Evaluating common descent requires evaluating the morphology, genome, and reproductive behavior of every extremely distinctive group of species, or of a great many. You don't need to look at each individual species, but you at least need to rule out convergent evolution and (late) lateral gene transfer as adequate explanations of homology. (And, OK, aliens.) How many LessWrongers have put in that legwork?

Evaluating the age of the universe requires at least a healthy understanding of contemporary physics in general, and of cosmology. The difficulty isn't just understanding why people think the universe is that old, but having a general enough understanding to independently conclude that alternative models are not correct.

That's a very basic sketch of why I'd be surprised if LessWrongers could better justify those two claims than the mere claim that global temperatures have been rising (which has been in the news a fair amount, and can be confirmed in a few seconds on the Internet) and a decent assessment of the plausibility of carbon emissions as a physical mechanism. Some scientific knowledge will be required, but not of the holistic 'almost all of biology' or 'almost all of physics' sort indicated above, I believe.