So far this year the extent of sea ice floating on the northern polar ocean has been off-the-chart low … again. It’s currently about a million square kilometres below the extraordinary record melt year we saw in 2012. That is zero at the bottom of the chart:
Arctic sea ice extent, from the US NSIDC
‘Extent’ is a measure of gross ice covered area — the area of ocean with at least some floating ice. That chart¹ is from the US National Snow and Ice Data Centre, which defines extent as the area of ocean and tidal waterways with at least 15% floating sea ice (excluding glacial ice tongues and shelves). That’s not the same as net sea ice area, for which you need to subtract all the included watery bits (quite a bit harder to measure).
Followers will know that I don’t find sea ice extent all that interesting. It says little about how much ice is still there, which also depends on ice concentration (the proportion of actual ice cover within the ‘extent’ area) and on ice thickness. The measure we want is sea ice volume, which of course is even harder to measure. Sea ice volume is now approximately measured for part of the year by satellite remote sensing (CryoSat-2), but the best continuous volume estimate is still that from the data assimilation model run by the Polar Science Centre at the University of Washington, called PIOMAS.
PIOMAS northern sea ice volume
I’ve written at length about that before. Suffice to say that the recent drop puts the anomaly trace right back on the parabolic trend line. Our planet is on track to lose its northern polar ice cap — this decade, or next. No later.
Notes
- A sensor on a US military satellite that NSIDC used to estimate sea ice extent failed earlier this year. They are currently using a different sensor on a newer satellite, which is not yet fully cross-calibrated. As a result the extent trace since March is provisional (shown dot-dash), but I don’t expect it to change much, based on other extent estimates from independent satellite remote sensing sources.
