"Things You Wouldn't Know If We Didn't Blog Intermittently."
It doesn't have to go to the beginning of time... how about to the beginning of modern record keeping? 1979 seems like a mighty arbitrary year. I seem to remember that was about when we were being told to prepare for the 'coming ice age'. If the graph showed from 1950 to present how would it be diferent?
This is probably data from the PIOMAS model, which only goes back into the 1970s. Other data sources do provide measurements from farther back in time. http://www.realclimate.org/index.php/archives/2012/04/arctic-sea-ice-volume-piomas-prediction-and-the-perils-of-extrapolation/
Good thing the ice has been melting for the past 20,000 years or so.What was it like 500 years ago, 1000, 3000, 10000, 25000?If that yellow line trends up for the next 3 or 4 years the whole graph from 79 to now (whenever that is) is going to be far less dramatic.
Hmm.. this data doesn't seem to match other data sets. For a quick comparison I went to the University of University of Illinois Sea Ice Dataset for the Northern Hemisphere 1870-2011 at http://arctic.atmos.uiuc.edu/SEAICE/ and plotted a quick comparison of quarterly averages from 1979 and beyond. UofI provides the data in a text file, and it was pretty trival to copy it into an Excel file and get a plot of quarterly averages. The data seems a bit different.. For example, in 2000 from the above chart, it appears the monthly average for extent of sea ice plotted is somewhere between 6-7 million km^2. However, if you look at the full data set from the University of Illinois they show the quarterly average is between 8-15 M million km^2. That's a big difference!!There's a plot of this data on the UoI page at http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/seasonal.extent.1900-2010.png -- so my math seems to check with this data set. Hmm... That's a big difference in the data.. Almost 100% difference in the amount of sea ice included in the analysis. Why? I suspect the authors of the chart only pulled the data from part of the Arctic region, and not the full Arctic ice pack data.
Seems that you might also include an Antarctic Sea Ice chart for comparison.