Open Educational Resources are teaching, learning and research materials in any medium - digital or otherwise - that reside in the public domain or have been released under an open license that permits no-cost access, use, adaption and redistribution by others with no or limited restrictions. Source: The William and Flora Hewlett Foundation.
Environmental Studies resources on MERLOT - MERLOT provides access to curated online learning and support materials.
Environmental Science resources on MERLOT.
Resources related to the environment on OER Commons - OER Commons is a public digital libraryof open educational resources.
The chapter on Earth & Environmental Sciences from the Open Educational Resources (OER) Directory available on Pressbooks.
Other "environment" titles available on Pressbooks.
Dacus Library has a Research Guide on OER that contains lots of information about OER and links to collection of OER materials.