Dark of the Sun by C Q Yarbro
The explosion triggers Tsunamis that destroy ships and seaports. Airborne ash causes crop failures. During the bleak, seemingly endless winter that follows, people blame gods and devils for their troubles. The vampire Saint-Germain is unwilling to feed on the starving and knows that women who fear the supernatural do not welcome vampire lovers. Can a vampire starve to death? Or will his true nature attract a darker fate before he reaches sanctuary in Transylvania?