Couldn't positive feedbacks account for late 20th century warming?

As most of you probably know, the rise in solar irradience correlates very nicely with the warming we had from 1900 to 1940, and indeed many scientists feel it was responsible for the vast majority of the warming during this time: http://www.mps.mpg.de/images/projekte/sun-climate/climate.gif So how is it not... show more As most of you probably know, the rise in solar irradience correlates very nicely with the warming we had from 1900 to 1940, and indeed many scientists feel it was responsible for the vast majority of the warming during this time:
http://www.mps.mpg.de/images/projekte/su...

So how is it not possible that this increase in temperatures indirectly spawned positive feedbacks (water vapor, ice albedo, oceanic CO2 release, PDO/ENSO...etc) which accounted for most of the warming of 1970-today?
Update: Thanks Capn T...that report is about 'sunspot cycle length' though, which is different from total solar irradiance.
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