Three-quarters of Arctic sea ice lost in 30 years? We check.
- 07 Jun 2012, 14:45
- Verity Payne
© Lars Witting/ARC-PIC.COM
Over the weekend an Observer Magazine
article featuring a Greenpeace stat - that 75 per cent of
Arctic sea ice has been lost over the last 30 years - prompted
bloggers, who suggest that even "[f]ive minutes or less of
checking would have prevented this blunder."
We took up the suggestion and spent rather more than five
minutes checking both the 75 per cent figure, and the responses to
it. The outcome? A salient lesson that when quoting facts and
figures, particularly about Arctic sea ice, being clear and quoting
sources is vital in order to avoid misunderstandings.
First though, the story so far:
Sunday's Observer magazine featured an
article about bottom-baring Britpop icon Jarvis Cocker and his
new role as frontman for the Greenpeace campaign to bring about a
moratorium on exploiting the Arctic resources exposed by melting
sea ice. Cocker, we are told, has become concerned about the fate
of the Arctic since visiting it in 2008:
"Not that I'm a massive expert, but when
I heard that they wanted to dig it up, I thought: hold on a minute
- that's not good."
Beneath the Observer article is a Greenpeace-sourced list of
'[c]hilling facts about the Arctic', begining with:
"Of the Arctic sea ice, 75% has been
lost over the past 30 years. Last year saw sea-ice levels plummet
to the second-lowest since records began."
This short statement caused a fair amount of discord in the
blogosphere over the weekend, with climate skeptic bloggers
Haunting the Library disputing the figure and branding it a
"total departure from both reality and common sense". On the other
hand, climate modeller and
blogger William Connelly seems to back the 75 per cent
So where has this figure come from? And is it right?
For a clue as to where it comes from, the 75 per cent figure,
which was attributed to Greenpeace, also appeared in a Greenpeace
blogpost back in February:
"If there's one fact to remember which
underlines the urgency in protecting the Arctic it's this: in 30
years we've lost 75 per cent of the Arctic sea ice."
According to the
blogpost, the 75 per cent figure comes from research into sea
ice volume, and not the more commonly discussed 'sea ice extent'
(which is roughly the same as 'area').
It's obviously more difficult to measure volume, where a lot of
change happens below the surface, than extent, where satellite
measurements of the Arctic can be used.
But sea ice volume is an important measurement,
as we have written in the past, and as Greenpeace highlights in
"Some people compare Arctic sea ice
amount by looking at the surface area of the ice and calculating
how much it contracts by. But if you want to know how much of the
ice is actually left, you've got to look at the volume - which is
both the area and the thickness of the ice."
The Greenpeace blogpost links to the University of Washington's
Science Center website.
Faced with the difficulties of directly measuring sea ice
volume, The Polar Science Center produces an assessment based on
computer modelling - specifically something called the Panarctic
Ice Ocean Modeling and Assimilation System (PIOMAS). The 75 per
cent figure appears to be taken from the
PIOMAS site, which reads:
"Monthly averaged ice volume for
September 2011 was 4,200 [cubic kilometres]. This value is 66%
lower than the mean over this period, 75% lower than the maximum in
1979, and 2.0 standard deviations below the 1979-2011 trend."
It's worth noting that this statement is somewhat ambiguous.
Arctic sea ice goes through a yearly seasonal cycle, growing to a
maximum volume in the cold winter months and shrinking to a minimum
volume in the warm summer months; according to PIOMAS, the average
sea ice volume changes from 28,700 cubic kilometres in April to
12,300 cubic kilometres in September.
Hence the PIOMAS website, which describes ice volume in
September 2011 (a summer minimum) as 75 per cent lower than 'the
maximum in 1979', could be read as comparing a minimum with a
maximum, which would mean the 75 per cent figure was a misleading
comparison that compares summer to winter.
On the other hand, it could mean a comparison between the
biggest summer minimum volume, which occurred in September 1979,
and the smallest summer minimum volume, in September 2011. In that
case, noting that the second was 75 per cent smaller would be a
reasonable comparison to make when discussing PIOMAS results.
To check, we contacted PIOMAS scientist Dr. Axel Schweiger. He
explained that the 75 per cent figure was actually comparing
minimum to minimum - it refers to a comparison between the
September 1979 minimum ice volume and the September 2011 minimum
So, as Dr. Schweiger told us:
"[W]hen referring to one particular
measure of "Arctic sea ice loss" : PIOMAS ice volume, then that
statement [the 75% figure from the PIOMAS website] is correct."
This is confirmed in the graph below, where the red line shows
the PIOMAS average monthly arctic sea ice volume for September, and
the blue line for April. Note that the September 2011 volume is
only about a quarter of the September 1979 volume:
PIOMAS mean monthly arctic sea ice volume for April and
September. Dashed lines parallel to linear fits represent one and
two standard deviations from the trend. Error bars are estimated
based on comparison with thickness observations and model
sensitivity studies (
Schweiger et al. 2011) Source:
Picking two numbers exaggerates the noise
It seems to us that while the claim that there's been 75 per
cent Arctic sea ice loss isn't out and out incorrect, as some
bloggers have argued, that doesn't mean it's the most appropriate
measure to highlight in a soundbite - for a few reasons.
First, as Schweiger told us, comparing just two single points in
time rather than a trend 'exaggerates the noise'. Looking at the
decadal trends in sea ice volume - i.e. an average of what the ice
has been doing over each decade, rather than just a comparison of
the start and end points - produces more confidence. According to
PIOMAS, decadal trends suggest that over the past few decades sea
ice volume has diminished by around 3,100 cubic kilometres per
However, there's a significant degree of uncertainty in this
measurement, of around +/- 1,300 cubic kilometres per decade. It's
this kind of uncertainty that might suggest caution with presenting
the PIOMAS figures, which are only one bit of scientific evidence,
PIOMAS might be bang on the money, but as WUWT and Haunting the
Library point out, other scientists appear to disagree - Met Office
Chief Scientist Julia Slingo is quoted in a
Guardian article from earlier this year disputing the 75 per
"[Slingo] said that suggestions the
volume of sea ice had already declined by 75% already were not
credible. 'We know there is something [happening on the thinning of
sea ice] but it's not as dramatic as those numbers suggest.'
"The problem, [Slingo] explained, was that researchers did not
know the thickness of Arctic sea ice with any confidence. [Slingo]
hoped a new ice-monitoring satellite launched in 2010, Cryosat2,
would help with more accurate measurements."
Sea ice: extent, area or volume?
Sea ice volume is a useful measurement of how the Arctic is
changing, and it's important to remember that the sea ice is
changing beneath the surface when assessing its state of health. In
his email to us Schweiger highlighted:
"It is important to distinguish between
ice-extent, area, and volume. Because the ice has shrunk in extent
and thinned, the loss in volume is greater than in extent
There can be very little doubt that Arctic sea ice is in
decline. The Arctic is losing
its older sea ice, leaving younger, thinner ice in its place.
So a measure like sea ice volume, which includes this thinning, is
probably more representative of the changes taking place in the
Arctic than sea ice extent, which doesn't include this
And, as Schweiger told us:
"No matter how you slice it, there is
very solid evidence from both observations and modeling that shows
that the Arctic Sea ice has not only shrunk in extent but also
thinned, thus yielding a loss in ice volume."
But ultimately, this example indicates that complicated
scientific conclusions don't survive the process of being reduced
to soundbites particularly well.