Environmental Ship Index will also evaluate CO2 performance, the first one with a quality mark in shipping

The index does this based on historical data already included in the index. The ESI compares the fuel consumption and the nautical miles travelled in the past three years (e.g. 2013, 2014, 2015) to that of the following year (2016). If relatively less fuel has been consumed - in other words, if sailing was more efficient - then less CO2 has been emitted. ESI has chosen for this method because it is actually very difficult to measure CO2 emissions.

Environmental Ship Index
The ESI is a score that is issued at the request of ship owners as of 1 January 2011. Ships that score high on the Environmental Ship Index receive a premium from the participating ports and other organisations that reward sustainable shipping. As of 1 April 2017, the index comprised 5497 ships and 50 bonus providers.