Humans are too frail for the Earth
Chixulub demonstrated that adequately.
At some point, we must grasp this nettle and progress beyond our cradle on this pale blue dot, or it will become our grave.
The factors in the article Raul linked to are reasonably well-known, though some of the information about stability of medicines was news to me. The solutions may be difficult, expensive, and slow, but they exist or can be imagined. We just need to be launching kilotonnes into space on a regular basis.
The bottom line, is that we must find ways round these problems, or we will know the answer to the Fermi Paradox