45gm CO2e per passenger-kilometer
For CO2 from travel by ferry using
http://www.co2balance.com/
And for distances using
http://www.groningen-seaports.com/images/x_images/distances.swf
| From | To | Naut m | NautM/km | Km | Return Kms | CO2 | CO2/km |
| Amsterdam | Hull | 254 | 1.85 | 470.41 | 940.82 | 40000 | 42.52 |
| Amsterdam | Oslo | 410 | 1.85 | 759.32 | 1518.64 | 90000 | 59.26 |
| Amsterdam | Murmansk | 1400 | 1.85 | 2592.8 | 5185.6 | 230000 | 44.35 |
Ferry travel appears to be about 45gm CO2e per passenger-kilometer