This book is divided into 6 parts, spanning 38 years. It's set in the near future (at the time of writing) and describes what could happen if mankind fails to take action and continues to upset the natural balance of the planet. In particular, the book discusses in some detail what would happen if the algae within the oceans is killed off, reducing the composition of the air we breathe (there would be less oxygen).
This environmental message is told in a fairly accessible way. Although some specialised scientific issues are discussed between characters, it's done in a manner that most people can understand without having to follow all the technical details.
The story follows a group of people over the years as they attempt to warn the relevant authorities about the possibility of oxygen depletion, and then describes how they, and society, deal with it as the situation becomes increasing worse. The story itself is fairly well thought out but it's hard to say what type of book this is. In some respects it's a work of fiction, but based on fact, some of it reads like a thriller, but it becomes more like apocalyptic fiction as the story progresses. It's quite a long book and takes a while to get to the more interesting sections, and there are some major plot lines that ultimately don't really go anywhere, but overall it's an interesting concept and well written (even though I thought the ending was slightly unbelievable personally).