@Dan:
Nonsense.
The Chicxulub crater, caused by the asteroid impact that led to the Cretaceous extinction, released an energy of about 500 million Megatons, which is about 80,000 times the energy of all the nuclear weapons in the world (about 6,000 Megatons).
A nuclear catastrophe might very well wipe out humanity, but ALL life on Earth? Hardly. The Earth has survived much, much worse in the past.