- Bishop Hill blog

The BBC's Inside Science programme looked at the Ship of Fools expedition yesterday. We were treated to the unedifying sound of presenter wondering about whether what he called "deniers'" views on sea ice extent in the Southern Hemisphere carried any weight. We were led to beleive that they did not. Normal BBC fare I hear you say.

Some of what was said was bizarre though. Interviewee Professor John Turner of the British Antarctic Survey said the rise in Antarctic sea ice extent was less than 1% and was therefore well within the bounds of natural variability. I have no problem with the second part of that statement, but 1% seemed very low to me. Here is the relevant graph from the Cryosphere Today website.

The Antactic sea ice has an annual mean of 9 million km² or so and the current anomaly is 1.395. So we are at 14 or 15% above normal, not less than 1%. Am I missing something, or does Prof Turner have a problem with his mathematics?

The audio is attached below.

Update on Jan 10, 2014 by

Bishop Hill

Prof Turner emails:

Based on the Bootstrap version 2 passive microwave sea ice data the annual mean Southern Hemisphere sea ice extent increased at a rate of 147 × 10**3 km**2 per decade (1.2% per decade) over 1979-2011, which is significant at p<0.01.

I think that's him agreeing that the figure he gave on the BBC was wrong, but I've emailed back asking for clarification. Remember folks, there is nothing wrong with errors if they are corrected.

Update on Jan 10, 2014 by

Bishop Hill

The British Antarctic Survey Twitter account just pointed me to this paper.

@etzpcm Here is a paper you might find useful with 1% per decade fig: http://t.co/q4E0lcUJh2. Thanks to you and @aDissentient for contact.
— BAS_News (@BAS_News) January 10, 2014

I'll see if I can get a copy, but the tweet makes it clear that when Prof Turner said there had only been a 1% increase in sea ice, he meant 1% per decade. Moreover, the paper was submitted to the journal in January 2009 and was published a few months later. It is therefore unlikely that it includes data beyond 2007.

Here's the longer term record of the Antarctic sea ice anomaly:

As you can see, things have moved on quite a bit since that time. That said, his original email said 1.2% up to 2011, so I'm still not entirely clear.

Update on Jan 10, 2014 by

Bishop Hill

Another email from Prof Turner

- the trend was calculated from the annual mean total Southern Hemisphere sea ice extent.
- the percentage increase was based on the average annual SIE for 1979-2011 since there is no reliable whole Antarctic sea ice data before 1979.

If you include the 2012 data the trend increases to 157 × 10**3 km**2 per decade (1.27% per decade).

2012 didn't have the largest annual mean in the record, but it did have the largest daily ice extent, which occurred on 24 September. Our paper on this event is attached. In 2013 there was an even greater extent of Antarctic sea ice in September, so many people are trying to understand the mechanism or mechanisms responsible for the changes we are seeing.