The problem with Irreversible Global Warming (particularly the -> desert style) then it is something that you can't have in large amounts so that makes it a very rare mechanic. and very Rare mechanics are bad because they have to be very powerful to be significant. I could see a situation where you had "permanent" changes on tiles ie Desert<->Plains<->Grassland where there was a slow level of random change and Global Pollution tended to push it in one direction. (you could also have certain (biological) resources get depleted and possibly rediscovered/reintroduced. (similar to the way mineral resources are)... Global Pollution would drive that mechanic in the direction of Less resources. The ideal is that 1. whatever change Global Warming does it should be a change that through chance or time (and nothing else) should possibly reverse 2. It should be common 3. It should be weak (this is balanced by the commonness of it 4. It should be connected to overall polluting practices of the civs 5. either Environmentalism or Ecology would be required in the Long term to prevent it cousing significant problems (so once you get Medicine+Assembly Line, its either go Environmental... push through to Ecology... or just get the Win (the short term problems it would couse would be much less than the long term problems it would cause) 6. effectively totally avoidable+repairable... ie now matter how bad it has gotten, if you take the right actions it will EVENTUALLY (Very Long term) eventually correct itself.