International shipping in a world below 2oC

doi:10.21203/rs.3.rs-2958063/v1

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International shipping in a world below 2^oC

https://doi.org/10.21203/rs.3.rs-2958063/v1

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published 22 May, 2024

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In recent years, the decarbonisation of international shipping has become an important policy goal. While Integrated Assessment Models (IAMs) are often used to explore climate mitigation strategies, they typically provide little information on international shipping, which accounts for around 0.75 GtCO₂/yr. Here, we perform the first multi-IAM analysis of international shipping, drawing on the results of six global models. Results indicate the need for decreasing emissions in the next decades, with reductions up to 88% in 2050. This is primarily achieved through the deployment of low-carbon fuels. Models that represent several potential low-carbon alternatives tend to show a deeper decarbonisation of international shipping, with drop-in biofuels, renewable alcohols and green ammonia standing out as the main substitutes of conventional maritime fuels.

Physical sciences/Energy science and technology/Energy modelling

Earth and environmental sciences/Climate sciences/Climate change/Climate-change mitigation

Scientific community and society/Energy and society/Energy supply and demand

The goal of limiting the increase in global mean temperature to well below 2^oC and preferably below 1.5^oC was reaffirmed at COP27¹. According to the Intergovernmental Panel on Climate Change (IPCC), pathways that meet these goals require rapid and deep greenhouse gas (GHG) emissions reductions in all sectors throughout the first half of this century. Decarbonisation efforts can, however, vary significantly across sectors^2,3. Some sectors, including cement and steel production and long-distance air and maritime transport, entail CO₂ emissions that are quite difficult to eliminate, especially with fast-growing demands^4,5. While ‘hard-to-abate’ sectors were responsible for around 29% of energy/industry CO₂ emissions in 2019^5–7, their share can drastically increase over the next few decades^2,3. Demand-side and efficiency measures can contribute to decarbonising these sectors⁸, but face significant barriers and constraints their potential is intrinsically limited. Many model-based mitigation pathways consistent with the Paris Agreement, therefore, rely on Carbon Dioxide Removal (CDR) strategies to offset emissions⁹.

Maritime shipping is a major hard-to-abate sector that accounted for CO₂ emissions between 0.95 and 1.06 Gt/yr during the 2012–2018 period. Around 71% of this total came from international shipping. Non-CO₂ gases currently play a minor role in the sector, typically accounting for no more than 2% of total CO₂-equivalent emissions¹⁰. While international shipping emissions have been fairly stable since 2008, it would be wrong to conclude that the sector has already reached its emission peak. The growth of shipping emissions was slowed down by the 2008-09 financial crisis, which brought international seaborne trade down from 42 to 40 trillion tonne-miles (Tt-nm)¹¹. After this, emissions kept declining until 2014 due to considerable efficiency gains despite the recovery of demand, enabling a temporary decoupling of emissions and transport activity. Since then, however, further increased shipping activity outpaced efficiency gains, and emissions have gradually increased^10,11. Just as it went from 10 Tt-nm in 1970 to around 60 Tt-nm in 2020^11,12, international seaborne transport work could reach 90–180 Tt-nm by 2050^10,13, even if there are several sources of uncertainty¹⁴. With the current carbon intensity (11.7 gCO₂/t-nm in 2018), this would represent international shipping CO₂ emissions estimated at 1.1–2.1 Gt/yr in 2050.

In 2018, the International Maritime Organization (IMO), responsible for regulating global maritime transport, reached an agreement to reduce international shipping GHG emissions by at least 50% by 2050 compared to 2008 (hereinafter IMO2050). As part of a broader strategy that also involves energy efficiency and carbon intensity goals, IMO2050 aims to ensure an emission pathway compatible with the temperature limits defined by the Paris Agreement^15,16 (since international shipping was not explicitly covered by the treaty).

