If in any year the receipts from taxes or other sources should from some unforeseen cause become insufficient for the ordinary expenses of the City, or if any calamity or other public emergency necessitates the making of extraordinary expenditures, the Council may by ordinance issue on such terms and in such manner as the Council determines emergency debt certificates to run not to exceed three years. A tax sufficient to pay principal and interest on such certificates with the margin required by law shall be levied as required by law. The ordinance authorizing an issue of such emergency debt certificates shall state the nature of the emergency and be approved by a unanimous vote of the Council. It may be passed as an emergency ordinance.