MAY 1980 vs. MAY 2008 SEA ICE CONCENTRATION

NATIONAL SNOW AND ICE DATA CENTER

PERSONAL CORRESPONDENCE; REPRINTED BY THE WEATHER CHANNEL WITH PERMISSION

First a prelude on the difference between "area" and "extent.”

"Area" basically sums the actual area covered by any sea ice. Think of

a slice of Swiss cheese - for "area," you'd only count the cheese part

and not the holes.

"Extent" sums up the total area covered by at least 15% sea ice. In

this case, you're counting the entire slice of Swiss cheese, holes and

all - as long as the cheese part comprises at least 15% of the slice.

There's a more thorough explanation here.

Now, the next thing is that two different types of satellite sensors

have been used over the period from 1979-present. The older sensors,

the Scanning Multichannel Microwave Radiometer (SMMR) flew from

1979-1987, when it was replaced by the Special Sensor Microwave/Imager

(SSM/I). These satellites have very similar characteristics and the

data between them are consistent, except in one way.

Satellite sensors are not always able to obtain data everywhere on

earth, due to limitations in the sensors and the satellite orbit. One

common area where data cannot be obtained is around the poles, commonly

called the "pole hole". This is the case for both SMMR and SSM/I.

However, the area of the pole hole is much larger in SMMR than in

SSM/I. This can easily been seen in the grey circular area in images

with the May 1980 (SMMR) pole hole much larger than the May 2008 (SSM/I)

pole hole. See the left side here.

Now in the "Total area" values of those left side images, these both

refer to the area of ice that the satellite can "see" - i.e., it doesn't

include any ice within the pole holes, even though we know there is ice

there (but we don't know how much, so we can't put a number to it). In

SMMR this area is 1.19 million sq km, while in SSM/I it is 0.31 million

sq km. Thus, SSM/I potentially "sees" 0.88 million sq km more possible

ice that SMMR does not. Of course, that full 0.88 million sq km is only

realized if the area is all 100%, but that close to the North Pole,

there is close to 100% ice (90-95% is a reasonable number - e.g., ~0.8

million sq km).

Thus, any SSM/I area value can be expected to be on the order of 0.8

million sq km higher relative to SMMR. Thus, though the given value for

the area for May 1980 and May 2008 is 10.9 million sq km, this

equivalence is an effect of the different size pole holes. If a similar

size pole hole existed in 1980 as currently, the May 1980 value would be

somewhere around 11.7 million sq km. Thus, you cannot legitimately

intercompare the total area values between SMMR years and SSM/I years,

as has been done by the Global Warming Hoax site.

Now, for the anomaly calculation, obviously, you can't do a proper

anomaly with two different sized pole holes. So, the larger SMMR pole

is applied to calculate the monthly anomaly for both SMMR and SSM/I

periods. This can be seen (perhaps with some difficulty) in the right

side of the above linked images, where the white (no change) area around

the pole, due to the pole hole, is the same size in both images.

Because we're now comparing the same potential region, the anomaly

values are consistent. Thus the anomaly values are a more

representative comparison and indeed May 2008 has less ice than May 1980.

This difficulty in looking at area is one reason why NSIDC consistently

focuses on "extent" when discussing long-term trends or variability with

the public. (We distribute the area fields as well because there is

potential scientific value if used properly). Now, you might think that

extent doesn't really help because there is still the pole hole.

However, while we can't say how much ice there is within the pole hole,

we can be completely confident that locations that near to the pole,

there is at least 15% ice over the entire pole hole, whether it be the

smaller SSM/I hole or the larger SMMR hole. Thus, we can consider the

entire hole in both as "ice-covered" and count that region as part of

the total sea ice extent. This means that the total potential

ice-covered area is the same for both SMMR and SSM/I - namely the entire

Arctic all the way to the pole - and whatever extent values there are

for any given month or year are completely consistent and can

legitimately intercompared.

That is why if you look at the extent fields, you see that May 1980 had

an extent of 14.0 million sq km, while May 2008's extent was 13.2

million sq km of ice, as seen in the left-side here.

Thus we were definitely not back to 1980 conditions this past May.