However, the role of shipping in a world of deep decarbonisation is still unclear (Fig. 1). First, while energy efficiency may still help lower the carbon intensity of ships, its potential is intrinsically limited. Measures already present in the sector (e.g., speed reduction, improvement of hull design and propulsion system optimization) and measures that are still to come (e.g., increased usage of digitalization to optimize shipping routes) can provide a significant abatement of CO₂ emissions^17,18. Still, an efficient shipping sector almost completely fuelled by fossil energy (as is the case today) would entail relatively high CO₂ emissions. As such, low-carbon fuels are regarded as a key mitigation option for shipping¹⁹. Candidate alternative fuels are diverse both in terms of production routes and final energy carriers²⁰. Using some of them depends on implementing new motorisation options, such as dual-fuel engines and electrochemical powertrains. Contrastingly, the existing fleet is almost entirely based on compression ignition engines, which can only work with bunker- and diesel-like fuels²¹. The long lifespan of ships and the low average age of existing vessels therefore imply significant technological inertia. Furthermore, low-carbon shipping would require fuel-specific bunkering infrastructure¹⁹, in contrast with today’s standard global hubs of Heavy Fuel Oil (HFO) and Marine Gas Oil (MGO)²². In view of these challenges and of the important linkages between shipping and the global energy system, we argue that international shipping emissions scenarios should be put into the context of global GHG emissions scenarios.

So far, only a few studies have looked at the decarbonisation of international shipping, and often based on sectoral modelling only^{19, 28–30}. Economy-wide Integrated Assessment Models (IAMs) have historically paid little attention to shipping¹³. These models have been used to explore the consequences of different long-term climate mitigation strategies, with significant impacts on climate governance and policy^{3,9,39,40, 31–38}. Currently, IAMs are the backbone of scenario analyses reviewed by the Working Group III (WGIII) of the IPCC, which focuses on mitigation response strategies to global warming^3,9,41,42. In recent years, the IAM community has started to further explore the specificities of international maritime transport, which also made possible a better representation of the sector’s demand and mitigation options within models^8,13,43,44. These improvements allow an integrated perspective of the sector’s decarbonisation strategy, adding value to the existing literature. Instead of seeing shipping as an isolated sector, IAMs can treat maritime transport as part of a broader response strategy covering the rest of the energy sector (and, in some cases, agriculture and land use) and its complex linkages with maritime transport. Thereby, it becomes possible to evaluate the role of shipping in futures with varying storylines and climate targets.

In this paper, we explore possible futures of international shipping in terms of energy carriers and GHG emissions under an IAM framework. To that end, we perform the first multi-model comparison of this sector, relying on the results from six global IAMs. Rather than thinking of maritime emission reductions as the final objective, our analysis sees international shipping as part of a wider challenge, in which the goal is to halt the increase of global mean temperature. We argue that a myopic mitigation strategy in the shipping sector could not only cause harmful economic impacts but also imply spill-over effects on GHG emissions⁴⁴. As such, one of the objectives of our analysis is to compare shipping emission reductions observed in IAM-based scenarios to that suggested by IMO2050, which is currently the landmark of maritime emissions.

Using the same socioeconomic assumptions and carbon budgets, we develop scenarios of the global energy system for 2020–2100 and concentrate our analysis on international shipping. The IAMs used in this exercise differ not only in terms of overall structure and modelling dynamics but also in the way they represent shipping mitigation options (see Supplementary Information – section 1). Four of our models (COFFEE, IMAGE, PROMETHEUS, and TIAM-UCL) have relatively high technological resolution in the shipping sector, meaning that they include numerous low-carbon maritime fuel production routes and vessel motorisation options. The other two models (IMACLIM-R and WITCH) focus on specific low-carbon alternatives in their modelling structure, with IMACLIM-R mainly relying on lignocellulosic biofuels and WITCH on hydrogen and hydrogen-based synthetic fuels. One of the purposes of our analysis is precisely to assess the impact of having a few or several mitigation options for shipping represented in IAMs.

Our scenario design does not impose any sectoral storyline but harmonises input data by using the Shared Socioeconomic Pathway no. 2 (SSP2)⁴⁵. We work with three main scenarios groups, with the reference group (NDC) assuming the fulfilment of Nationally Determined Contributions (NDCs) as stated in the 2015 pledges. In these scenarios, total global emissions are not restricted in any way. The other scenario groups (C1000 and C600) also assume the fulfilment of NDCs but additionally impose carbon budgets of 1,000 and 600 GtCO₂ for the period 2020–2100. While C600 scenarios can be seen as in line with a warming slightly above 1.5^oC by 2100, C1000 scenarios would reflect a world likely below 2^oC by that time (see Supplementary Information – section 2). Both C1000 and C600 scenarios follow a peak-budget dynamic, i.e., net negative emissions are not allowed after reaching the point of net zero⁹.

