Alok Jha of the Guardian wrote about Air New Zealand’s trial of jet fuel based on jatropha berries here. This note looks at the percentage of the world’s land area that would have to be devoted to the crop in order to provide for the total needs of aviation, an industry that uses about 5% of the world’s oil.
Background Jatropha is a medium-sized tropical shrub that will grow on a wide variety of soils including saline degraded land. Its berries contain about 40% oil which can be used as a potential substitute for both diesel and kerosene. Its proponents claim that because it will grow on poor tropical soil and can tolerate drought and some frost it will not drive food crops off productive land.
Opponents of jatropha point to production data from field trials which show that although the mature shrub will yield 2 tonnes per hectare on poor soils it will give up to 12.5 tonnes in better conditions and where water and fertiliser are applied. Their contention is that this yield difference will mean that large-scale farming of jatropha will inevitably gravitate to good land on which food would otherwise be grown.
How much land would be required to provide enough jatropha oil for the world’s aviation fleet, if it is grown on poor quality land?
|Total demand for aviation kerosene||About 240 million tonnes per year (extrapolated from OECD use)|
|Jatropha berry yield||2 tonnes per hectare|
|Processing losses (estimate)||15%|
|Kerosene replacement per hectare||0.68 tonnes|
|Number of hectares needed to replace kerosene||About 350 million hectares|
|Percentages of world land area|
|Percentage of all land area||About 2.5%|
|Percentage of all arable land||About 18%|
|Percentage of all pastoral land||About 9%|
So it is conceivably possible to grow enough jatropha to provide all the world’s needs for aviation fuel. It would require the berry to be grown on about 2.5% of all the globe’s land area. About 13% of the world’s area currently grows crops and if jatropha were grown on this land, it would use almost one fifth of total arable land. (Typical unirrigated yields would probably be substantially higher – perhaps 4 tonnes per hectare – so this calculation is somewhat unfair. The percentage of arable land used might be lower than 10%.)
Aviation fuel uses about 5% of the world’s oil so to replace the world’s entire demand for crude would require almost all the world’s non-desert land to be devoted to this single crop. This is not surprising – the energy we use from oil each day is about twenty times the energy we use from food. So if we grow oil instead of food, we would need twenty times as much land.