The <Science_Parameters> field allows for the specification of Earth science keywords that are representative of the data service 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 <Science_Parameters> field consists of a 7-level hierarchical classification of science keywords, defined as follows:

• <Science_Category> is the highest keyword category. The default is "EARTH SCIENCE".
• <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

• <Science_Term> is next level in the hierarchy under <Science_Topic>. There can be several controlled <Term> keywords for each <Science_Topic>. 
• <Science_Variable_Level_1> is the next level in the hierarchy under <Science_Term>. There can be several controlled <Science_Variable_Level_1> keywords for each <Science_Term>.  
• <Science_Variable_Level_2> is the next level in the hierarchy under <Science_Variable_Level_1>. There can be several controlled <Science_Variable_Level_2> keywords for each <Science_Variable_Level_1>..
• <Science_Variable_Level_3> is the next level in the hierarchy under <Science_Variable_Level_2>. There can be several controlled <Science_Variable_Level_3> keywords for each <Science_Variable_Level_2>.  
• <Detailed_Variable> is an uncontrolled free text field that allows the metadata author to specify any keywords to more exactly describe the measurement represented by the data.

<Science_Parameters>
<Science_Category>EARTH SCIENCE</Science_Category> (by default)
<Science_Topic>Topic keyword</Science_Topic>
<Science_Term>Term keyword</Science_Term>
<Science_Variable_Level_1>Variable Level 1 keyword </Science_Variable_Level_1>
<Science_Variable_Level_2>Variable Level 2 keyword </Science_Variable_Level_2>
<Science_Variable_Level_3>Variable Level 3 keyword </Science_Variable_Level_3>
<Detailed_Variable>text</Detailed_Variable>
</Science_Parameters>

• <Detailed_Variable> is a free-text field, which may contain up to 80 characters.
• <Science_Category>, <Science_Topic>, <Science_Term>, <Science_Variable_Level_1>, <Science_Variable_Level_2>, and <Science_Variable_Level_3> must be selected from the set of controlled Earth science keywords.

• The <Science_Parameters> field is required field and may be repeated as often as necessary.
• The following fields within <Science_Parameters> are required and may not be repeated within the <Science_Parameters> field:
• <Science_Category>
• <Science_Topic>
• <Science_Term>

• The following fields within <Science_Parameters> are optional and may not be repeated within the <Science_Parameters> field:
• <Science_Variable_Level_1>
• <Science_Variable_Level_2>
• <Science_Variable_Level_3>
• <Detailed_Variable>

Multiple <Science_Parameters> can be specified for each metadata record. All applicable <Science_Topic>, <Science_Term>, <Science_Variable_Level_1>, <Science_Variable_Level_2>, and <Science_Variable_Level_3> science keywords should be selected that describe the data set.

<Science_Parameters>
<Science_Category>Earth Science</Science_Category>
<Science_Topic>Atmosphere</Science_Topic>
<Science_Term>Precipitation</Science_Term>
</Science_Parameters>

<Science_Parameters>
<Science_Category>Earth Science </Science_Category>
<Science_Topic>Climate Indicators</Science_Topic>
<Science_Term>Teleconnections</Science_Term>
<Science_Variable_Level_1>North Atlantic Oscillation </Science_Variable_Level_1>
</Science_Parameters>

Specification of <Parameters> with two different <Detailed_Variable> keywords in the same metadata record.  Note that the <Parameters> field must be repeated.

<Parameters>
<Category>Earth Science</Category>
<Topic>Biological Classification</Topic>
<Term>Animal/Invertebrates</Term>
<Variable_Level_1>Fish</Variable_Level_1>
<Detailed_Variable>Demersal Fish</Detailed_Variable>
</Parameters>

<Parameters>
<Category>Earth Science</Category>
<Topic>Biological Classification</Topic>
<Term>Animal/Invertebrates</Term>
<Variable_Level_1>Fish</Variable_Level_1>
<Detailed_Variable>Catch per Unit Effort</Detailed_Variable>
</Parameters>

Specification of two <Parameters> with the same <Variable_Level_1>, but different <Term>.  Both <Parameters> may be in the same metadata record.

<Parameters> 
<Category>Earth Science</Category>
<Topic>Cryosphere</Topic>
 <Term>Sea Ice</Term>
 <Variable_Level_1>Sea Ice Concentration</Variable_Level_1>
</Parameters>

<Parameters> 
<Category>Earth Science</Category>
<Topic>Oceans</Topic>
 <Term>Sea Ice</Term>
 <Variable_Level_1>Sea Ice Concentration</Variable_Level_1>
</Parameters>