Researchers use a variety of simulation models to explore how shipping can achieve its climate targets. However, the models are based on different assumptions and metrics, and the underlying details are not always fully disclosed. This makes it difficult to compare the results of different simulations and studies. A new scientific study aims to bring some order to the chaos.

Which fuels will dominate the shipping industry of the future? How quickly will the fleet be renewed? What happens to emissions when the price of carbon rises or new IMO regulations are introduced?

To answer questions like these, researchers are increasingly using sophisticated simulation models. But a new study published in the Journal of Marine Science and Engineering shows that the results produced by these models are often difficult to compare.

The researchers behind Towards Net-Zero Shipping: A Review of Simulation Models and a Harmonization Framework reviewed 100 scientific papers and selected 14 studies that simulate the effects of different climate measures in shipping. The review reveals major differences between the models in terms of their structure, assumptions and units of measurement. Moreover, the details are not always fully reported, making it difficult to determine what actually lies behind the results.

One model, for example, may assume a particular development in global trade, fuel prices and energy efficiency, while another may make entirely different assumptions. Some models allow factors such as vessel orders and fuel choices to change within the model itself, while others treat the same factors as fixed inputs.

The study illustrates the problem by comparing two previous studies. Their projected emissions trajectories differ dramatically. In one model, which describes a scenario without climate measures, emissions increase and remain well above 2008 levels in 2050. In the other study, climate measures cause emissions to fall towards zero. However, the studies are based on different assumptions regarding transport demand, energy efficiency improvements, lifecycle emissions from fuels, fuel availability and policy measures. This makes them more or less impossible to compare directly.

To address the problem, the study proposes a common framework for simulation models. The proposed framework is not a standalone simulation tool, nor does it prescribe any particular type of model. Instead, it structures variables and standardises units across four areas frequently found in the studies reviewed: transport demand, ports, vessels and fuels. This means that factors such as transport volumes, vessel capacity, energy efficiency, vessel lifetime, fuel type, fuel costs and lifecycle emissions from fuels can be described in a consistent way.

The point, then, is not that all researchers should use the same models or make the same assumptions. On the contrary, models may need to differ depending on the questions they are designed to answer. But those differences need to be made clear, the researchers write.

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