Global warming - or rather anthropogenic climate change - has been a significant issue throughout the history of civilization, even if it has only become popularised in relatively recent times. However human-induced climate change and pollution in general have not been adequately represented in a civ game since at least civ3, and even then it was an unrealistic, simplistic, whack-a-mole style nuisance. What I would like to see in a civ game is a much more detailed and fleshed out pollution mechanic. This would mean including not just pollution muck on squares and tiles turning to desert, but other effects such as:
-Rivers and lakes drying up/ being depleted through excessive irrigation
-soil degredation due to agriculture (salinity, erosion, silting etc)
-polluted waterways (rivers, lakes and ocean)
-spreading Civ4-style fallout from nuclear wars and meltdowns
-nuclear winter effects e.g. tiles turning to desert and tundra
-desertification due to excessive agriculture, depeleted waterways, and deforestation I still cant get over the fact that in civ4 you could chop every forest AND jungle (!) in sight and it would have no side effects on climate whatsoever.
-FFH Armageddon Counter-style environmental damage/climate change meter, so that climate change and other pollution events get more severe and frequent as the counter increases
-human-induced climate change turning tiles not just to desert, but jungles and forests to plains, coastal grassland squares to swamps, tundra to grassland, and even low lying coastal tiles to ocean tiles (including drowning cities).
-slums, a kind of pollution that results from rapid urban population growth and low levels of human welfare could spread in villages and towns (or trading posts if you really must) surrounding cities.
To make all this work properly, it would probably be necessary to have a SMAC-style weather and climate system in the game, including dynamic global climates which gradually got warmer or colder over time (depending on what the player chooses at the start of the game).
Perhaps there could also be a new UN resolution for a "Global Climate Protection Agreement" - reduces pollution worldwide but also reduces every civ's production and gold by 10-20% for x turns.