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.