If the goal is reducing carbon emissions, making shipping emit half as much (650 Megatonne to 325 Mt) would be less of a gain than making trucking emit only 80% of its carbon (2,230 Mt to 1,830 Mt).

The question is which is easier to do (ROI)... to cut the shipping fuel carbon footprint by half, or over the road trucking (that's about 1/4th of all the shipping) by 20%? For that matter, moving 25% of the over the road trucking to rail would accomplish that too.