The Nitrogen Fix

By Hal Clement

Something terrible happened in the past. Somehow, Earth's atmosphere has lost most of its oxygen and life on Earth has been devastated. Now the only animal life left are a few struggling settlements of humans. There is plant life, of a sort, and people are able to survive by maintaining greenhouses full of crops and plants that provide food and oxygen. But life is very harsh and life spans greatly shortened.
Kahvi and Earrin are nomadic traders, traveling by raft to and from seaside communities. They have a young daughter and Kahvi is pregnant. They have a small greenhouse in which they sleep and recharge their oxygen equipment. They also have a traveling companion, Bones.
Although Kahvi and Earrin are sure they are the aliens and Bones is the native, the reverse is the actual truth. Bones is an observer from another star system, on Earth to study its native life and atmosphere.
A group of young scientists from the former city of Boston has developed the hypothesis that Bones and his fellow aliens on Earth are the ones responsible for the destruction of the oxygen as part of a plot to seize the world for themselves. To that end, they have captured a small alien and are now attempting to capture Bones. They don't know that the aliens are mere observers and are on Earth just to gather information. But Kahvi and Earrin have known Bones for years and trust the alien, even leaving it to watch over their young daughter.
It all comes to a head when the radicals take not only Bones but Earrin too and Kahvi has to try to save both her husband and Bones from the well-meaning but mistaken young scientists.

I liked this book. It paints a picture of a very different world in which people are barely managing to hang on, a real survivor tale, with the added interest of strange, peculiar aliens. The humans have made science a forbidden subject, blaming it for the collapse of the atmosphere. The young scientists in the story are revolutionaries, going against the established wisdom in order to attempt to save their damaged world. And even though they are the "bad guys," at least they are trying to do something, unlike their elders. Very interesting and exciting story.