Environmental Ship Index (ESI)

The Environmental Ship Index (ESI) is a voluntary port industry scheme that scores ships based on their environmental performance. Developed by the International Association of Ports and Harbors (IAPH) in 2011, it evaluates emissions such as nitrogen oxides (NOx), sulphur oxides (SOx), and greenhouse gases to identify vessels that outperform International Maritime Organization (IMO) standards. Participating ports use ESI scores to offer incentives—typically reduced port dues—to ships demonstrating lower emissions.