Everything you ever wanted to know about Arctic sea ice (but weren't enough of a geek to ask)
- 12 Aug 2011, 16:00
- Christian Hunt
"New
warning on Arctic sea ice melt" says one headline; whilst "
Increase in Arctic ice confounds doomsayers" announces another.
Media coverage of Arctic sea ice can all get a bit confusing - and
we're about to see another round of it, as Arctic sea ice extent
reaches its annual summer low in September.
September 2007 saw a record low for the extent of Arctic sea
ice. The most recent data for the National Snow and Ice Data Centre
(NSIDC) assessment
of average Arctic sea ice extent during July 2011 was even lower
than at the same time in 2007 - the lowest average extent reached
over July since the start of the satellite data record.
So will this year's summer low be another record breaker?
Speculation has already begun, and so has the inevitable 'Arctic
death spiral' debate (exemplified
here and
here). Record lows and 'death spirals' are undoubtedly
newsworthy, but often serve to confuse. The accompanying debate
tends to reduce understanding of the main point - that there is an
indisputable declining trend in Arctic sea ice over the last 3
decades.
With that in mind here are ten things you might want know about
the Arctic ice cap...
1. Arctic sea ice coverage is shrinking
The Arctic ocean is the smallest and shallowest ocean on the
planet, and is host to a floating ice cap which grows and shrinks
with the seasons. The sea ice shrinks in the Arctic summer,
reaching its smallest extent around the middle of September, and
grows again in the Arctic winter, peaking around April. But as well
as this annual cycle, it's become clear to scientists monitoring
the region that the ice cap is getting smaller, with both summer
and winter ice extent reducing since satellite records began:
This is perhaps not particularly surprising, as the ice floats
on an ocean that is getting warmer, and is surrounded by air that
is warming rapidly. The
IPCC
assessment in 2007 was that "Average arctic temperatures
increased at almost twice the global average rate in the past 100
years".
The US scientists who monitor the Arctic sea ice at the NSIDC,
put it pretty
clearly:
"Arctic sea ice is declining at an
increasing rate in all months of the year, with a stronger decline
in summer months. Researchers who study climate and sea ice expect
that at some point, the Arctic Ocean will lose its ice cover
completely in late summer. A variety of evidence suggests that
Arctic sea ice is declining because of climate warming resulting
from increased greenhouse gases in the atmosphere. Because
greenhouse gases persist and are expected to increase, scientists
see no reversal to the downward trend in ice extent."
According to a
review of recent Arctic scientific research, the extent of sea
ice coverage in the Arctic is at its lowest point for several
thousand years.
2. Scientists say that man-made Arctic warming is
responsible for the long-term decline in sea ice
A group of scientists have modelled
natural and man-made Arctic temperature changes
responsible for the long-term decline in sea ice and found that
the Arctic warming is caused by human activities. They say:
"We find that the observed changes in
Arctic and Antarctic temperatures are not consistent with internal
climate variability or natural climate drivers alone, and are
directly attributable to human influence. Our results demonstrate
that human activities have already caused significant warming in
both polar regions, with likely impacts on polar biology,
indigenous communities, ice-sheet mass balance and global sea
levels."
3. Summer 2007 disturbed many scientists and other
observers
The low point in sea ice in summer 2007 was unusual. There was
an abnormally rapid drop in ice area, to a level that was
significantly below anything seen before. This graph from the Japan
Aerospace Exploration Agency shows this in context - you can see
that the black line (2007) falls significantly below those for
other recent years:
The 2007 fall jarred with the expectations of scientists who had
predicted a more gradual decline - and this raised questions about
whether a new phase of rapid collapse had begun.
One suggestion regarding why 2007 was so low is that rough
weather conditions acted to break up the ice more swiftly than
normal. But the same level of decline hasn't been repeated in
subsequent years - yet. A glance at the figures for 2011 (the red
line) in the graph above indicates that we may be on a similar path
this year.
