Decarbonizing Heavy-Duty Transport: Challenges, Synergies, and Progress

Heavy-duty transport occupies a critical position in the global energy transition, enabling trade, mobility, and industrial value chains, while also being among the most difficult sectors to decarbonize. With transport volumes continuing to grow, the challenge is defined by two binding capacity constraints.

The first is access to clean energy. While electrification remains the most energy-efficient end state for the main transport modes where technically viable, its role in heavy-duty transport is constrained by infrastructure costs and long fleet asset lifetimes. In addition, as electrification accelerates across multiple sectors, many regions are likely to face persistent grid-capacity constraints through to 2050. As a result, molecular fuels, such as biofuels and hydrogen-based fuels, are expected to carry the bulk of the decarbonization burden in heavy‑duty transport through the mid‑century.

Road transport dominates inland freight and accounts for around 11% of global emissions. According to the International Road Transport Union (IRU), roughly 80% of the road transport pathway to carbon neutrality is expected to come from battery-electric propulsion, supported by a progressively decarbonized grid. However, global fleet turnover implies that diesel trucks will remain in operation for at least one to two decades, even under accelerated transition scenarios, stressing the importance of drop-in fuels, efficiency, and operational measures rather than wholesale drivetrain change.

Aviation, maritime shipping, and rail freight share this reliance on molecular fuels. Over the next one to two decades, deep emissions reductions will depend primarily on sustainable fuels and hydrogen, rather than full electrification.

From a system-level perspective, there is a case for shifting freight from road to more energy-efficient modes, notably rail and maritime transport, wherever possible. Such a shift would materially reduce overall energy demand, as road transport is the least energy-efficient freight mode, while rail is five to nine times more energy-efficient. Across all modes, there is an opportunity for increasing technical and operational efficiency.

Taken together, scaling sustainable biomass and renewable hydrogen are critical to enabling the pathway to net zero for heavy-haul transport.

The second capacity constraint lies in the transport system itself. In many major cities — which serve as key terminals and bottlenecks in global supply chains — road capacity is already saturated, and further expansion is either infeasible or would require substantial emissions. Rail offers considerably higher space efficiency: A double-track railway can carry freight volumes comparable to roughly 15–20 motorway lanes, depending on train length and utilization. Coastal and river shipping, wherever geographically possible, can also help decongest the road network in and around major cities, particularly when combined with rail terminals for onward transport.

Aviation Route

For the foreseeable future, aviation decarbonization will be dominated by fuel substitution rather than propulsion electrification. Sustainable aviation fuels and, in the longer term, hydrogen-based propulsion combined with efficiency improvements represent the principal mitigation levers, with outcomes tightly coupled to the availability of low-cost sustainable biomass and green hydrogen across the wider energy system.

Aviation is currently responsible for approximately 2 to 2.5% of global energy-related CO₂ emissions, equivalent to roughly 800 to 900 million tonnes annually, with emissions having largely rebounded to pre-pandemic levels. Its climate impact extends beyond CO₂ through nitrogen oxides and non-CO₂ effects such as contrail formation, which increase total radiative forcing. The state of the art in aviation decarbonization remains dominated by incremental efficiency improvements and fuel substitution rather than direct electrification.

Battery-electric aircraft are limited to very short-range applications due to energy-density constraints, leaving sustainable aviation fuels (SAF) as the primary near- to medium-term mitigation option. However, SAF currently accounts for well below 1% of global jet fuel consumption, far short of the shares assumed in net-zero pathways, which typically require 20 to 30% SAF penetration by 2040–2050. IATA expects 60% of the pathway to net zero to be attributed to SAF, 25% by carbon removal (including MBM), and 3 to 6% each from efficiency improvements, hydrogen, and operational measures, respectively.

Aircraft fleets have long lifetimes, and certification processes are slow and rigorous. Alternative zero-carbon propulsion systems, such as hydrogen combustion or fuel cells, remain confined to sub-regional or aircraft carrying 10 passengers or fewer. Battery electrification is limited to short flights (around 45 minutes) for light-sport aircraft. At the same time, SAF production is constrained by limited sustainable feedstocks, high capital costs, and competition for renewable hydrogen with other sectors such as steel, chemicals, and power storage.

These constraints highlight a critical systems insight for policymakers: Aviation decarbonization is inseparable from the scale and cost trajectory of renewable electricity and hydrogen. Power-to-liquid synthetic fuels, in particular, depend on abundant low-cost renewable power, linking aviation outcomes directly to broader energy-sector investment decisions. Recent developments, including SAF blending mandates, long-term offtake agreements, and public support mechanisms, signal policy momentum, but supply growth remains well behind stated climate targets. IATA expects SAF capacity to ramp up until 2030, but even this would represent only around one-tenth of the volume required by 2050.

