Encyclopedia of Earth

The Encyclopedia of Earth is "a new electronic reference about the Earth, its natural environments, and their interaction with society. The Encyclopedia is a free, fully searchable collection of articles written by scholars, professionals, educators, and experts who collaborate and review each other's work. The articles are written in non-technical language and will be useful to students, educators, scholars, professionals, as well as to the general public."

Sample articles include:

Biodiversity
Causes of Climate Change
Transpiration and Evaporation