Size matters: Exploring the importance of vessel characteristics to inform estimates of shipping emissions

The decarbonisation agenda is placing increasing pressure on retailers to directly and indirectly influence greenhouse gas emissions associated with full supply chains. Transportation by sea is an important and significant element of these supply chains, yet the emissions associated with shipping, particularly international shipping, are often poorly accounted for. The magnitude of emissions embodied in a product is directly related to the distances involved in globalised product chains, where shipping can represent the most emission intensive stage per tonne of goods transported. Specifically, limited choice of ship type and size within assessment tools negates a fair estimate of product chain emissions. To address this, the correlation between ship emissions and size is quantified for a sample of United Kingdom (UK) port callings to estimate typical UK emission factors by ship type and size and to determine how well existing global data and available databases represent UK shipping activity. The results highlight that although ship type is a crucial determinant of emissions, vessel size is also important, particularly for smaller ships where the variance in emission factors is greatest. Existing, globally averaged data correlating ship size with emissions agree well with the UK data. However, the relatively higher proportion of smaller ships satisfying a UK demand for short sea shipping results in a skew towards higher typical emission factors, principally within the general cargo, product and chemical tanker categories. This bias is most visible when emissions per individual ship calling are estimated. Incorporating existing global correlation curves split by ship category into assessment tools would improve product chain estimates. However, user knowledge of likely ship size and type is an important prerequisite given the wide range of emission factors at the lower end of the scale, which could be a pivotal determinant in the absolute product chain emissions estimate.

► Ship emission baselines can be used to inform studies but require prior knowledge.
► Region specific conditions alter average shipping emission factors.
► Region specific conditions are clearer when individual callings are examined.
► Relationship between ship size and emissions frustrates estimating mean emissions.