Deconstructing Earth

In order to simplify the setting in the Sol Frontiers project I 'D like to write in a cataclysm that lets us take present real world nations out of the picture and allows for new nations on the Moon or Mars to be more dominant in the setting. There seems to be a lot of information on the net for what happens if an asteroid strikes. I'd go with asteroid or comet but would include some other stuff like limited nukes like north Korea initiates war believing the need to go south to escape coming ice age. US aegis systems knock down most Korean missiles but one gets through and takes out San Fransisco. Toss in resulting world wide fires and a good old fashioned mega volcano eruption to plunge earth into a new job ce age.