Clean Air Act
I don't know about ice cores, but there is a plausible theory out there that states that a period of recent cooling that seems to defy the industrialization-caused warming trend may have been caused by increased industrial particulates blocking sunlight, and that the Clean Air Act may have neutralized these, causing another upward warming trend.
I haven't seen Gore's movie, and won't, but I suspect this is what he's talking about, or trying to.
Of course, this implies that the sunlight level is one of the causes of warming, but that's another thread.