NicoInternational shipping accounts for about 1.7% of global greenhouse gas emissions. This number includes passenger trips, but the vast majority is from freight trips.
Shipping emissions come almost entirely from the oil used to power ships, usually the cheapest and dirtiest possible. In freight shipping in particular, cost is all important, and weight almost negligible.
For a breakdown of current international policies and developments, we recommend the IEA page on international shipping.