Things get warmer, but then if the Gulf Stream disappears, things will also get colder. And then you can argue what causes it. The world is heating up, but then it does from time to time. The hole in the ozone layer is not talked about so much now. While CFCs are banned among some nations, there are other ozone destroying gases, and anyway, developing nations may not be able to use alternatives.
Global winter is another way of getting cold.
This is caused by complete cloud cover over the Eartyh for several years. This gets mentioned in mass extinction theories quite a lot. We could get this through super volcanoes, asteroid collision, or really serious nuclear war. We also have minor weather problems, such as smog caused by traffic, or forest fires, or increased warmth within cities, or climate change when forests get cut down. Disease Disease is one of the original Horsemen of the Apocalypse.