To the best of my knowledge in Civ 4 the effects were all negative and to be actively avoided (unless for some particular reason you really wanted a tile to become desert). However, if your example were implemented, you might get the interesting phenomenon of the equatorial climes become less habitable (e.g. plains to desert would decrease food yield) while the northerly/southern regions become more habitable (e.g. tundra to plains would increase production). If widespread enough, that could cause a major change in fortunes for the civs involved. I think that could make for a great end game, as formerly fertile equaltorial civs, facing famine etc. look elsewhere, and northerly/southern civs start getting some "unwelcome visitors": after all, competition for resources has been a major driver of war, and no resource is as valuable as good land (people have to live somewhere that can support them). Edit: Didn't see Murky's post (2 previously) when I wrote this, and I'll bow to his/her greater knowledge of the ecological factors!