In addition to our global analysis, in the Supplementary Information (section 5), we present detailed results for the European Union (EU). This regional analysis was performed using the PRIMES-Maritime model and is consistent with C1000 and C600 global carbon budgets.

Stable or lower international shipping emissions in carbon budget scenarios

Under current policies (NDC), our IAMs project international shipping emissions to stabilise or rise in the long-term (Fig. 2), with values in the range of 0.6–1.4 MtCO₂/yr in 2050, 0.7–1.7 MtCO₂/yr in 2070 and 0.8-2.0 MtCO₂/yr in 2090. From a decomposition perspective, this uptrend is due to total shipping activity, which is projected to continuously increase through the century (panel a). Higher reference emissions are associated with more pessimistic efficiency assumptions (e.g., TIAM-UCL).

On the other hand, most models show international shipping emissions to fall significantly in the Paris-compatible scenarios, even with rising activity. This is made possible by efficiency improvement and fuel switch. For instance, with a carbon budget of 1,000 GtCO₂, international shipping emissions lie between 0.3 and 1.2 GtCO₂/yr in 2050. Particularly, the most technology-detailed models indicate reductions relative to the 2008 level. While COFFEE and IMAGE show cutbacks lower than that of IMO2050 (26 and 34%), PROMETHEUS shows a reduction of 65%, going beyond IMO2050 and even the Sustainable Development Scenario (SDS) of the International Energy Agency (IEA)⁴⁶. IMACLIM-R shows a lighter decrease (9%) in shipping emissions, while TIAM-UCL and WITCH show increases of 49 and 16% in 2050. In the case of TIAM-UCL, this effect is the combination of a fast-rising energy demand with a relatively slow development of low-carbon fuels, whose development takes place mostly after 2050. This time lag makes the variation of TIAM-UCL emissions go from the most positive in 2050 (+ 49%) to the most negative in 2070 (-58%). As such, WITCH is somehow the only exception, depicting international shipping emissions as approximately stable throughout the century in its C1000 scenario. This is partly due to the low technological granularity of the shipping sector in WITCH, which mainly includes blue/green hydrogen-based fuels as potential energy alternatives. Considering these limited mitigation options, the inter-temporal economic growth optimization of the model prioritizes the abatement of CO₂ emissions elsewhere.

With a carbon budget of 600 GtCO₂, reductions are more pronounced in 2050. COFFEE and IMAGE show reductions of 43 and 59%, in line with IMO2050 and the SDS scenario, while PROMETHEUS depicts emissions as low as those of the IEA Net Zero Emissions by 2050 Scenario (NZE), with a cutback of 88%. In C600 scenarios, even low-shipping-resolution models show at least stable emissions in 2050. The IMACLIM-R cutback is very close to that observed in its C1000 scenario, in which the low-carbon fuel production already reaches its full potential. In TIAM-UCL, the more stringent carbon budget engenders a faster maritime fuel transition, making international shipping emissions 2% lower in 2050 compared to 2008. Finally, the C600 scenario from WITCH is quite similar to its C1000 scenario, with high fossil fuel deployment and a 2% increase in emissions in 2050.

The five models whose time horizon is 2100 project international shipping emissions to approximately stabilize, in both C1000 and C600 scenarios, in the second half of the century. This pattern is due to the rise of Bioenergy with Carbon Capture and Storage (BECCS) after 2040 (see Supplementary Information – section 3). As explained before, our carbon budget scenarios do not allow net negative emissions. However, this does not exclude the use of some level of CDR strategies in these mitigation scenarios. This finding should be tempered by the uncertainties surrounding the large-scale deployment of BECCS (e.g., land and water requirements^47,48). Restrictions on the diffusion and implementation of this technology would mean a higher decarbonisation pressure on the shipping sector.

