We are told that shipping is the most efficient way of transport. Basically, the energy consumption and emissions from shipping are so low that it should not be seen as a problem. Or?
Well, like many other discussions the reality is more complex. While it is very efficient per ton-km, shipping still consumes a lot of energy. One single large container ship can use 200 ton of diesel per day, this means that the daily emissions for one single sailor is more than the yearly emissions of two Americans.
The International Maritime Organization (IMO) says sea shipping makes
up around 3% of global CO2 emissions which is slightly less than Japan’s
annual emissions, the world’s 5th-highest emitting country. And with current trends CO2 emissions from ships will increase by up to 250% in the next 35 years, and could represent 14% of total global emissions by 2050. These calculations don't include all the infrastructure needed for the shipping, the harbours, the ships themselves. And of course most goods moved by ship will anyway be loaded on a truck in the few major harbours. Maritime shipping seems to provide a typical example of Jevons paradox.
Unfortunately maritime transportation is not part of the Paris agreement or in the national greenhouse gas inventories. So it is largely forgotten or disregarded in the climate debate.
Kiln have produced this fantastic interactive map showing movements of the global merchant fleet over the course of 2012.It gives you an idea....