As you can see, the current melting is far below any melting episode of the 1920s and 1940s. The historical data shows that the Arctic melting really started in the 1950s, disproving the relatively common claim that 1979 was a peak year for Arctic sea ice. The unusual nature of the current melting is further demonstrated by comparing ice extent maps today with monthly ice maps compiled by the Danish Meteorological Institute (DMI) between 1893 and 1956.
Here's the July 1938 DMI map compared with satellite map of sea ice extent on July 26, 2013 from the NSIDC:
|July 1938 Arctic sea ice|
|July 26, 2013 sea ice extent from NSIDC|
Extending the comparison further into the past, here's the main results from Kinnard et al. (2011), which reconstructed Arctic sea ice extent back 1,450 years:
|Graph from http://www.skepticalscience.com|
The main message: Not only is the recent Arctic melting unprecedented over the past 143 years, it's unprecedented over at least the past 1,450 years, a time which includes the Medieval Warm Period. And there's no way this is any sort of natural cycle.