[giantg2]

You should probably look at some prepper forums.

Best guess is someplace remote, decent soil, good supply of water, somewhat mountainous (CO2 is heavier than O2 although probably negligible, lower temperatures higher up, better chance of rainfall if weather patterns shift and you are located between 2 bodies of water), and not too far south but also not too far north (not sure how fast ocean currents would slow and what affect that would have in each location). Probably best to find a small group of trustworthy and like minded individuals/neighbors to plan with.