At the moment, the environment in Civ games is essentially indestructible. You can farm, mine and colonise every inch of the planet with no ill-effects, there's no limited carrying capacities, no soil-exhaustion, no resource degradation and no societal collapses from overexploitation. This is not very accurate because all through human history, the fates of different civilisations have been inherently and inescapably tied to their material surroundings and how they exploit them. In Civ IV you transcend nature, you don't learn to live within it. Perhaps this just reflects the hubris of our age, but either way, this subordination is the place of the natural environment in all Civilisation games to date.
That's why we have static terrains... the implications of dynamic terrains are far-reaching for what sort of an idea of human civilisation and human progress the game contains.
Thing is, however unrealistic this non-environmental narrative of history and development is, I'm not sure a Civilization game based on the (far more accurate) environmentalist/materialist narrative of history would be very fun compared to the essentially idealistic and technology-centric view of human societies and human progress Civilization puts forth (mankind pulling itself up through its own ingenuity rather than because of mundane anthropological and biological reasons). It might be more realistic to have land clearing and over-exploitation undermine you in the long-run but that doesn't sound fun, and if we're going to go that route Civ would need a deeper rethink than just counting farms and cottages and throwing tile-changes at you.
. Basically just move a mass of land down a square and fill the gap with water or something.
