Cogitator, the sun is the sine qua non for earth's climate (and for all life on earth to exist). To claim, therefore, that the sun is only responsible for 10 to 30% of earth's climate variability (as you and Hathaway do) brings me to...
Definition Two — Folly: a lack of understanding; foolish belief.
The realization that you don't understand. If the Sun didn't vary, it wouldn't cause climate variability. Under that circumstance, climate variability would be solely due to factors internal to the climate system.
Howver, because the Sun varies a little, a small part of the observed total climate variability is attributable to the variability of the incoming solar radiation.
It's not that hard a concept. And it's not a foolish belief.
Hathaway’s explanation of variability.
I suspect he is comparing the variability with a base line of sun influence. Kind of like if a pot of water is on simmer, the water is 112 degrees farenheit, but if you turn up the heat you get a rolling boil which is a little hotter. If you consider the sun’s base line is a simmer, then the variability is anything more than that.