As far as geo-engineering goes, I think what they'd be wanting to do is cause a super-volcano or three. You really don't want to be pulling asteroids down - just a couple of finely place nukes will do. Get a volcano going, put a few tons of sulphur in the air and you have a nice cooling effect. Just be careful of the unintended tsunami. Should probably just evacuate Iceland and keep popping the volcanoes until it's cool again.