Fossil fuel shift associated with model granularity for shipping

Total fuel demand for international shipping is relatively convergent across models, reaching 10–16 EJ/yr in 2030. Over a longer time, discrepancies are more relevant, with a range of 7–23 EJ/yr in 2050 and of 12–28 EJ/yr in 2070. This is due to differences in activity projections and energy efficiency assumptions. Scenarios with high activity and low efficiency gains (e.g., NDC scenarios from IMACLIM-R and TIAM-UCL) have a total annual demand of 20–23 EJ/yr in 2050, representing an increase of over 100% compared to current international shipping energy consumption. At the opposite end, IMAGE and PROMETHEUS, whose modelling includes demand responses to carbon pricing, have fuel demands of 7–8 EJ/yr in 2050, slightly below the current consumption. Compared to their NDC scenarios, this represents fuel demand reductions of 25–50%, with part of these reductions coming from avoided fossil fuel demand. In the case of COFFEE, although the transportation of coal, oil and gas is modelled in detail, no significant impact is observed on fuel demand in carbon budget scenarios. As shown in panel (a) of Fig. 2, the variation of the activity level across scenarios in COFFEE is small, since the mitigation strategy indicated by the model includes the conservation of a relatively large international fossil fuel market.

As a hard-to-abate sector, international shipping tends to have higher shares of fossil energy in deep mitigation scenarios when compared to sectors such as electricity supply, heating, and road transportation. Our results confirm this (Fig. 4), depicting conventional maritime fuels as relevant energy sources until the second half of the century. The magnitude of this fossil energy gap between shipping and the rest of the energy system is partly associated with the level of detail of the shipping sector in each IAM, most notably in carbon budget scenarios. For instance, in C1000 scenarios, while high-shipping-resolution models show fossil energy with a share of 46–67% in international shipping in 2050 (versus 34–65% in global primary energy), low-shipping-resolution models indicate a fossil share of 64–96% in the same year (versus 46–55% in primary energy). Similarly, in C600 scenarios, high-shipping-resolution models indicate maritime fossil shares in the range of 18–63% in 2050 (compared to 34–52% in primary energy), while low-resolution models point to higher values (58–88% in international shipping compared to 28–47% in primary energy). This finding is even more relevant when considering that total fuel demand is lower in carbon budget scenarios from COFFEE, IMAGE, and PROMETHEUS (which is also due, as explained, to detailed modelling of shipping demand and/or energy efficiency options). With higher maritime fuel demand, low-carbon energy carriers would likely have an even bigger share in carbon budget scenarios from these IAMs.

A diverse portfolio of low-carbon fuels, with LNG playing a minor role

Our models point to a set of various mitigation responses, with the choice of specific alternative maritime fuels strongly related to the global trends observed in the different IAMs. Carbon budget scenarios from TIAM-UCL, for example, include aggressive electrification based on solar and wind power, with these sources reaching ~ 300 EJ/yr of primary energy in 2070 and ~ 350 EJ/yr in 2090. This context favours the adoption of maritime fuels based on renewable electricity, especially ammonia (with an energy density advantage compared to hydrogen), which reaches around 9 EJ/yr in 2070 and 18 EJ/yr in 2090. In contrast, pathways resulting in higher biomass shares in primary energy (such as IMACLIM-R and IMAGE) show a preference for bio-based maritime fuels, especially synthetic liquids (e.g., bio-alcohols and Fischer-Tropsch hydrocarbons). However, models that rely on bio-based fuels to decarbonize transports always face a competition for feedstock in generating negative emissions with the power sector⁴⁹. This largely explains the limit reached by low-carbon maritime fuels in IMACLIM-R scenarios. Carbon budget scenarios from COFFEE and PROMETHEUS show lower fuel demand and a more diverse maritime energy mix. In COFFEE scenarios, for example, vegetable oils and renewable alcohols are the first options for fuel switch in the 2040s. This is due to their higher technological maturity both in terms of production and use. In the second half of the century, green ammonia (~ 3 EJ/yr) and residual lignocellulosic biofuels (~ 2 EJ/yr) become the dominant alternative energy carriers. PROMETHEUS scenarios depict a faster transition and richer fuel portfolio based on ammonia, hydrogen, and oilseed-based fuels, while lignocellulosic bioenergy rapidly become the most important low-carbon alternative, reaching more than 2 EJ/yr in 2050 in its C600 scenario.

