The Nitrogen Fix, by Hal Clement

Rating: 3
Pages: 289

It is the far future, and Earth's atmosphere has changed. There is no free oxygen. The only living creatures left breathe nitrogen, except for a few small communities of people and their jealously-guarded oxygen producing plants. People live in enclosed greenhouses with their plants, and only venture outside with breathing masks and oxygen tanks.

Two wandering gatherers, Kahvi and Earrin, live at sea on a raft. They are friends with Bones, one of the large nitrogen-breathing natives of Earth. This friendship creates friction whenever Kahvi and Earrin visit cities to sell the glass and copper they've gathered; the city folk dislike the natives.

When making one large delivery of scavenged glass, Kahvi and Earrin are attacked by a radical group who wish to experiment on Bones. They think that Earth once had an atmosphere with free oxygen, and believe that Bones and his species are extraterrestrials who are responsible for changing the atmosphere.

Clement paints a nice picture of people surviving in a nitrogen atmosphere. Their existence is tenuous and fragile. A cracked dome, a fire, a plant disease, are all that stand between life and death. There is no doubt that mankind will soon be extinct.

Then there is the mystery of Bones. He is not a native of Earth, and is indeed an extraterrestrial. But what is he doing on Earth? He considers himself an Observer, charged with gathering as much information as possible about Earth. He is most interested in Earth's changed atmosphere. But his purpose on Earth is unknown.