For a study I am working on this semester while on sabbatical at Harvard University, I wanted to try to estimate book publishing trends over time related to climate change, renewable energy, biodiversity, sustainable development, and population growth. After consulting with several librarians, my best option was to estimate these publishing trends by way of the database which compiles library holdings across countries. In the figure below, based on a search of the WorldCat database, I plot the number of books by year and catalog subject term, using library holdings as a proxy for books published per year.

The trends are interesting. Notice the explosion of books during the 1970s on energy conservation and renewable energy and then the leveling off during the 1980s.  Overall, there are far more books published on energy conservation and clean energy than the other subjects.  Also notice following the 1992 Rio Environment Summit the rise in books published focused on sustainable development and biodiversity.  Though starting in the mid-2000s as climate change-related books spiked, publishing on these other topics declined, perhaps representing the tendency during this period for climate change to be a meta-problem through which most other environmental issues were viewed and discussed.  Finally, despite its linkages since the 1970s to environmental degradation and later to climate change, overpopulation as a topic has not received nearly the same focus by book publishers and authors as the other topics tracked.

What do readers think of these trends?  Know of other ways to estimate book publishing trends over time?  Ideas on other interpretations of these trends or uses for the database?