Thanks to BilllaurelMD who has agreed to crosspost his weekly update on Arctic sea ice here at Climaticide Chronicles. [JR]
During the past week, the Arctic sea ice melt has continued, but at an accelerated pace, as covered by Kossack, JohnnyRook, in this diary. In that post, a National Sea Ice Data Center (NSIDC) news release was mentioned; it was titled “Race between waning sunlight and thin ice”. More on that after the jump.
Before we talk about the changes from 3 August 2008, here’s today’s webcam view from the North Pole, from 13:59 and some seconds universal time (UT) today, 10 August 2008.
The Arctic Sea Ice Concentration graphic for today appears below.
“Race between waning sunlight and thin ice”
What was the NSIDC talking about? Basically, they’re referring to the decreasing solar angle above the horizon as the sun slowly descends as it circles in the sky toward its long twilight at the North Pole. As this is happening of course, the South Pole awaits the long dawn; these both come at the autumnal equinox, September 22 at 21:29 UT this year.
Surface Energy Balance and Albedo
The decreasing angle of the sun means the same amount of solar energy is spread out over a larger area, leaving less per unit area available for, say, melting some ice. Depending on the condition of the surface (and the resulting albedo, that is, the ratio of solar energy striking the surface absorbed to that *reflected* to space) and the amount of cloudiness (which radiates energy back down to the surface based on *its* temperature), the net energy at the surface of the North Pole may go negative (and the surface begin cooling) *before* the sun sets (the earth emits infrared radiation at a rate based on the surface temperature to the 4th power. NOTE: Incoming energy also is used for melting ice and snow, evaporation of water from liquid and/or ice surfaces (known as sublimation), sensible heating of the water and/or ice (that is, changing their temperature), and through turbulent eddy transfer, heating the air above the surface.
The net cooling begins earlier if there is ice and snow at the surface (lots of solar radiation being reflected away rather than absorbed), rather than large areas of water absorbing almost all of what solar energy exists. This includes melt ponds like the one in the web cam picture above.
Also, as we know, thin ice needs less energy to melt; and water rather than ice and snow at the surface absorbs more solar radiation, increasing the amount of energy available. The positive feedback that results from reduced sea ice volume (not just reduced area, but reduced area *and* thickness) becomes obvious.
This week’s sea ice concentrations compared to last week and last year
The first row gives today’s sea ice concentrations for this year and last year; the bottom row gives last week’s data for the same years. Again there are large areas of relatively low concentration, and the areas of high concentrations of sea ice (say 80% or more) has decreased considerably. At least qualitatively, the area covered with high concentrations looks to be less than in 2007 at this time.
|8/10/2008 sea ice extent||sea ice concentration||8/10/2007 sea ice extent|
|8/03/08 sea ice extent||8/03/07 sea ice extent|
|same color code as above|
Concentrations of *loose* sea ice (which is what we find at this time of year) depend on winds as well as the amount of solar energy available and the ambient temperature. All else being equal, converging winds will increase sea ice concentrations, diverging winds decrease them.
Comparison to long term 1953-1991 climatology
The climatology for sea ice concentration for 10 August is shown below, for the period from 1953-1991. Ice data before 1972 is less available (1972 onward is from satellite, and is very good after 1979). The color code for concentration is the same as above. Note that this climatology is earlier than for the areal sea ice coverage (which is from 1979-2000), so it’s not directly comparable, though evidence seems to show that ice coverage didn’t change much from the 1950s through the 1970s.
Areas that are dark blue in the climatology graphic can be assumed to have never been free of ice in the 1953-1991 period. It’s clear than many areas free of ice now had never been free of ice during that period, including much of the area north of AK and some of the channels in the Arctic Archipelago (north of Canada, wherein lies the so-called Northwest Passage, which opened up last year).
Areas to watch for sea ice loss
If you go back to previous weeks, the losses of sea ice cover (15% or greater concentration per 25 km2) seem to at least loosely correspond with areas that previously had relatively low concentrations. Using that as a rule of thumb for now, the areas highlighted in the graphic below should be watched for rapid decreases.