AER: the Annual Efficiency Ratio
The deadweight-based carbon intensity metric: how AER is calculated, which ship types use it, and why its capacity denominator penalises high payload utilisation.
The Annual Efficiency Ratio (AER) is the supply-based operational carbon intensity indicator that uses deadweight as the capacity term, expressing annual CO2 emissions per deadweight tonne-mile. It is one of the two indicators assigned by the CII guidelines, the other being cgDIST, which uses gross tonnage instead.
Under MEPC.352(78) paragraph 2.5 the AER applies to bulk carriers, tankers, container ships, gas carriers, LNG carriers, general cargo ships, refrigerated cargo carriers and combination carriers. It is calculated from the fuel oil consumption and distance data already reported under the IMO data collection system, so it requires no additional measurement.
Its structural weakness is the denominator, and it is the substance of the industry critique of the CII. Because the metric uses deadweight capacity rather than cargo actually carried, it measures capacity moved rather than cargo moved. IMO Secretariat document MEPC 82/INF.25 found the AER to be the least accurate of the metrics considered, because it assumes full loading in all situations including ballast legs, with the consequence that ships with higher payload utilisation are penalised. See what is CII and slow steaming .