Where does that 20% number come from? https://en.wikipedia.org/wiki/Environmental_impact_of_shipping https://en.wikipedia.org/wiki/Environmental_impact_of_ship

Where does that 20% number come from? https://en.wikipedia.org/wiki/Environmental_impact_of_shipping https://en.wikipedia.org/wiki/Environmental_impact_of_shippi... indicates that up to 4% of climate change gasses come from ships. https://en.wikipedia.org/wiki/Environmental_impact_of_transport https://en.wikipedia.org/wiki/Environmental_impact_of_transp... has a breakdown of CO2 per ton-mile. It shows that ships are the most efficient followed by zeppelins and trucking is the source of 20% of the UK's carbon emissions.