The <Parameters> field allows for the specification of Earth science keywords that are representative of the data set being described. These keywords are important for the precise search and retrieval of information from the GCMD. The author must select these keywords from the controlled set of science keywords. The <Parameters> field consists of a 7-level hierarchical classification of science keywords, defined as follows:

• <Category> is the highest keyword category. The default is "EARTH SCIENCE".
• <Topic> is the next highest level in the keyword hierarchy under <Category>. There are 14 Topics:
  • Agriculture
  • Atmosphere
  • Biosphere
  • Biological Classification
  • Climate Indicators
  • Cryosphere
  • Human Dimensions
  • Land Surface
  • Oceans
  • Paleoclimate
  • Solid Earth
  • Spectral/Engineering
  • Sun-Earth Interactions
  • Terrestrial Hydrosphere
• <Term> is next level in the hierarchy under <Topic>. There can be several controlled <Term> keywords for each <Topic>. For example, <Topic>Atmosphere</Topic> <Term>Aerosols </Term>.
• <Variable_Level_1> is the next level in the hierarchy under <Term>. There can be several controlled <Variable_Level_1> keywords for each <Term>. For example, <Topic>Atmosphere</Topic> <Term>Aerosols </Term> <Variable_Level_1>Aerosol Optical Depth</Variable_Level_1>.
• <Variable_Level_2> is the next level in the hierarchy under <Variable_Level_1>. There can be several controlled <Variable_Level_2> keywords for each <Variable_Level_1>. For example, <Topic>Biological Classification</Topic> <Term>Animals/Invertebrates</Term> <Variable_Level_1>Arthropods </Variable_Level_1> <Variable_Level_2>Crustaceans</Variable_Level_2>.
• <Variable_Level_3> is the next level in the hierarchy under <Variable_Level_2>. There can be several controlled <Variable_Level_3> keywords for each <Variable_Level_2>. For example, <Topic>Biological Classification</Topic> <Term> Animals/Invertebrates </Term> <Variable_Level_1>Arthropods</Variable_Level_1> <Variable_Level_2>Crustaceans</Variable_Level_2> <Variable_Level_3>Decapods </Variable_Levele_3>
• <Detailed_Variable> is an uncontrolled free text field that allows the metadata author to include any keywords to specifically describe the measurement represented by the data. For example, <Topic>Biological Classification</Topic> <Term> Animals/Invertebrates </Term> <Variable_Level_1>Arthropods</Variable_Level_1> <Variable_Level_2>Crustaceans</Variable_Level_2> <Variable_Level_3>Decapods </Variable_Level_3> <Detailed_Variable>Lobsters </Detailed_Varieble>
  

• <Detailed_Variable> is a free-text field, which may contain up to 80 characters.
• <Category>, <Topic>, <Term>, <Variable_Level_1>, <Variable_Level_2>, and <Variable_Level_3> must be selected from the set of controlled Earth science keywords.
  

• The <Parameters> field is required and repeatable.
• The following fields within <Parameters> are required and may not be repeated within the <Parameters> field:
  • <Category>
  • <Topic>
  • <Term>
• The following fields within <Parameters> are optional and may not be repeated within the <Parameters> field:
  • <Variable_Level_1>
  • <Variable_Level_2>
  • <Variable_Level_3>
  • <Detailed_Variable>

Multiple <Parameters> can be specified for each metadata record. All applicable <Topic>, <Term>, <Variable_Level_1>, <Variable_Level_2>, and <Variable_Level_3> science keywords that describe the data set should be selected.