Suppose the panic immediately before and during the cataclysm caused the already heightened tensions between China and the US to develop into full scale nuclear war. And thus, as a result of this nuclear war, the entire world’s atmosphere has been clogged and choked by trillions of tons of dust, debris, dirt, ash, and smoke that blots out the sun - causing whats known as a nuclear winter.
Nuclear winters at their worst have the capacity to reduce the average temperature of an area by about -40 centigrade for a period of several years. Basically, this would be a mode that causes winters to be extremely, severely cold while all the other seasons are just winter under regular circumstances with varying degrees of severity. Snow all year round, temperatures always below freezing.
On top of this, it is never EVER sunny, and vision radius is reduced due to there being less sunlight penetrating through the thick layers of ash above. Tough luck if you rely on solar panels for your cars, kitchen appliances, or CBM power. Also, as a result of the cold and lack of sunlight, plants of all kind die in droves. You can’t plant anything for agriculture, triffids / fungi quickly die, foraging underbrush for food becomes next to impossible.
It could also introduce a few new weathers: Grey snow and Black rain. Basically, snow or rain that has been heavily contaminated by fallout and which causes small amounts of radiation exposure for those caught within it without protection. It would also have the chance of randomly contaminating (outdoor) tiles with radiation to simulate the effects of fallout clouds spreading throughout the world. This would also give actual reason to hunker down underground in LMOE shelters or basements, on top of underground being (in this case) substantially warmer than above-ground.
If possible to code, this would likely result in all above-ground sources of water like lakes, swamps, and rivers being contaminated and unusable (as well as killing any fish in them). In such a mode, anything but groundwater would be “irradiated water” that cannot be boiled into clean water, but can be purified with a charcoal purifier.
Of course, because this game takes place in the countryside/suburbs of New England, no cities are directly hit by the bombs. They are spared total destruction, but not spared the effects the bombs have on the climate, nor the fallout.