While some NDC scenarios indicate a relevant role for Liquefied Natural Gas (LNG) (2–5 EJ/yr in 2050), carbon budget scenarios do not show natural gas as a major energy alternative for shipping. As a fossil resource, LNG has a limited climate mitigation potential, and the investment in low-carbon fuels provides a more efficient decarbonisation pathway across models. TIAM-UCL is somehow an exception, using significant amounts of natural gas in carbon budget scenarios to cope with a fast-growing demand, which cannot be entirely met by renewable-based ammonia.

The demand for international shipping is likely to increase over the century, bringing an upward trend to its GHG emissions. In contrast, deep mitigation scenarios of the global energy system show that international shipping emissions must stabilize or decrease to be compatible with the long-term temperature goals of the Paris Agreement. With decreasing energy efficiency potentials, this must be achieved with the deployment of low-carbon fuels from 2030 onwards, reaching up to 88% of the sector’s final energy in 2050. While several fuel production routes and final energy carriers can be considered, they all face challenges upstream (e.g., high production costs, land use competition and low technological maturity) and/or downstream (e.g., low energy density and applicability to the existing fleet). This implies that no single fuel has an unlimited potential and that relying on a single option might limit the emission cutback of international shipping. As put into perspective by our results, models that represent only a few low-carbon maritime fuels tend to keep a larger share of fossil energy in vessels in carbon budget scenarios, possibly underestimating the true decarbonisation potential of the sector and contributing to higher reliance on CDR strategies. Drop-in biofuels (e.g., FAME-biodiesel, SVO, HVO-diesel) and renewable alcohols (e.g., ethanol and methanol) seem the most promising short-term candidates, while ammonia and synthetic energy carriers (especially lignocellulosic biofuels) become essential towards 2050 and beyond. International shipping has a high level of technological inertia, which means that if the industry is to be prepared for a significant amount of renewable energy from 2030 onwards, it is essential to start investing in low-carbon fuels and new motorisations as soon as possible.

Model exercise

Our work evaluates the role of international shipping as part of the much broader challenge of limiting global warming to relatively safe levels. To this end, we use IAMs to develop low-carbon scenarios for the energy and industrial systems (and, in some cases, for agriculture and land use systems as well), focusing our results analysis on international shipping, a sector whose modelling has been recently improved across these models.

Integrated Assessment Models

Integrated Assessment Models (IAMs) describe key processes in the interaction of human development and natural environment. Typically, they are designed to assess the implications of achieving climate objectives, such as limiting global warming to 1.5 or 2^oC^9,50. The six global models used in this study are COFFEE^51,52, IMACLIM-R^49,53, IMAGE^13,34, PROMETHEUS^52,54, TIAM-UCL^55,56 and WITCH^57,58.

COFFEE is a process-based IAM⁴¹ that utilizes intertemporal linear programming optimization with perfect foresight to model global energy, agricultural, and land-use systems. The energy sector is at the core of the model. Each of the model’s regions has a detailed representation of energy extraction and conversion technologies, and individualised estimates of energy resources (both in terms of volumes and costs) in the form of cost-supply curves. Divided into five main sectors (energy, industry, transportation, buildings, and agriculture), the model accounts for all primary energy produced by energy systems and its later transformation into secondary and final energy. The detailed modelling of international shipping is one of the most recent refinements in COFFEE. The approach used for modelling this sector focused on accurately representing the demand and potential alternative fuels. Shipping demand is based on the representation of 31 products/product groups, with most of them modelled endogenously. The energy modelling of ships is based on 10 illustrative motorizations, ranging from conventional two-stroke diesel engines to advanced electrochemical powertrains. Meanwhile, candidate fuels are grouped into eight categories, with each one potentially applicable to different powertrain types. In their turn, technological routes that produce these fuels are represented in each COFFEE region, ranging from technologically mature processes (e.g., oil refining, vegetable oil extraction) to advanced energy conversion processes (e.g., Fischer-Tropsch synthesis). The improvement of ship energy efficiency over time is modelled exogenously based on conservative assumptions. Detailed information on COFFEE can be found in subsection 1.2 of the Supplementary Information.

