The <Science_Keywords> 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 data. The author
must select these keywords from the controlled set of science keywords.
The <Science_Keywords> field consists of a 7-level hierarchical
classification of science keywords, defined as follows:
Multiple <Science_Keywords> 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.