Why
isn’t the Arctic sea ice free already?
Roz
Pidcock
23
September, 2014
Diminishing
Arctic sea ice is perhaps the most iconic impact of climate change.
There's a good chance that before too long we'll lose it altogether
in summer unless we cut emissions, scientists say.
But
according to polar scientist Dr Dirk Notz, the question isn't why is
Arctic sea ice melting so fast, it's how come we have any left at
all?
Notz
told a Royal Society conference
in London this morning that with the additional heat earth is
absorbing Arctic sea ice "should be long gone by now".
So
why isn't it?
Arctic
decline
Arctic
sea ice cover is declining by about four
per cent per decade.
The seasonal low at the height of summer is shrinking particularly
quickly, at more like 11.5 per cent per decade.
The National Snow and Ice Data Centre announced this year's Arctic sea ice minimum on the 17th September. At 5.02 million square kilometers, it was the 6th lowest on record. Orange line is 1981 to 2010 average extent for that day. Source: NSIDC
The
National Snow and Ice Data Centre announced this year's Arctic sea
ice minimum on the 17th September. At 5.02 million square kilometers,
it was the 6th lowest on record. Orange line is 1981 to 2010 average
extent for that day. Source: NSIDC
Dirk
Notz, a polar scientist at the Max Planck Institute for Meteorology
in Germany, told today's conference that if you consider how much
heat the Arctic is absorbing, it should be more than enough to melt
the sea ice completely.
Globally,
we've already seen 0.85
degrees
of warming since 1850. But temperatures are rising twice
as fast
in the Arctic than in the rest of the world, with the region warming
about two degrees Celsius over the same period.
Accelerating
ice loss
In
the time since satellites started monitoring the region 30 years ago,
we've seen at least enough heat absorbed in the Arctic to make all
the sea ice in the Arctic a whole metre thinner, Notz calculates.
And
as soon as sea ice starts to thin by that much, a number of processes
kick in to accelerate the pace of ice loss even more, he says.
Thinner
sea ice is weak and easier for passing storms to break up, for
example. And as sea ice retreats, sunlight that would have been
reflected is absorbed
by open water instead. The less sunlight is reflected, the more heat
the ocean absorbs, and the faster sea ice melts.
Scientist
know both these processes are happening. Arctic sea ice is thinning
and the region is reflecting less heat back into space. In theory,
these processes working together should accelerate ice loss until
there's none left to melt, Notz says:
"With
[these factors] and nothing else, Arctic sea ice melt should be much
faster. There shouldn't be any sea ice in the Arctic"
But
that hasn't happened, which means something else must be going on.
Positive
feedbacks
On
top of the overall decline in sea ice over the last few decades, the
amount we see from one year to the next varies a lot. You can see
this in the spikes and troughs in the graph below of the decline in
sea ice extent over the past 30 years.
Scientists
are seeing something unexpected in the way Arctic sea ice behaves
year to year.
After
a year with a big drop in sea ice - such as 2007 and 2012 - the
amount of sea ice seems to be swiftly replaced the following year. As
Notz puts it:
"Sea
ice in one year is closely connected to sea ice the year after …
Strong loss in one year is followed by a recovery".
This
suggests there's something going on to stabilise the sea ice system
each time it drops particularly low, or rises particularly high.
Notz
says the likely explanation is that when sea ice disappears, it's
much easier for the surface of the ocean to kick heat out to the
atmosphere, cooling the water down again to conditions that favour
sea ice to form or grow thicker again. And thin ice grows much faster
than thicker ice, Notz says.
A
sea ice free Arctic
Such
large natural variability makes it difficult to predict Arctic sea
ice behaviour from one year to the next, says Notz. Natural
variability is the main reason scientists see a mismatch between what
climate models predict will happen and what happens in reality,
especially over the relatively short period of time over which
scientists have reliable satellite data.
But
while Notz's research suggests Arctic sea ice may be more resilient
than expected from one year to the next, the longer-term picture is
clear.
If
emissions stay high, almost all climate models predict the Arctic
will become sea ice-free in summer by mid-century. But pinpointing a
precise year is far more difficult, Notz says:
"Because
it's a chaotic system, there will always be large uncertainty as to
when Arctic sea ice will be gone in summer."
The
prospect of sea ice-free summers has wide-ranging consequences. It
would affect people
and ecosystems,
unlock new shipping
routes
and expose more fossil
fuel reserves.
All
the available information from climate models puts the likelihood
that we'll be seeing sea ice free Arctic summers by 2040 at around 30
per cent, Notz estimates.
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