To make that mod realistic, you shouldn't have oil just run out, because that's not what happens. When you hit the peak, the rate of production declines thereafter year by year, but production doesn't actually stop. The U.S. domestic production hit peak in 1970, but the U.S. is still producing oil, just not as much of it as we once did.
In game mechanics, what you could do is have peak oil announced, kind of like a random event, and the player could choose to invest in countermeasures. You could have a new building, call it Improved Infrastructure or something like that, which greatly improves energy efficiency and switches to alternative power. Make it cost half the hammers under Environmentalism, maybe. Any city that doesn't have it will suffer something like:
1) A surtax on gold production that increases over time, representing the cost to buy increasingly-expensive and scarce oil;
2) Declines in productivity of farms, mines, towns, etc. reducing them to pre-Biology, pre-Railroad, or just generally sucky level, these declines not happening all at once but also getting worse over time;
3) An increase in the support costs for all oil-based military units that grows over time; and/or
3) Increased unhealthiness/unhappiness because of the massive inflation caused by zooming fuel prices.
As for global warming, you could simulate that by:
1) Rising sea levels affecting coastal cities, requiring them to build expensive sea walls or other things to keep from disappearing. (Probably have to run Universal Suffrage and rush them in many cases.)
2) Farm failures as climate change makes it impossible to grow the same stuff in the same places. The failure would last for a certain number of turns until farmers learn how to grow new stuff (this could maybe be rushed with an outlay of gold). It would also be a recurring problem unless/until the warming was halted.
3) Increased storm activity, mainly affecting coastal cities again. Treat it like a random event, but with an increasing chance of happening.
4) An epidemic random event with a chance increasing over time.
The obvious way to fight global warming in-game is to pass the UN resolution requiring Environmentalism as a global civic. That cuts the rate in half, maybe. Then there might be a further reduction for every coal plant replaced by either hydro or nuclear, and for every city that builds Improved Infrastructure (see above).
Then we get into overpopulation and new plagues that try to compensate for it . . . and what about poor civs that don't have the tech to do any of this stuff?
It's getting a little too real in here . . .
