The National Snow and Ice Data Center is changing the way they display Arctic Ice information graphically. For years they have used the period from 1979 to 2000 as a base data set. They are increasing the period to 30 years ….as the site states:
Until now, we have used the 22-year period 1979 to 2000 when comparing current sea ice extent to past conditions. When NSIDC first began to monitor and analyze sea ice extent, a longer period was not available. Since the satellite record is now extended, we are choosing to move to a more standard 30-year reference period, from 1981 to 2010.
A 30-year period typically defines a climatology (comparsion period) and is the standard used by organizations such as the World Meteorological Organization (WMO) and the U.S. National Oceanic and Atmospheric Administration (NOAA). Thirty years is considered long enough to average out most variability from year to year, but short enough so that longer-term climate trends are not obscured.
I was surprised by the explanation. 30 years does not, in my view, average out most variability from year to year, the time is way too short. An easy way to demonstrate this is to look a the climate history since 1850, courtesy of East Anglia University in the UK.
Let’s start a 30 year window in 1878 and end in in 1907. Now compare that to one that starts in 1908. Consecutive 30 year data sets display completely different trends The first set shows .4 degree C of cooling in 30 years while the second set shows an equivalent amount of warming.
If we had started in 1850 the data would have shown a warming trend, 1860 somethings else. 1911 is a particularly good place to start if I want to show lots of natural warming. If we assume that most of the warming since 1950 is man caused, then the data from 1976 to 2006 looks spectacular. But it also looks a lot like the data from 1910 to 1940.
I’m glad the National Snow and Ice Data Center is using a longer time period, but I don’t think it accomplishes the things they say it does. They are matching a standard others use…but the standard is of little real value.