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The most commonly applied MESSAGE transport sector representation is very stylized and essentially includes fuel switching and price-elastic demands (via MACRO linkage) as the main responses to energy and climate policy.

In this stylized transport sector representation fuel switching is a main option, i.e. different final energy forms that provide energy for transportation can be chosen from. In addition to the alternative energy carriers that serve as input to these stylized transportation options, their relative efficiencies are also different. The useful energy demand in the transportation sector is specified as internal combustion engine (ICE) equivalent demands which therefore by definition has a conversion efficiency of final to useful energy of 1. Relative to that the conversion efficiency of alternative fuels is higher, for example, electricity in 2010 has about a factor of three higher final to useful efficiency than the regular oil-product based ICE. The overall efficiency improvements of the ICE in the transportation sector and modal switching over time is implicitly included in the demand specifications, coming from the scenario generator (see section on demand). Additional demand reduction in to response to price increases in policy scenarios is then coming in via the fuel switching option (due to the fuel-specific relative efficiencies) as well as via the linkage with the macro-economic model MACRO as illustrated in Figure 1 below.

To reflect limitations of switching to alternative fuels, for example as a result of limited infrastructure availability (e.g., rail network) or some energy carriers being unsuitable for certain transport modes (e.g., electrification of aviation), share constraints of energy carriers (e.g., electricity) and energy carrier groups (e.g., liquid fuels) are used in the transport sector. In addition, the diffusion of speed of alternative fuels is limited to mimic bottlenecks in the supply chains, not explicitly represented in MESSAGE (e.g., non-energy related infrastructure) (add link). Both the share as well as the diffusion constraints are usually parametrized based on transport sector studies that analyze such developments and their feasibility in much greater detail.

Figure 1: Schematic diagram of the stylized transport sector representation in MESSAGE.

The demand for international shipping is modeled in a very simple way with a number of different energy carrier options (light and heavy fuel oil, biofuels, natural gas, and hydrogen). As discussed in the demand section, the demand for international shipping is coupled to global GDP development with an income elasticity.

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