Significant progress in curbing CO2 emissions was made over the last decade in electricity generation, residential, and other sectors of the economy while emissions from transportation continues to grow.  Moreover, emissions from freight transportation—versus passenger transportation—are projected to accelerate well past mid-century international climate goals. 
Without a widespread and concerted effort, transportation is set to overtake energy as the most carbon-intensive sector by 2040. 
To understand emissions attributable to the movement of freight, however, we have to dig deeper than sector- or industry-level statistics, which hide the complexity of freight transportation. Unfortunately, there is a glaring gap in the disclosure of freight-related emissions by companies, even by “sustainability leaders” that participate in programs like CDP. 
One place to start is to look at the carbon intensity of each transportation segment. This can capture important nuances such as the difference between long-haul routes in heavy-duty trucks compared to more urban and regional freight moved by light-and medium-duty vans and trucks.
The most obvious result is rail and maritime transportation are significantly more efficient than freight moved on the road and in the air. On the road, light-duty commercial trucks serving short-haul and final-mile segments are significantly less efficient than heavy-duty trucks moving larger, consolidated loads.
These results are not surprising. They also can be taken to the (carbon) bank by shifting freight from road/air to rail/ocean when possible. This is where procurement and logistics can have a significant impact.
In addition to carbon intensity, it’s important to consider the quantity of freight moved by each mode of transportation.
These results are striking. Trucking contributes the vast majority of CO2 emissions in the U.S., and is projected to double internationally by 2050—though the overall proportion of emissions from trucking is projected to decline compared to maritime transportation.
Internationally, the majority of freight moves across oceans—and to a lesser extent coastal and riverine waters—accounting for over 30% of international freight emissions in 2015.
What does this mean?
We need lower carbon solutions in trucking. It is both the most significant proportion of total emissions and one of the carbon intensive modes—especially short-haul and final-mile routes by light- and medium-duty vans and trucks.
Ocean freight is an important opportunity as well. The mode is efficient, but so much freight moves on the ocean that ships contribute a significant share of CO2 globally, and so small improvements can have a big impact.
Alan McKinnon takes on these challenges in Decarbonizing Logistics, providing a thorough survey of decarbonization strategies for this critical sector.
 US EPA (2020). Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2018. Report # 430-R-20-002.
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 US DOT Bureau of Transportation Statistics. Freight Facts and Figures. Retrieved February 25, 2020.