From the IJIS thread -
Now that sea ice extent has strung together a string of century drops, I wonder about the thoughts of those who have been claiming extent is a meaningless/useless metric.
Off to make a cherry pie.
I don't think extent is meaningless....but it is CERTAINLY LESS MEANINGFUL than volume. Volume has ALWAYS been the most important measure in my mind.
No need to shout.
Your speculation and arguments are not unreasonable, however.
Volume? Extent? I don't think anyone is going to win this argument but I also doubt that it intrinsically is an either/or question.
Getting beyond beliefs (I am always wary of those) some fundamentals apply.
No one currently measures either concentration or extent over the whole region. Folks measure microwave emissions at some combination of wavelengths, polarities, and resolution. Based on correlations (comparatively crude ones, if you measure their success against high resolution imagery) they publish rough estimates of extent. Based on a longer list of physical measures, input into a computer model, other folks publish even rougher estimates of volume. Any one of these estimates can be compared against itself over time, or (after cross comparison of years of data) with other estimates, giving some indication if the status quo has changed or not. Nobody is counting ice flows and subtracting those less than 15% over the entire Arctic. If you try it in any particular spot, comparison with the pantheon of published numbers are likely to be controversial. Used carefully, these comparisons can always provide insight but the further conditions on any given day are from the conditions on which the correlation or model was designed, the less the resulting numbers have to do with physical reality.
Back to reality (which we cannot currently measure), ice volume is a exact indicator of the locked in latent heat of fusion that would be needed to return the Arctic to liquid. The heat imbalance in the Arctic always operates against ice volume - a given number of joules results in a given amount of ice gained or lost. Extent (or 2D measurement based on whatever ice fraction cutoff you select) is the driver of the effect of ice. At its thickest, the depth of the pack is small compared to the ocean layer it influences. Ice sets the temperature of nearby water to the freezing (melting) point - 2D measures of ice drive what area of the ocean is stable at the local freezing point, and what area is free to store sensible heat as rising water temperature. Presence of the thinnest frazil or the deepest keel both mean nearby water is at the local freezing point (for the local salinity). Solar Albedo is also primarily a function of 2D area of ice. There are lots of second order effects but in all but thin ice, solar albedo is not strongly affected by thickness. In the winter season thickness does strongly affect net heat flow from the ocean (and over-ice air temperature), but that effect changes little as thickness moves into the multi-year range.
Heat added to or removed from ice always acts on its volume, but (particularly during the melting season) the most significant physical effects (including most negative feedback loops) of having ice or not are generally a function of its physical 2D presence.
Lots of strange things happen in the space between mature, unbroken ice and open water (the marginal ice zone - the part of the Arctic where ice is present but where wave action still occurs). The changing fraction and behavior of part of the pack which can be considered MIZ is hugely interesting. Both obviously exert influence but changes in the MIZ are not related in a simple or direct way to either extent or volume. It doesn't help that in the MIZ, both volume and extent are harder to measure with certainty. The MIZ is a function of local mechanical weather energy available, and local mechanical strength of ice available to resist it. A mild storm acting on 50cm ice can break up the same area as a huge storm over cold 3m ice. The effect (on ocean temperature and albedo) of a given concentration and size distribution of floes is not very sensitive to whether they are 50cm or 3m thick. Ice is there until it is not (which may help explain the lack of dramatic secondary effects from the huge decrease in volume in 2012).
I would hope that we don't get so wrapped up in details that we lose sight of the many (disproportionately influential) people in the world who claim nothing is changing at all, and certainly not because of CO
2 that we have been returning to the atmosphere at an ever greater rate.