The Textile Markup (2.4) Reference for Textpattern
Superscript is indicated by a surrounding carets ^, and subscript text by surrounding tildes ~ after a preceding space.
To avoid spaces around the super- and subscripted text, square brackets have to be used. The above example then should be rewritten as:
Global warming in the 21[^st^] century is mostly caused by CO[~2~] in the atmosphere.
or a chemical formula:
Calcium borate: Ca[~3~](BO[~3~])[~2~]
code into the box above.