From back of book: Five hundred years in the future, humanity is no longer confined to Earth. Colonies have been established on the inner planets and the Moon. But even as mankind spreads farther out among the stars, another species has taken an interest in the mother planet....

The alien Tarsalans have come to Earth, and they want to stay. After negotiations between the aliens and Earth's rulers go horribly wrong, a mysterious green sphere begins forming around the Earth--one that prevents all sunlight from reaching the planet below. Temperatures cool. Plant life and crops wither and die. Worldwide famine sweeps the globe. Anarchy reigns as political systems fall, one after another.

Only two scientists--one isolated on the Moon's colony, the other trapped on a dying Earth--possess the minds and the means to discover the nature of the alien phytosphere. If they cannot find a way to destroy it in time, there may be nothing on Earth left alive....

(Source: Roc)