Two types of perennial grasses offer the greatest global potential for producing lower-carbon aviation fuels, according to a new analysis. The study provides some of the most comprehensive evidence yet about which biofuel crops to grow and where in order to gain the greatest environmental benefits.
An international agreement, the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA), requires airlines to purchase carbon offsets or to swap out petroleum-based kerosene jet fuel for renewable biofuels. The aim is to cap the carbon emissions of international air travel at 2019 levels, and, ideally, achieve net zero international aviation by 2050.
But when it comes to renewable aviation fuel, the devil is in the details. “Although biofuels are made from renewable biological resources, such as crops and grasses, this does not necessarily make them less carbon intensive than fossil fuels,” says study team member Neus Escobar, an agricultural engineer and researcher at the International Institute for Applied Systems Analysis in Austria and the Basque Centre for Climate Change in Spain.
A big reason for that is that large-scale production of biofuels can require converting forests or grasslands to crop monocultures, potentially increasing carbon emissions by decreasing the amount of carbon stored in the soil and vegetation.
CORSIA’s benchmark is that sustainable fuels must reduce greenhouse gas emissions by at least 10% compared to fossil kerosene. But there’s no CORSIA-approved protocol for calculating greenhouse gas emissions from land-use change involved in biofuel production.
In the new study, Escobar and her collaborators used satellite and modeling data to calculate the land-use change emissions associated with growing six different biofuel crops – soybeans, maize (corn), switchgrass, miscanthus, jatropha, and reed canary grass – in different locations around the world. They mapped the global variability and yields of the different crops and their effect on soil and vegetation carbon, sketching out regions of the globe where each crop could be grown to make CORSIA-compliant biofuel.
Of the six crops, soybeans have the highest average land-use change emissions, the researchers report in the journal Science of the Total Environment. Jatropha, a tropical evergreen shrub, and miscanthus, a widely distributed group of grasses, have the lowest average land-use change emissions.
The researchers then linked their land-use change calculations with other data on carbon emissions across the life cycle of biofuels to determine where crops could be planted to yield fuels in accordance with CORSIA’s 10% benchmark. There are large swaths of the globe where miscanthus, switchgrass, and jatropha-based fuels could decrease greenhouse gases by the required amount, they found.
But again, details matter, such as local soil and climate conditions that enable high crop yields and minimal soil carbon loss. Then there’s the question of how much carbon the biofuel crop itself sequesters, whether the crop is annual or perennial, and how much fertilizer is needed to grow it.
“Our study highlights that not only the type of crop, but also the production location is critical, as this determines the achievable feedstock yields, the land uses that can be converted to biofuel feedstock, and the overall environmental performance in terms of resource use and emissions,” says Escobar.
The largest amounts of CORSIA-approved biofuels could be produced from miscanthus and switchgrass, a grass species native to North America, Escobar and her collaborators found. But at most, such fuels could take the place of only about one-fifth of the current kerosene market – leaving that net-zero 2050 goal very much up in the air.
Source: Escobar N. et al. “Spatially-explicit land use change emissions and carbon payback times of biofuels under the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA).” Science of the Total Environment 2024
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