I’ve got a question for readers: can someone explain to me how the NSIDC sea ice projections were actually made? I can’t get from A to B. Relevant information is in their monthly reports for April 2008 and May 2008.
Remember how much we’ve heard about new ice and why this was expected to lead to 2008 ice meltback surpassing the 2007 record. Let’s try to follow their projection method step by step,
First, we start with March ice, which is the yearly maximum. They say:
Average sea ice extent for March 2008 was 15.2 million square kilometers.
Next we learn:
Figures 4 and 5 indicate that relatively thin, first-year ice now covers 72% of the Arctic Basin, including the region around the North Pole; in 2007, that number was 59%.
By simple multiplication, the area of new ice in March was 10.94 million km^2 (old ice: 4.26 million km^2).
Next we learn:
about 30% of first-year ice typically survives the summer melt season, while 75% of the older ice survives
Their projection using average survival rates is reported as follows:
If we apply the survival rates averaged over all years to current conditions, the end-of-summer extent would be 3.59 million square kilometers.
Can anyone explain this to me? If I multiply opening new ice (10.94 MM sq km) by 30%, I get 3.28 MM sq km; and multiplying opening old ice (4.26 MM sq km) by 75%, I get another 3.19 MM sq km, for a total of 6.48 million sq km – a figure which, by the way, looks like a pretty good estimate to me of where we might end up this year. If you inadvertently divide the surviving old ice by 10, you get something pretty close to 3.59 million sq km ( 3.28 + 3.19/10).
I don’t pretend to be familiar with these calculations and I could easily have been wrongfooted somewhere along the way. So if anyone can figure the calculation out, I’d appreciate it. BTW wouldn’t it be nice to be able to simply look at an archived script and be able to see for yourself what they did?