If in any year the receipts from taxes or other sources should from some unforseen cause become insufficient for the ordinary expenses of the city, or if any calamity or other public emergency should subject the city to the necessity of making extraordinary expenditures, the council may by ordinance issue and sell on such terms and in such manner as the council determines emergency debt certificates to run not to exceed two 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 at least four members of the council. It may be passed as an emergency ordinance.