National level greenhouse gas emission allowances established under the Kyoto Protocol. One AAU represents one metric tonne of CO2 equivalent greenhouse gas. The amount held by each country is determined by the 1990 emissions baseline and national Kyoto target for 2008-2012. AAUs are tradable at state level only. A country failing to meet its Kyoto emissions reduction target can buy AAUs from another Kyoto Annex I country that has met its target and holds a surplus.