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:

July average Arctic sea ice extent

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:

Yearly Arctic sea ice extent JAXA 

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:

Mar Sept Arctic sea ice extent Cryoclim

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):

PIOMAS Arctic sea ice volume anomaly

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):

Arctic sea ice minima 2010

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.

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...

ARCUS predictions for 2011 sea ice minimum

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