Plot run through.... remember this is a very rough outine!

How about having a Fracking mishap ( the process of injecting liquid at high pressure into subterranean rocks, boreholes, etc., so as to force open existing fissures and extract oil or gas. 
This will cause massive earthquakes in the US and extending out to other countries. 

Meanwhile, a radioactive asteroid shower lands which causes a giant heat wave and makes everyone miserable, cranky and short tempered.

Somewhere in the world, a team of mad scientists creates a virus, makes large batches of this virus which are placed in hot air balloons. When the hot air balloons, land, they explode to release the virus to those around the area.

All of the countries blame each other for sending the virus which will result in a nuclear war.

The nuclear bombs are released causing mutations as well as causing the Earth to change on it's axis. 

The change in the Earth's axis changes the weather patterns that brings on the Ice Age.