Technically its easy to prevent an asteroid impact. Its the money that gets us at this point. Given enough lead time, turning it a fraction of a degree or slowing it a by a few inches per hour is all it takes.
I personally think a gravitational tractor is the way to go.
I'm all in favor of recent budget increases for the Near Earth Object(NEO) tracking program. An estimated 10% of NEO's are currently unaccounted for. That's too big a margin for catastrophic error. We have the technology and brainpower to do much better.
Being clobbered by an asteroid again is a 100% certainty at some future time. So is so-called "climate change." The difference is we may be able to prevent an asteroid hit while climate change requires adaptation, not tax increases and international wealth redistribution.