From the Wikipedia:
Any impact would be extremely detrimental to an area of thousands of square kilometres, but would be unlikely to have long-lasting global effects, such as the initiation of an impact winter.
Also, this. Looks like Russia has a lot more at stake then America does. Looking at that picture the only downside for the US is that there will be no more poverty in central America, drug trafficking from south Amerika just got al lot harder and US finally get that wider panama canal they'v always wanted. In other words: what's holding up that asteroid!