Maritime Deep Dive

While maritime fuels do not have a lower carbon intensity per unit of fuel than aviation fuels, shipping remains far less carbon-intensive per tonne-kilometre due to its high energy efficiency. Measures to further increase energy efficiency are under implementation. Furthermore, electrification is expected to be implemented for short-haul voyages and on-board electricity demand. However, the brunt of climate performance is expected to be taken by scalable low-carbon fuels, including biofuels and hydrogen-derived options such as methanol and ammonia.

International shipping accounts for roughly 2 to 3% of global greenhouse gas emissions and carries over 80% of global trade by volume. The International Maritime Organization began regulating shipping emissions in the early 2000s with mandates to improve technical and operating efficiencies.. Efficiency gains through vessel design, slow steaming, and route management have moderated emissions intensity. However, disruptions on global shipping routes originating from conflicts, droughts, and regional politics have increased the miles sailed per voyage. As a result -total emissions have continued to track global trade growth.

Decarbonizing shipping is complicated by fuel uncertainty and infrastructure lock-in. Multiple fuel pathways — including ammonia, methanol, biodiesel, ethanol, pyrolysis oils, hydrogen, and partial electrification — are under active development, each with different implications for vessel design, safety standards, bunkering infrastructure, and upstream energy supply. This diversity harbors both opportunity and risk: while it allows innovation, it also delays convergence and raises the cost of global coordination. For global shipping, this reinforces the importance of multilateral frameworks, such as those under the IMO, to provide regulatory clarity and a global cost of carbon.

There is a significant uncertainty as to the best decarbonization options for the industry: While direct electrification is the most energy efficient choice, current battery energy density limits its use to short-haul voyages. However, renewable energy is already increasingly applied to decarbonize port operations and batteries are increasingly integrated into hybrid systems to meet on-board power demands. Several types of biofuels are already enabling the energy transition. However, competing demand from other industries makes it unlikely that sustainable biomass will be available in sufficient amounts for all sectors. Furthermore, while biomethane and waste based biodiesel are economically viable, other waste-derived biofuels remain too expensive or technically immature. Electrofuels, obtained from electrolytic hydrogen, are expected to be required to bridge the gap to zero emissions. However, these require large volumes of renewable electricity and water, and are often associated with massive infrastructure projects for on- and off-shore wind, solar, and industrial hydrogen hubs. As for biofuels, the sustainability of electrofuels cannot be taken for granted. Recent years have seen the first commercial orders for methanol- and ammonia-ready vessels, pilot bunkering infrastructure, and expanded shore-power mandates, indicating a transition from concept to early deployment. However, fuel availability at scale remains the dominant constraint.

Rail: A Road to Net Zero

At first glance, rail stands apart as the most mature example of transport electrification. Rail is energy-efficient, carries a significant share of passenger and freight activity, while accounting for a disproportionately small share of transport emissions. In regions with extensive electrification, rail operations are already largely decarbonized at the point of use, particularly in dense passenger networks, where track utilization is high. However, most long-distance heavy-haul freight remains unelectrified outside Western Europe, India, and China.

In regions such as North America, Russia, and Australia (where rail is the backbone of inland freight with market shares ranging from 50-90%) and in Africa and Latin America (where around 85% of inland freight still moves by road) freight rail often operates on low-utilization, non-electrified lines, and full electrification faces weak economics and long carbon payback times. In regions with carbon-intensive grids (e.g. in Poland more than two-thirds of the energy production comes from coal and gas power plants), further electrification infrastructure would increase CO2-emissions compared to drop-in biofuels until grids are decarbonized.

As a result, the rail industry is pursuing a dual approach: electrifying where economically and climatically optimal — primarily on passenger or mixed‑traffic lines — while developing advanced energy-management systems and efficient onboard energy sources such as batteries, biofuels, and hydrogen for low-density operation.

Battery technology continues to improve, but current ranges between recharges are insufficient for most freight use cases, and the highest-energy-density chemistries rely on critical minerals. For the medium term, original equipment manufacturers (OEMs) invest heavily in hydrogen-powered locomotives. In the short term, rail shares key structural challenges with aviation and maritime transport. Drop-in biodiesel and hybrid trains serve as transitional options, using modular designs that enable future conversion to hydrogen-based propulsion.

Despite these challenges, rail retains strong inherent emission advantages. It is five to nine times more energy-efficient than road transport and far more land-use efficient: a double-track corridor can move the same volume of goods as an 18-lane motorway.

A shift to rail is a road to net zero.

