(Image source: PIOMAS)
Despite an ongoing and precipitous trend of sea ice loss, it still happens now and then. As cold air invades and re-freeze sets in, pace of new ice formation spikes and we get the potential for a bit of ‘recovery’ in sea ice area and extent by March-April following record summer losses. The trend for sea ice area and extent for these months is still down, however. According to NSIDC, extent is falling at a pace of about 2.5% per decade during March and April. This pace of loss is quite modest when compared to summer losses, even though the long-term trend is still down.
Sea ice volume, on the other hand, is an entirely different story. The measure of total ice area + ice thickness as determined by PIOMAS has crept steadily and rapidly downward during both winter and summer months. Though the total loss below the 1979-2012 average is slightly greater for summer (approx. 9,000 cubic kilometers) it does not greatly exceed the loss seen during winter months (about 7,000 cubic kilometers) since 1979. (Percent losses for summer exceed 25% per decade while percent losses for winter are about 13% per decade. So the pace of summer volume loss in this measure is still much greater.)
There is, however, one small wrinkle in this observation. Winter sea ice volume measurements have tended to cluster in a given range before taking large steps down during certain years. Summer, on the other hand, has shown more steady and consistent melt with large step years spaced out by many years of more moderate melt.
So it was little surprise when sea ice volume tied and slightly exceeded March 2011 and 2012 measurements for brief periods during the winter of 2013, edging above the record low value by about 70 cubic kilometers on certain days.
Now, PIOMAS shows pace of volume melt rapidly increasing through mid April. And this new melt has brought Arctic sea ice volume back into record low territory, edging about 80 cubic kilometers below the record set in 2011 for that time.
Sea ice volume is the critical measure now that validation from satellite instruments has clarified the accuracy of PIOMAS modeling. Area and extent only measure the surface as visible from above. But the total proportion of remaining ice is captured by current volume measurements. And what PIOMAS is showing, at this point, is that sea ice volume in the Arctic is currently lower than it has ever been in modern reckoning.