Sea Ice Area, Cryosphere Today, 1979-2000 in gray [click to enlarge]

It looks increasingly likely that we’ll match or beat the 2007 record for Arctic sea ice area. That means we should easily set the record for volume, since the ice is considerably thinner than 4 years ago. The death spiral continues.

Whether we will set the record for sea ice extent, which tends to get most of the media coverage, is a tougher call. Extent and area are diverging more than usual, as I’ll discuss below. But either way, the next few weeks should be pretty fascinating to watch.

As meteorologist Dr. Jeff Masters explained yesterday, “Arctic sea ice poised to undergo record decline in mid-August”:

A strong high pressure system with a central pressure of 1035 mb has developed over the Arctic north of Alaska, and will bring clear skies and warm southerly winds to northeast Siberia and the Arctic during the coming week, accelerating Arctic sea ice loss. Widespread areas of northeastern Siberia are expected to see air temperatures 4 – 12°C (7 – 22°F) above average during the coming week, and the clockwise flow of air around the high pressure system centered north of Alaska will pump this warm air into the Arctic. Arctic sea ice extent, currently slightly higher than the record low values set in 2007, should fall to to its lowest extent for the date by the third week of August as the clear skies and warm southerly winds melt ice and push it away from the coast of Siberia. This weather pattern, known as the Arctic Dipole, was also responsible for the record sea ice loss in 2007, but was stronger that year. The weather conditions that led to the 2007 record were quite extreme–one 2008 study led by Jennifer Kay of the National Center for Atmospheric Research showed that 2007′s combination of high pressure and sunny skies in the Arctic occur, on average, only once every 10 – 20 years.

The 2011 summer weather pattern in the Arctic has not been nearly as extreme as in 2007, but the total sea ice volume has declined significantly since 2007, leading to much loss of old, thick, multi-year ice, making it easier to set a new low extent record with less extreme weather conditions. The GFS model is predicting that the Arctic Dipole will weaken by 8 – 15 days from now, with cloudier weather and weaker high pressure over the Arctic. This should slow down the rate of Arctic sea ice loss to very near the record low values observed in 2007. It remains to be seen if 2011 Arctic sea ice extent will surpass the all-time low set in September 2007; it will be close, and will depend on the weather conditions of late August and early September, which are not predictable at this time. It is already possible to sail completely around the North Pole in ice-free waters through the Northeast Passage and Northwest Passage, according to sea ice maps maintained by the UIUC Cryosphere Today website. This marks the fourth consecutive year–and the fourth time in recorded history–both of these Arctic shipping routes have melted free. Mariners have been attempting to sail these passages since 1497. This year, the Northeast Passage along the north coast of Russia melted free several weeks earlier than its previous record early opening.

Much of the reporting on the Arctic is in terms of “sea ice extent.” That’s what the National Snow and Ice Data Center (NSIDC) reports in its widely tracked daily update. As the NSIDC explains in its FAQ:

A simplified way to think of extent versus area is to imagine a slice of swiss cheese. Extent would be a measure of the edges of the slice of cheese and all of the space inside it. Area would be the measure of where there is cheese only, not including the holes….

The most common threshold (and the one NSIDC uses) is 15 percent, meaning that if the data cell has greater than 15 percent ice concentration, the cell is considered ice covered; less than that and it is said to be ice free.

For a longer discussion of area vs. extent, see here.

Right now, there is a record divergence between area and extent, as Neven reports in his must-read Arctic Sea Ice Blog. “When the pack gets compacted area and extent will come closer together. But in the melting season the gap gets greater. This is because ice is melting, gets spread out and melt ponds start to form.”

Here are two plots from the International Arctic Research Center in corporation with the Japan Aerospace Exploration Agency (JAXA).

As you can see, 2011 has matched 2007 in area, but lags noticeably in extent.

Neven writes:

I don’t think this is just the ice pack spreading a lot. I think that in a significant part of the Arctic – the Pacific side to be exact (the Beaufort, Chukchi and East Siberian Seas) – the ice is, to quote Professor Peter Wadhams from this BBC interview a few years back, “just melt[ing] away quite suddenly”. Or to link back to the title of the last SIE update: ‘flash melting.’ And it’s showing up in area numbers first.

If so, both area and extent will continue to plummet in the coming weeks. Stay tuned.By Joe Romm