Cross-Sector Strategies for the Paris Outcomes

Viewed through a system-level lens, aviation, maritime shipping, road, and rail freight converge on a common decarbonization reality: Over the coming decades, access to sustainable biomass and renewable hydrogen is likely to be a limiting factor for deep emissions reductions in heavy-duty transport. While direct electrification remains the most energy-efficient solution where feasible, its contribution is structurally limited in these modes by grid capacity, infrastructure costs, utilization rates, and long asset lifetimes.

This makes the allocation, certification, and scaling of sustainable fuels a central policy challenge — not only for transport, but also for agriculture, industry, and power systems competing for the same resources.

For global policymakers and energy leaders, including the United Nations and UNIDO, heavy-duty transport represents anchor demand for renewable electricity, green hydrogen, and sustainable fuels, requiring integrated energy-industrial strategies rather than isolated sectoral interventions. This is essential to accelerate clean industrial development and deliver Paris-aligned outcomes at the scale required.

Dr. Anita Sengupta is the CEO/Founder of Hydroplane Ltd. Roberta Cenni is the Head of Biofuels at the Mærsk Mc-Kinney Møller Center for Zero Carbon Shipping. Ulf Larsson is the Technical Director of Global Rail & Transit at WSP Global Inc.

References

1. Intergovernmental Panel on Climate Change (IPCC). Sixth Assessment Report, Working Group III: Chapter 10 – Transport. IPCC, 2022. https://www.ipcc.ch/report/ar6/wg3/chapter/chapter-10/
2. International Air Transport Association (IATA). Net Zero Roadmap: FlyNetZero. IATA, 2023. https://www.iata.org/en/programs/sustainability/flynetzero/roadmaps/
3. International Civil Aviation Organization (ICAO). ICAO Environmental Report 2022. ICAO, 2022. https://www.icao.int/icao-environmental-report-2022
4. International Council on Clean Transportation (ICCT). Global Shipping Greenhouse Gas Emissions. ICCT, 2023.
5. International Energy Agency (IEA). Aviation – Energy System. IEA, 2023. https://www.iea.org/energy-system/transport/aviation
6. International Energy Agency (IEA). Aviation and Climate Change. IEA, 2023.
7. International Energy Agency (IEA). Rail – Tracking Clean Energy Progress. IEA, 2023.
8. International Energy Agency (IEA). The Role of Ports in Energy Transitions. IEA, 2023.
9. International Maritime Organization (IMO). Fourth IMO Greenhouse Gas Study 2020. IMO, 2020.
10. International Maritime Organization (IMO). IMO Strategy on Reduction of GHG Emissions from Ships. IMO, 2023.
11. International Renewable Energy Agency (IRENA). Decarbonising Shipping: All Hands on Deck. IRENA, 2021.
12. International Road Transport Union (IRU). Green Compact: Delivering Decarbonisation and Development for a More Sustainable World. IRU, 2023. https://www.iru.org/system/files/IRU%20Green%20Compact%20-%20Delivering%20decarbonisation%20and%20development%20for%20a%20more%20sustainable%20world.pdf
13. International Road Transport Union (IRU). Pathway to Carbon Neutrality. IRU, 2023.
14. International Union of Railways (UIC). Global Rail Sustainability Report 2023. UIC, 2023. https://uic.org/IMG/pdf/2023_global_rail_sustainability_report_web_1_1_.pdf
15. International Union of Railways (UIC). Global Rail Sustainability Report 2024. UIC, 2024. https://uic.org/IMG/pdf/2024_global_rail_sustainability_report_v3.pdf
16. REN21 Secretariat. Renewables Global Status Report 2025 – Transport Factsheet. REN21, 2025. https://www.ren21.net/gsr-2025/downloads/pdf/demand/GSR_2025_Factsheet_Demand_Transport.pdf
17. United Nations Framework Convention on Climate Change (UNFCCC). Transport and Climate Change. UNFCCC, 2022.
18. United Nations Industrial Development Organization (UNIDO). Industrial Decarbonization and the Role of Hydrogen and Sustainable Fuels. UNIDO, 2023.

About the Council of Engineers for the Energy Transition (CEET)

This article is part of the Energy Insights Series published by the Council of Engineers for the Energy Transition (CEET). The CEET is a global, high-level body of engineers and energy systems experts, created under the auspices of the United Nations Secretary-General, with the goal of building coalitions and energy pathways for comprehensive decarbonization.

It should be acknowledged that these materials are for discussion purposes only, given the rapidly changing landscape of the energy transition and the various contexts in which they are relevant. CEET members are participating in their individual capacity and expertise without remuneration. Their professional affiliations are for identification purposes only, and their views and perspectives, including any statements, publications, social media posts, etc., are not representative of the United Nations, SDSN, or UNIDO.