Aviation could consume almost all available biofuel for decarbonization – maritime shipping needs to broaden its own strategy
The aviation industry and the maritime industry have both made commitments to reduce their net greenhouse emissions to near-zero by midcentury. A variety of measures of varying effectiveness and reliability can be deployed to lower the industries’ net emissions – efficiency improvements, mode-shifting, on-shoring, and out-of-sector offsets, to name a few – but the most important, most impactful step will be to deeply reduce the carbon intensity (CI) of the fuels that power airplanes and marine vessels.
A wholesale shift to low carbon intensity energy carriers poses huge quantitative and qualitative challenges for the two industries, particularly because they need to move in concert. The challenges associated with the shift and the tools that might be used to address them are connected in ways that have important implications for each sector’s decarbonization strategy. One such implication has to do with biofuel, an energy carrier that is central to both industries’ plans, in particular because biofuels can be blended with conventional shipping and aviation fuel. In recent model runs that consulting firm DNV prepared for the International Maritime Organization (IMO), maritime sector demand for biofuel reaches 2.0-7.8 Exajoules (EJ) by 2050 – or roughly 47% to 181% of the 4.3 EJ of biofuels currently produced worldwide.1
The problem, in short, is that the amount of biofuel the two industries expect to use going forward far exceeds the amount of existing biofuels, and scaling new climate-beneficial biofuels and supply chains entails significant challenges. Airlines have fewer alternatives when it comes to energy carriers so they will be highly motivated to outspend marine shipping companies (and every other industry) for all the biofuel they can get. The implication for the maritime sector is clear: the role that biofuels can play in its energy transition will be small at most.
As the International Maritime Organization finalizes a set of industry-wide, midcentury decarbonization targets this month, a measure of its commitment to those targets will be the extent to which it acknowledges the competing demands that will frustrate biofuel-based strategies and prioritizes the development of other low-carbon energy carriers and compatible propulsion systems.
Massive demand for biofuel in both industries faces limited supply and climate-beneficial supply challenges
The total amount of energy that the aviation sector and the maritime sector will need in 2050 is projected to be about 38 EJ – 24 EJ for aviation and 14 EJ for maritime. Any number of things – upturns or downturns in the global economy, major shifts in demographics, policy, or consumer behavior – could cause the actual level of energy demand from the two sectors to be substantially higher or lower than 38 EJ, but the challenge remains unavoidably stark: the aviation and maritime sectors are set to consume roughly the same amount of energy each year as India currently does, and all that energy will need to be clean.
And not just any form of clean energy will do. The process of moving a cargo-laden, 200,000-tonne vessel across ten thousand kilometers of ocean favors energy-dense fuels, especially because the amount of space dedicated to on-board energy storage affects the amount of paying cargo a ship can carry. Aircraft have even less leeway. Because liquid fuels that pack a lot of energy into a small and lightweight package help mitigate the fundamental difficulty of getting and staying aloft, energy-dense hydrocarbons power the vast majority of aviation applications and will continue to do so for the foreseeable future. Battery-powered aircraft are under development, but they face formidable constraints around size and range.
For years, then, the aviation industry has looked to biofuel as the energy carrier it could lean on to achieve deep emission reductions. It popularized a generic-sounding term for the fuel – “sustainable aviation fuel” or SAF – that appropriates biofuels’ promise of a carbon-neutral, closed-loop future while also keeping some distance from biofuel’s real challenges. Regardless of how it’s marketed, though, SAF made from biogenic matter is still biofuel – which means that bio-SAF production at scale poses many of the same challenges to food markets, water quality, and ecosystem health that have made the mass production of ethanol and biodiesel for on-road vehicles so problematic.
The environmental benefit of conventional biofuels on a per-unit basis typically gets smaller as overall production levels get bigger. Fuels made from wastes like crop residue and other advanced feedstocks can deliver real reductions in net greenhouse gas emissions, but the supply of suitable waste biomass is limited, and it remains to be seen if other advanced feedstock supply chains can be significantly scaled. The vast bulk of biofuels made today come from commodity oilseeds and starches like soy and corn grown with emitting inputs on prime farmland. Producing exajoules worth of biofuel is easier and cheaper when the feedstocks are cultivated, harvested, and processed on huge tracts using industrial agricultural equipment and practices. Sustainable intensification and strategic perennialization of agricultural land could increase climate-beneficial feedstock supply, but expanding biomass supply chains could cause direct or indirect conversion of grasslands and forests into agricultural land, depending on how the biomass is grown.
