Champagne has about the same alcohol content as other French white wines, but its alcohol becomes effective more quickly because the dissolved carbon dioxide in the wine goes straight to your bloodstream. Your circulation reacts by speeding up, just as it does when you are running, to exchange carbon dioxide for oxygen. So a faster bloodstream carries the alcohol around your system. You giggle sooner, but the effects pass off more rapidly.
From How To Enjoy Your Wine by Hugh Johnson (1985)