On breaking up an asteroid, there was this discussion back in 2010:
In which I wrote the following:
…from Grey’s BOE calcs, one can see that when dealing with a small asteroid (on the order of 100 meters diameter), breaking it up prior to impact may be beneficial. The total atmospheric temperature rise due to burnup is negligible, and we would save ourselves from an impact that could kick up climate-changing amounts of debris into the atmosphere. When dealing with a much larger one (on the order of 10 km diameter), we’re just about screwed either way: this embodies energies on the order of 15 million megatons (the largest nuclear device ever detonated was around 50 megatons). A surface impact would cause ridiculous seismic events and kick up life-extinguishing amounts of debris; if instead we could somehow shatter a meteor that large prior to impact, the atmospheric temperature rise would be on par with the worst clime-change forecasts, except it would happen literally overnight.
Move from a 10km-diameter bolide to a 20km one, and the energy (and therefore the temperature rise from atmospheric burnup) increases by a factor of 8, to almost 100 Kelvins. Anything not living in the ocean would be dead in fairly short order, and I imagine the eventual increase in temps of the oceans’ surface layers would upset the marine ecology so much that life throughout the oceans might eventually die off completely.