As detailed by CATF in Decarbonizing Aviation: Challenges and Opportunities for Emerging Fuels (2022), the International Energy Agency (IEA) published a determination that the global supply of “sustainable” biofuel could roughly triple between 2021 and 2030. Setting aside doubts that such an increase could occur without driving significant land use change-related GHG emissions, CATF compared IEA’s 2030 supply estimate to the projected energy demand from the aviation sector and found that the entire global supply of biofuels would amount to 69% of the energy that aircraft will need in 2030, 59% in 2040, and 52% in 2050. A subsequent report from CATF, Decarbonizing Aviation: Enabling Technologies for a Net-Zero Future (2024), outlines the extent to which the aviation industry will need to rapidly develop and deploy other energy carriers like synthetic SAF and hydrogen, given that all the world’s liquid biofuel can only meet about half of the sector’s mid-century energy demand. The process of commercializing synthetic hydrocarbons will be slow and expensive; the same is true for introducing hydrogen aircraft. Employing high-quality offsets from direct air capture or biomass carbon removal systems (which may produce hydrogen as a co-product) will also require a significant amount of time to scale. As such, the aviation industry will want to mitigate the related financial and logistical challenges by maximizing its use of bio-SAF, so it is unlikely to get outbid for the available supply of biofuel, especially by industries that can turn to other options.
The marine shipping industry must look beyond biofuels to decarbonize
Fortunately, marine shipping is one of those industries that has enough operational flexibility to avoid getting into a bidding war with the aviation sector. In Managing the Transition to Zero-Carbon Marine Fuels (2024), CATF shows how ship owners can gradually but significantly reduce their emissions by deploying dual-fuel engine technologies and phasing in low-carbon fuels.
For a variety of reasons, ammonia may be a compelling option for fuel-shifting in the maritime shipping sector. The production of ammonia is massively scalable because its constituent parts – nitrogen and hydrogen – are nearly inexhaustible (although isolating hydrogen is energy-intensive). However, like biofuels, optimal engineering practices and management strategies must be implemented to avoid high reactive nitrogen losses, which could negate any GHG emissions reductions. Ammonia can be used as fuel in both purpose-designed and modified internal combustion engines (ICEs), either neat or in a blend with petroleum fuel. Marine vessels are especially well-positioned to use ammonia-fueled ICEs because they can accommodate heavier engines and larger fuel tanks more easily. Many ports already site and build ammonia storage and handling equipment, which could help lessen the chicken-or-egg problem that often confounds the transition to alternative transportation fuels. Finally, more work needs to be done to test ammonia’s safety as an energy carrier, but marine vessels are already fueled and operated by professionals who can be trained to handle it safely.
Turning marine sector targets in achievable strategy
Country representatives, industry groups, and nongovernmental organizations are gathering in London for the 83rd convening of the International Maritime Organization’s Marine Environment Protection Committee. The agenda is anything but dry: the delegates are set to approve measures “aimed at driving the international shipping industry’s transition to achieve net-zero GHG emissions by or around, i.e. close to, 2050.” The measures to be endorsed are a global pricing mechanism for greenhouse gas emissions and “a goal-based marine fuel standard that will phase in the mandatory use of fuels with less GHG intensity.”
The carbon intensity standard will be a powerful tool for eliminating maritime sector emissions, provided the IMO designs the standard to account for global, multi-sector trends in the energy market. Because one of those trends is the growing mismatch between demand for low-carbon biofuels and the volume at which biofuels can be sustainably produced, a carbon intensity standard for the maritime sector that assumes significant access to biofuels is destined to fail.
The maritime sector’s new midcentury targets are commendable. But targets require execution, and execution requires commitment and foresight. Industry and governments can demonstrate their commitment to the mid-century targets by supporting the development of viable decarbonization pathways that don’t depend on access to biofuel (e.g., by limiting the extent to which biofuels can earn compliance credits under a clean fuel standard or by subsidizing the use of synthetic fuels), and they can demonstrate foresight by steering investment toward dual-fuel engines, on-board storage technologies, bunkering facilities, and other technologies that support the transition to ammonia and/or other massively-scalable low-carbon fuels.
Why is the biofuel supply crunch so often disregarded?
Industry leaders and policymakers overlook or ignore biofuels’ inability to meet a significant amount of the energy demand from multiple sectors for a handful of reasons:
- Consumption levels of non-conventional fuel are still very low, so the implications of an eventual supply shortfall do not feel immediate.
- Farm economies are under pressure around the world, and policymakers are reluctant to discuss limits to its growth.
- Lifecycle carbon accounting for biofuels is complicated, confusing, and controversial. Disagreements abound with respect to the total supply of biomass that can be used to make low-carbon energy, and producers and users have relied on the resulting uncertainty to make it appear that biofuels can serve much more demand than is likely possible.
- Sectoral decarbonization plans are being developed independently without sufficient consideration of other sectors’ needs, strategies, and relative market power.
1 Comprehensive Impact Assessment of the Basket of Candidate Mid-Term GHG Reductions, DNV for IMO, July 2024 (MEPC 82/INF.8/Add.1)
