We are pleased with the increased interest and use of the Global Change Master Directory's (GCMD) Earth science keywords. At this time, we are requesting that the following citation be used by those individuals and groups using the GCMD keywords.

Olsen, L.M., G. Major, K. Shein, J. Scialdone, R. Vogel, S. Leicester, H. Weir, S. Ritz, T. Stevens, M. Meaux, C.Solomon, R. Bilodeau, M. Holland, T. Northcutt, R. A. Restrepo, 2007 .
NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 6.0.0.0.0

DISCLAIMER: We are in the process of completing keyword revisions, associated with MD9.7's extension of the keyword taxonomies and the subsequent re-indexing of DIFs that are affected by these changes. Users should not integrate these keywords into their systems until the revisions are complete. However, we welcome your access to these keyword pages, provided that they are not shared outside your community at this time. Thank you.
http://gcmd.gsfc.nasa.gov/Resources/valids/archives/keyword_list.html

The GCMD Earth science keywords evolve as new metadata records are added and existing records are modified to meet the changing needs of the Earth science community.   These keywords were first offered to the public on April 24, 1995 as Version 1.0.0.   At that time, they were organized in a hierarchy constructed of TOPICs, TERMs, and VARIABLEs.   Changes at the TOPIC level (top level) required a change in the key version number.   TERM changes were indicated through the second digit, with VARIABLE level changes indicated using the third digit of the version number.   A TOPIC change was made in early 2003 when the new TOPIC, Climate Indicators, was added as Version 4.2.2.   A fourth change was made in July 2004 when the TOPIC name, Radiance/Imagery, was changed to Spectral/Engineering to more accurately describe the data sets behind the TOPIC.   The new version number thus  became "GCMD Keywords, Version 5.0.1".   A TERM addition was made in March 2005 when the  TERM, Paleoclimate Reconstructions, was added.  New TERMs were added in May 2005 (Sun-Earth Interactions) and in December 2005 (Climate Indicators).   New variables were added in 2006, bringing the version to "GCMD Keywords, Version 5.3.8".

In April 2007, the hierarchy was expanded to two additional levels.   A new TOPIC, Biological Classification, was added and inspired the extension of the science keyword taxonomy to five levels.   This added functionality arose from consultations with Ocean Biogeographic Information System (OBIS, http://www.iobis.org) representatives and resulted in the implementation of the OBIS taxonomic categories.  The TERMs and VARIABLEs under Biological Classification were further extended based on the following authoritative taxonomic sources:  Integrated Taxonomic Information system (ITIS) [2007] http://www.itis.gov, Global Biodiversity Information Facility (GBIF) [2007] http://www.gbif.org, and Species 2000 [2007] http://www.sp2000.org/.   In addition, the Topic, Hydrosphere,  was clarified by adding the modifier, "terrestrial".  Thus, the first digit for the keyword set will be "6" (Topic level changes) and the number of digits was extended to five to include Topic, Term, Variable1, Variable2, Variable3, and Detailed Variables.   The "Detailed Variables" serve as a critical component of the keyword taxonomy, by providing a level to specify uncontrolled keywords that the author chooses to include in the hierarchy.   Thus, the latest iteration should now be cited at "GCMD Keywords, Version 6.0.0.0.0".   For additional information see the GCMD Science Keyword Rules and the Procedures for Modification of the GCMD Keywords.