IMACLIM-R is a multi-sectoral Computable General Equilibrium (CGE) model representing the global economy as a set of 12 production sectors with input-output trade links. It is primarily based on macroeconomic theory, featuring consistent input-output accounting of both economic and physical energy flows. The demand for international shipping is influenced by the trade volume of physical goods but also by the price of freight transport. In its turn, this price is strongly influenced by energy carrier prices and energy efficiency. Maritime energy sources are determined by the relative prices of energy, also considering carbon taxation and exogenous hypotheses for energy efficiency improvement. Detailed information on IMACLIM-R can be found in subsection 1.3 of the Supplementary Information.

IMAGE is an intermediate complexity IAM that provides a process-oriented representation of human and earth systems, which are connected by emissions and land use. The model is driven by various factors, including demographic, economic, and technological development, as well as resource availability, lifestyle changes, and policy. The model's energy module simulates long-term trends in final energy use, depletion, energy-related GHG, and other air pollution emissions, as well as land-use demand for energy crops. The results are obtained using a single set of deterministic algorithms, which derive the system state in any future year entirely from previous system states. The model projects freight service demand using a constant elasticity of the industry value added for each transport mode, with international shipping being one of six freight transport modes. The competition between vessels with different energy efficiencies, costs, and fuel type characteristics is described using a multinomial logit equation. These substitution processes capture the price-induced energy efficiency changes, and over time, more efficient technologies become more competitive due to exogenous decreases in costs, representing the autonomous-induced energy efficiency changes. The model assumes that each type of vehicle uses only one fuel type. Therefore, this process also describes the maritime fuel selection. Detailed information on IMAGE can be found in subsection 1.4 of the Supplementary Information.

PROMETHEUS is a global energy system model covering in detail the interactions between energy demand, supply, and prices. Its main objectives are to assess mitigation pathways, analyse the implications of policy measures and quantify impacts of climate policies on energy prices. The model quantifies CO₂ emissions and represents abatement technologies and policy instruments (e.g., carbon pricing, efficiency standards). In terms of mathematical formulation, PROMETHEUS is a recursive dynamic simulation model, with investment and operation decisions mostly based on the current state of knowledge of parameters such as cost and performance. Recently, PROMETHEUS was enhanced with an improved representation of international shipping. Several technologies and emission reduction options were included in the model, with focus on low-carbon fuels. Maritime activity is split by segments (i.e., dry bulk, general cargo, container, and tankers). For tankers, the demand is endogenously estimated from interactions with the energy sector, while other segments have exogenous projections based on the literature. Emission reduction options include energy saving alternatives, speed reduction and the deployment of a wide range of alternative fuels. Detailed information on PROMETHEUS can be found in subsection 1.5 of the Supplementary Information.

TIAM-UCL is an energy-economy model of the global energy system that uses a linear programming optimization approach to explore cost-optimal systems. Features of its formulation include perfect competition and foresight. The representation of the global energy system encompasses primary sources from production through their conversion into final energy and utilization to meet service demands across a range of economic sectors. Using a scenario-based approach, the evolution of the system to meet future energy service demands can be simulated driven by the least-cost objective solution. Decisions around investments are determined based on cost-effectiveness, using the existing system in 2015 as a starting point. Energy resource potential, technological availability and policy constraints are other important aspects considered by the optimization. In TIAM-UCL, the transport sector is also fully based on this cost-optimisation paradigm, with international shipping being a part of the freight module. The demand for shipping is split by product group. For non-energy commodities, activity is exogenous, calculated and mapped using trade projections from an auxiliary sectoral model. For energy commodities, activity is endogenously estimated in TIAM-UCL, driven by the trade of fossil fuels and other energy carriers. Emission reductions are mostly achieved by the deployment of low-carbon fuels, whose selection is based on fuel and carbon prices. Ship and logistic efficiency are introduced exogenously in the model. Detailed information on TIAM-UCL can be found in subsection 1.6 of the Supplementary Information.

