DEATH
In Ben Bova's novel New Earth, Jordan Kell led the first human mission beyond the solar system.  There they discovered only devastation: the ruins of an ancient alien civilization.  But one alien Al survived, and it revealed to Jordan Kell that an explosion in the black hole at the heart of the Milky Way galaxy has created a wave of deadly radiation, expanding out from the core toward Earth.  Unless the human race acts to save itself, all life on Earth will be wiped out.

Kell and his team return to Earth to carry the warning.  But they arrive centuries after their departure and find that their world has changed almost beyond recognition.  Not only has a second wave of greenhouse flooding caused sea levels to rise, but society has been changed by the consequences of the climate shift.  Few people want to face Jordan Kell's news.  He must convince Earth's new rulers that the human race is in danger of extinction unless it acts to forestall the death wave coming from the galaxy's heart.
WAVE