Those who accept the consensus that the Earth is warming due to human activity (anthropogenic global warming or AGW) point to declining Arctic sea ice as one line of evidence to support this conclusion. Those who do not accept the AGW consensus claim that Arctic sea ice is not declining, or at least we have insufficient data to reach such a conclusion.

What I like about this controversy is that it is about data. It is a fake controversy, driven by political ideology, but none-the-less we can sink our teeth into the data and see which side has the better position.

Arctic sea ice varies throughout the year, growing in the winter and melting in the summer. Therefore any year-to-year comparisons need to take this seasonal variation into account. Scientists use the summer minimum as one measure of the extent of Arctic sea ice for that year. You can also look at the winter maximum.

Sea ice can be measured in square miles, essentially the amount of area covered by ice. It can also be measured in thickness, and the two measures can be combined to calculate the overall volume of ice.

Very much like global temperatures, year-to-year variation is also a very noisy system, bouncing around based upon short term weather patterns. You really cannot make any reliable statements about what is happening to Arctic sea ice from any change over one or a few years. Again like global temperatures, if we want to detect any long term trend we need to look at what is happening over spans of time of about a decade.

So let’s look at the data. Here is a graph of annual arctic sea ice since we started keeping records in 1979:

You can easily see the short term variability, but also the long term downward trend is quite clear. This one line of evidence by itself does not prove AGW. But if we ask the purely factual question – is there a downward trend in Arctic sea ice, the answer in unequivocally yes. Added to other lines of evidence this trend does support the conclusion of AGW.

Here is another graph that is more dramatic, looking at total Arctic ice volume:

You can see the entire animation here, watching Arctic sea ice volume change from 1979 to 2014. Given how absolutely clear the data is, what are those who deny AGW saying? Well, they are focusing on the last few years because 2012 saw an extreme minimum, and so the few years since then have been higher than the 2012 minimum.

Already you should see the fallacy of this line of argument. As I stated – you can’t look at short term trends with such a noisy system. We need to average the data over at least a decade before the long term trend becomes statistically meaningful (before the signal can be pulled from the noise). It is easy to see this graphically. The chart above and the animation both show the noisy year-to-year variation but with a clear long term downward trend.

Deniers are also cherry picking. The year 2012 was a recent minimum, so starting there gives a false impression of a recent increase. This is just regression to the mean, however. It’s meaningless.

Further, it is not even an upward trend. Sea ice rebounded in 2013 compared to 2012 but then decreased in 2014 and this year as well, although is still above the 2012 minimum.

Another way to look at sea ice extent it to compare the current level to long term averages. In 2015 Arctic sea ice is trending more than two standard deviations below the average for 1981-2010.

So what are the deniers saying? From Steven Goddard at Real Science:

Red shows the September 2012 minimum extent. Green shows the current extent, which is likely the minimum for 2015. The Arctic has gained hundreds of miles of ice over the past three years, much of which is thick, multi-year ice. Nobel Prize winning climate experts and journalists tell us that the Arctic is ice-free, because they are propagandists pushing an agenda, not actual scientists or journalists.

The cherry picking is blatant and shameless. The “ice-free” comment is a reference to prior predictions that the Arctic may be ice free by 2015. This was not a consensus, just the most extreme predictions, the kind that tend to grab headlines. Extrapolating trends into the future is always difficult, especially in a noisy system. This, of course, says nothing about the data regarding what has already happened to Arctic sea ice.

Conclusion

Arctic sea ice is undeniably decreasing over the last 36 years. The long term trend is clear. Those who want to deny this trend, however, focus on short term data because you can cherry pick any conclusion you wish.

You don’t have to read far into the comments of any article on AGW to see the motivation. The top comment right now on the Real Science article reads:

The global warming hoax has never been about science, but a power grab instead for the UN. They need an excuse to override any country’s existing laws and constitutions, such as ours, all in the name of some great pending catastrophe. “We can’t be bothered with your freedoms. We need to save the world.” Yeah, right.

AGW deniers don’t really have a legitimate scientific point to make. Their position is based on the politics. I have argued previously that it is a flawed strategy to argue against the science when you disagree with the politics. It is a losing proposition – because you can’t fight with data. You can confuse, obfuscate, misdirect, and distort but eventually you will be buried in actual data, at which time you lose all credibility.

Deniers are also pseudoskeptics. They try to use the language and argument style of skeptics, but they distort the details to their agenda. This comment takes the irony prize:

I agree with what Stevengoddard says: NSIDC starts their graphs in 1979, which was the peak ice from the last half century. Only a complete clown would be fooled by such a fraudulent use of data.

No, 1979 is the starting point for the graph because that is when we started collecting data. It was not a cherry-picked peak. Some commenters also argue about whether 2 or 3 standard deviations is the proper cutoff to consider a trend statistically significant. This is a strategy of doubt and confusion. There is no one objective cutoff – as we get more data over a longer period of time our confidence will increase. Right now we are greater than two standard deviations, which is a reasonable level of confidence. You can always delay conclusions by pushing the threshold higher and higher. It’s never enough for deniers, however.

Those who are concerned with the politics of global warming should stop denying the science – it’s bad science and bad politics. Accept the science and then propose solutions which are more in line with your political values.

No matter what your political values, Arctic sea ice is decreasing (as is global ice).