The Importance of Concentration

Last month, a number of well known web sites and commenters were getting themselves worked up with comments like “Arctic ice dropping at the fastest rate in history” and “Arctic ice is dropping like a rock.” I advised repeatedly that prior to July, looking at the extent graphs is pointless.

July is here now, and the rate of ice extent decline has dropped dramatically over the last week. To put this in perspective, according to JAXA data, the June 28-July 4 rate is -53361 km²/day. In 2007 during the same period, ice was lost at -123104 km²/day.

In other words, 2007 was losing ice 2.31X faster than 2010.

This can be seen most dramatically in the DMI graph, which measures only higher concentration ice (30%.)

http://ocean.dmi.dk/arctic/icecover.uk.php

Close up image below.

So why the dramatic difference in slope? One reason is that sea ice concentration is at the highest level in the satellite record. Compare below vs. 1980, when ice was considered very “healthy.” Current concentration is considerably higher.

Ice concentration is particularly important this time of year because the sun is relatively high in the sky. When the ice concentration is low, sun shines into the water in “Swiss Cheese” holes around the ice, warms it, and corrodes away the edges of the ice. This year, ice concentration has been close to 100% in most of the Arctic – which means very little sunlight is reaching the water in the Arctic Basin. As a result melt will occur more slowly than during low concentration years.

The videos below represent an exaggerated visualization of the process. The first video shows an idealized view of future Arctic Basin melt during 2010 – i.e. a single large circle of ice surrounded by water.

The next video shows what happens in years when the concentration is lower. The sun is heating the water between circles, and because of the smaller circles a much larger surface area of ice is exposed to warm water. Warmer water and more exposed surface area causes melt to proceed faster.

Conclusion : Cold temperatures, cloudy skies, favorable winds and high concentration ice – all point to continued slow melt over the next few days.