WITCH is a comprehensive tool designed to examine the interplay between climate change, energy systems, and economic development. It has a hybrid structure, combining top-down and bottom-up features. The top-down component includes a macroeconomic intertemporal optimization model while the bottom-up component captures technological details of the energy sector. The model generates optimal mitigation and adaptation strategies in response to climate damage or emission constraints. Strategies result from a maximization process involving regional welfare, capturing free-riding behaviours and interactions induced by externalities. An iterative algorithm implements the open-loop Nash equilibrium in a non-cooperative, simultaneous, open membership game with full information. The model uses a social planner to maximize the sum of regional discounted utility, with a constant relative risk aversion (CRRA) utility function derived from per-capita consumption. Climate impacts affect gross output, with fossil fuel and GHG mitigation costs subtracted from them. Energy services are provided by a combination of physical energy input and a stock of energy efficiency knowledge. Shipping demand for each region is the total global demand allocated with respect to its GDP share. Then, future demand is estimated using the elasticity of GDP. Elasticities are distinguished for different cargo types. The international shipping module within WITCH is currently in its early stages and remains highly aggregated. On the supply side, the maritime sector has access to conventional oil-based fuels and a few alternative fuels. Energy efficiency improvement is modelled exogenously. Detailed information on WITCH can be found in subsection 1.7 of the Supplementary Information.

Scenarios

We work with three sets of scenarios, resulting in a total of 18 scenarios. Reference scenarios (NDC) do not restrict total global emissions in any way but assume the fulfilment of Nationally Determined Contributions (NDCs) as stated in the 2015 pledges. These policies (e.g., GHG reduction targets, energy and land-use policies) are assumed to be fully implemented for the period 2010–2030 according to information from the NDCs^50,59 and considering the regional aggregation of each model. For the longer term (2030–2100), we assume that mitigation efforts continue at a pace consistent with that observed during the period covered by the NDCs. Demographic, socioeconomic, and technological assumptions follow the Shared Socioeconomic Pathway (SSP) no. 2, which describes a middle-of-the-road development in mitigation and adaptation challenges space. In many aspects, SSP2 can be seen as in line with historical trends^45,60.

The other two scenario groups (C1000 and C600) derive from the first one, but additionally impose carbon budgets of 1,000 and 600 GtCO₂ for the global economy in the period 2020–2100. Net negative CO₂ emissions (and therefore temperature overshoot⁹) are not allowed, meaning that budgets refer to the sum of annual net CO₂ emissions until the year of net zero (“peak budget” scenarios). The choice of carbon budget values is based on model capabilities and warming categories defined by the IPCC in its most recent assessment report^2,3 (see Supplementary Information – section 2). More stringent carbon budgets (e.g., 400 GtCO₂) were not assessed because most of our models do not find solutions for such low values.

In all three scenario groups, international shipping emissions are not restricted in any aprioristic way. As shipping is the focus of our analysis, leaving its emissions unconstrained is a way to compare the results of our models with existing sectoral targets such as IMO2050.

Results organization for shipping energy carriers

Since the modelling of fuel conversion processes is not identical across the six IAMs, we use energy carrier categories to harmonize and compare our results (see Supplementary Information – section 4). These categories seek to group energy carriers according to common features, such as feedstock type, energy density and applicability. The Conv category corresponds to conventional fuels based on petroleum, such as HFO and MDO. The Oilseed category represents fuels based on vegetable oils obtained from oily crops, such as palm, soybean, and sunflower, and eventually also from animal fats such as beef tallow. The D-synt bio and D-synt other categories include fully drop-in renewable fuels produced through advanced processes such as the Fischer-Tropsch synthesis that are often chemically indistinguishable from fossil products. While the former relates to biobased feedstocks, the latter includes every other type of resource, most notably renewable-based electricity. The three AG categories correspond to groups of alcohols and gases (e.g., ethanol, methanol, and LNG), whose use in ships is typically made possible using dual-fuel engines. As in the case of drop-in fuels, they differ from each other by the type of primary source. Finally, the H₂/NH₃ category includes hydrogen and ammonia, while the Elec category refers to the direct use of electricity.

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