice means more extreme weather events in the temperate zone (Francis and Vavrus 2012, Jaiser et al. 2012, Liu et al. 2012, Petoukhov et al. 2013, Tang et al. 2013).  And more extreme droughts, floods, heatwaves, etc are good for no one.