The season 2011 again revealed sea ice extent is the less informative the lower sea ice concentration is in the Arctic. Actually you may call it a red herring hiding the true nature of sea ice decline. It is time to put on 3D glasses and talk about volume and thickness.

To the end of this season the discussion focused on whether the ocean surface covered by sea ice was less or more this September or in 2007’s. That’s like comparing the buying power of different currencies by measuring the size of the banknotes.

These rose-colored spectacles called extent and area do not tell us winter sea ice currently looses thickness at an averaged rate of 10 centimeters and more per year. They filter out bottom melting during summer easily sums up to a meter during a month with a maximum rate of 10cm per day.

Finally both parameter prevent us from realizing that if melting trends continue or accelerate we’ll witness in a few years how millions of square miles of first year ice a few centimetres thick surprisingly turn into sea water just within a few days leaving only scattered floes of older ice.

Sea ice is not flat as paper, so why do we count records in square but not cubic kilometers? Actually you’ll find people counting pixels to guess the tiny difference between sea ice extent of 2007 and 2011 to make a point and ignore in the same time the Arctic lost thousands cubic kilometers of ice.

What is the reason for such a behaviour? Why is exact data of an inappropriate measurement more attractive than an excellent model of the real nature of Arctic sea ice? Is it because a denying voice is telling us models are bad, not reliable and do not count as a proof?

Well, there is a difference between reality and models, for sure. But if you look around there are models everywhere. A thermometer does not tell you the temperature of your living room by measuring the average speed of all air molecules. It simply displays the volume of a liquid and badly fails in direct sun light or if temperature gets below freezing point of that liquid.

So, to make a long story short: Essentially, all models are wrong, but some are useful. A quote taken from George Edward Pelham Box' book titled "Empirical Model-Building and Response Surfaces", he wrote with Norman R. Draper.

Strictly speaking even skeptics accepted sea ice extent as a model of the Arctic’s ice pack, knowing it’s accurate, ignoring it’s flat.

Sea ice extent is the result of an algorithm turning electromagnetic echos in the range of 6GHZ to 89GHZ into sea ice concentration. Difficulties arise from coastlines, atmospheric interferences caused by rain, water vapor and high winds, sensor noise and surface conditions. Also, different institutes use divergent thresholds, grids, land masks and tie-points.

Despite these obstacles extent provides its own usefulness since it pictures the navigable area very well and we have a record going back 30 years to the beginning of the satellite era, which tells us sea ice is on decline faster than natural processes can explain and mostly probably even faster than sediments thousands of years old ever recorded.

However, sea ice concentration was already low in past summers and became an additional issue. And that’s why the University of Bremen announced a new minimum record 2011 and the NSDIC will not.

Rigorous measurement of sea ice thickness would involve thousands of ice mass balance buoys or continuous and area-wide aerial survey with e.g. an EM bird. Both methods are costly and impracticable. Less impacting are observations using satellites flying on a polar orbit .

Freeboard is to sea ice thickness what brightness temperature is to extent. Archimedes described first that “Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.” Sea ice has very roughly a density of 900kg/m³, so an ice floe one meter thick shows a freeboard of 10cm.

Knowing the elevation difference between the top of the snow surface, local sea levels, and snow height and density above the snow/ice interface allows freeboard calculation. Once you know freeboard and determined local ice density computing thickness is trivial. The known unknowns are snow load and ice density because multiyear ice is lighter.

NASA’s ICESat mission delivered first spaceborn thickness data until the satellite reentered the atmosphere in August 2010. A second mission is planned to launch in early 2016. ICESat measurements of the distribution of winter sea ice thickness over the Arctic Ocean between 2004 and 2008 showed massive thinning and MYI was no longer regarded as the dominant ice type.

Along with observations from US-Navy submarines, oceanographic moorings, bouys and airborne data ICESat data played a major role in the validation of the Pan-Arctic Ice Ocean Modeling and Assimilation System (PIOMAS) developed at APL/PSC. This numeric model is known for its capacity to assimilate real data from observations.

PIOMAS indicates the Arctic lost 72% of its ice volume since 1979 and calculates an average of 5,000km³ for August 2011. In a recent paper quantizing all model errors the authors suggest that while sea ice extent was lowest in 2007 the minimum volume record happened in 2010.

Latest addition to this list of attempts to address sea ice thickness is ESA’s CryoSat-2 mission. In opposite to ICESat’s laser instrument a radar is used to measure freeboard down to centimeters. It must be said that while expectations are high, CryoSat is an opportunity mission and can only be called experimental. Also the satellite does not target sea ice volume directly but seasonal changes of thickness.

Nevertheless the potential outcome might provide deep insights into first spatial and temporal sea ice thickness distribution and second the technology actually needed to provide accurate results.

The validation campaign is still ongoing and apparently the spacecraft works better than specified. First results have been presented earlier this year in Paris and suggest a functional processing chain. The satellite was accurately placed at his designed altitude and enough fuel remained for many years of operation.

Relatively unknown is another ESA mission - SMOS - designed to observe the global water cycle. Due to the unique wavelength (1.4 GHz) beeing analyzed sea ice thickness retrieval up to 1.5m seems possible.

It is not like we know nothing about sea ice thickness. At any point in time several buoys are floating in the Arctic, collecting ice drift data and document the melting process. Submarines provide upward-looking sonar profiles and occasional EM-Bird flights give very accurate data. Numeric models are capable to combine the results and fill in the gaps.

A slowly growing satellite fleet reports freeboard from space and last not least scientists extract ice cores to measure directly the third dimension of sea ice.

All of this indicates a faster thinning of the sea ice cover than 2D extent charts propose. Most likely an ice free Arctic will have an impact on weather pattern all over the northern hemisphere when millions of square miles of sea no longer reflect incoming solar radiation and instead the Arctic Ocean absorbs the energy and heats up.

A realistic and scientific view on sea ice conditions in all dimensions is mandatory to estimated future consequences and plan adaptations to a warming planet.

Earth is not flat and neither is sea ice.