The transportation of freight by land, sea and air underpins the complex network of global trade in physical commodities. Greenhouse gas emissions from freight transportation are a significant component of global emissions and are predicted to grow in coming decades. However, the inclusion of freight transport in emissions accounts and environmental impact studies is often incomplete. Both data availability and difficulties in allocating freight emissions to specific commodity trades contributes to this. In this study, international freight movements by transport mode are estimated from the bottom-up by imputing global freight transport routes. Emissions are estimated from these freight movements and integrated with a global multiregional input−output model. This enables the calculation of carbon footprints that are complete with respect to freight emissions. We estimate that global freight transport emissions contributed 2.8 Gt CO2-equiv in 2012, or about 41% of total transport emissions. In general, freight footprints contribute about 9% to national emissions footprints. While trade in physical commodities (such as construction materials, food and fossil fuels) are associated with the largest embodied freight emissions, services (such as public administration, education and health) also require significant freight transport. Using a consumption-based allocation of freight transport emissions allows the decarbonisation of other sectors to be complementary to the decarbonisation of transport through reduction in demand, for example through material efficiency strategies. To drive decarbonisation in maritime transport it is critical to include bunker emissions in national emissions inventories, thereby completing the system boundary.
Commodities with the largest contribution to the freight emissions footprint are construction (15%), food and beverages (12%), electrical products and machinery (10%) and petroleum, chemical and nonmetallic mineral products (9%) and agriculture (6%). The provision of construction services is associated with the largest embodied freight emissions. This is intuitive since construction supply chains require the transportation of heavy bulk commodities such as sand, gravel and timber. Furthermore, construction requires transformed materials that also require bulk transport in their supply chains, for example steel, concrete and masonry products. While petroleum, chemical and non- metallic mineral products contribute 9% to the total freight emissions footprint, the total embodied tonne-kilometres amount to 11% of the total. This can be explained through oil and other bulk commodities being transported by sea, with relatively low emissions intensity. Similarly, agriculture ranks higher in embodied freight emissions than embodied tonne- kilometres. This may reflect that agricultural supply chains are relatively simple, with products delivered to final consumption quickly after being produced. This contrasts with food and beverage products, which require more transformations and production steps, as well as freight transport. Education, health and public administration services are also associated with significant transport emissions. Despite being service sectors, these categories have complex material requirements which involve freight transport.
Previously, I discussed the construction of a global freight transport adjacency matrix here. This research was recently published here in the journal of Environmental Science & Technology.