If in any year 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 certificates to mature within three years. A tax sufficient to pay principal and interest on certificates with the margin required by law may 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 majority of the members of the council. It may be passed as an emergency ordinance.