4. With the sea ice changing all the time, the word
'recovery' needs to be used carefully
Sea ice shrinks and grows every year, and over short periods of
time is subject to the same kind of natural variation as any other
part of the climate system.
Since the summer of 2007, the extent of sea ice has increased
again. It grew again in the Arctic winter, as expected, and
subsequent summer minima were greater than in 2007. This led some
to argue that the sea ice is 'recovering', and that we should
therefore stop worrying about it.
What these commentators missed, or simply didn't explain, was
that the sea ice was only 'recovering' temporarily in a way
consistent with a long-term decline:
For example - in a piece entitled '
So it appears that Arctic sea ice isn't vanishing after all' in
February 2008, Christopher Booker wrote that the NOAA graph of the
ice coverage,
"… which shows the ice shrinking from
13,000 million sq km to just 4 million from the start of 2007 to
October, also shows it now almost back to 13 million sq km."
What he failed to mention was that the ice declines and then
increases again every year as part of the cycle of summer and
winter.
5. Sea ice volume is also important - and we're getting
better at measuring it
Although attention tends to focus on the area or 'extent' of the
sea ice, what's going on under the sea is also important. Each year
some ice survives the annual melt, and it's this older and thicker
ice which is more resistant to the summer melt.
It seems however that it is not only the area of sea ice which
is in decline, it's probably also the volume.
Climate
modelling
work, which doesn't give a direct measurement of ice volume but
which is calibrated against measurements of ice thickness from
submarines, satellites and buoys, suggests the ice volume is
shrinking. The model produced the following graph of ice volume
'anomaly' (difference to long-term average):
Information about the ice thickness should get a boost this year
with the launch of 'Cryosat-2'
- a satellite which can measure sea ice thickness, and the first
easy system for getting regular pictures of how thick the ice is.
With this new data, we can expect more of a focus of how much ice
there is under the water, as well as showing above it.
6. It's not over till it's over
It's best not to be premature about announcing an ice minimum.
The NSIDC announced a sea ice minimum last year for the 10th September,
but had to issue an updated press release a few days later when the
ice kept shrinking - see the double dip in this graph for September
2010 (the blue line):
7. There are different views about the significance of
sea ice decline
For scientists, Arctic sea ice decline is a clear symptom of
climate change - and it has been suggested that eventually, the sea
ice will disappear entirely in summer. (Note that this is not the
same as saying 'the North pole will be ice free in summer' - that
could happen much more quickly). Some scientists believe that the
ice is in a faster state of decline, and could be sea-ice free in
as little as
30 years.
8. Changes in the Arctic affect the rest of the
planet
But why does the sea ice get so much attention? In part it's
because it's a clear and visual indicator of the effect that
climate change is having on the planet. It's also because any
warming is enhanced in the Arctic compared to the rest of the
globe, due to a number of processes as outlined in the Arctic Climate Impact Assessment report from 2004.
These include the ice albedo effect - warming causes melting,
exposing dark seawater that absorbs more sunlight, which leads to
more warming. This lack of ice allows more heat to be transferred
from the ocean to the atmosphere, exacerbating warming.
Additionally, polar warming changes atmospheric and ocean
circulation patterns which can cause more heat to be transferred to
the Arctic, again leading to more warming. We have written more
about the impact of Arctic warming here.
9. There are some great places to get information about
what's happening
For armchair (or laptop) science enthusiasts, the Arctic sea ice
is a dream come true, because of the excellent resources that
scientists have made available to see what's going on.
http://www.catlinarcticsurvey.com/
10. The bets are on
The Arctic Research Consortium of the US (ARCUS) have invited
people to offer their prediction of what the sea ice minimum
will be this year.
This is an interesting concept, as it allows the comparison of a
number of model and statistical predictions. It should be noted
however that it also includes 'heuristic' predictions (guesses).
and contributors include a climate skeptic blog. The best guess of
a blogger should not be given the same credence as a statistical or
model derived estimate, which has a sound statistical or scientific
basis and an estimated error range.
Anyway, here are some of the predictions - including Stroeve's
for the NSIDC. We'll see how accurate they are in September...
