hmm if the yellowstone caldera explodes, yes there might be a HUGE setback for the country and we might see our capital be moved down to the southernmost reaches of the 50 states excluding hawai'i. all this global warming issue concerns the whole world equally so that shouldnt be needed to be addressed as it isnt anything confined to our local country.
one other thing the US will have a trouble facing in the future will be water supply. no the earth is not running out of water. there is j ust more people that is using it but the amount of water on earth generally stay the same. fresh water supply will not be coming from the ground or mountains anymore in the future. ground water comes from water in the mountains that got seeped into the ground. the amount used is severely over the amount that gets replenished each year thus making ground water supply thin out at an alarming rate. the water from rivers also come from mountains and the problem here isnt because of people from many cities pulling water for themselves but rather the source. California's biggest assets are the snow capped mountains that borders nevada. there is less and less snow each year due to obvious reasons. less snow = less water that will run down the mountain = water crisis.
in order for the water problem be solved around the US, there must be tons and i mean like numbering in the hundred thousands of desalination plants along the coast of this country if the US wants fresh water to go to all its lands. plus there is another plus to desalinating water via reverse osmosis: it taste hell of a lot better than drinking river water. i live in San Diego and godamnit people in sin city dumps sewage and shit into the Colarado river and that water tastes like crap.