Large-scale plantations of the hardy jatropha tree, Jatropha curcas, could help sequester carbon dioxide through a process known as 'carbon farming', according to a study based on data gathered in Mexico and Oman that was published in Earth System Dynamics last month (31 July).
Each hectare of the tree could soak up 17-25 tonnes of carbon dioxide a year, they say, at a cost of 42-63 euros (about US$56-84) per tonne of gas, the paper says. This makes the technique competitive with high-tech carbon capture and storage.
Klaus Becker, the study's lead author and director of carbon sequestration consultancy Atmosphere Protect, says that a jatropha plantation covering just three per cent of the Arabian Desert could absorb all the carbon dioxide produced by cars in Germany over two decades.
"Our models show that, because of plantations, average desert temperatures go down by 1.1 degree Celsius, which is a lot," Becker says. He adds that the plantations would also induce rainfall in desert areas.
Jatropha, which is a biofuel crop, needs little water, and coastal plantations would be irrigated through desalination, Becker says.
He also envisages a role for sewage in such large-scale plantations.
"There are billions and billions of litres of sewage that are discharged into the oceans every week, but instead we could send that water to the desert and plant trees," he says. "In this situation, you wouldn't need any expensive artificial nitrogen [to fertilise the trees]."
The team has also been working in Israel's Negev desert, where they planted 16 tree species, which, they say, is preferable to a jatropha monoculture. "A diversity of trees is good for the environment, good for investors and good for preventing diseases," says Becker.
At another of the team's carbon farms — a jatropha plantation in Madagascar — the organic matter content of degraded soil has risen from 0.2 per cent up to three per cent.
Local people now harvest beans planted between the trees, providing a vital source of protein and creating a symbiotic exchange of nitrogen — fixed from air by beans — and shade provided by the jatropha trees.
"Previously, no one had the idea of using uncultivated land to plant these kinds of leguminous beans because they would not grow there. But after four or five years of applying cultivation techniques, the soil quality increases dramatically," Becker says.
Alex Walker, a research assistant at the Centre for Environmental Policy at Imperial College London, United Kingdom, describes carbon farming as a "common-sense approach to rising carbon dioxide levels, with potentially positive biodiversity impacts".
He adds: "It will grow on non-arable land, and so not compete with food production, but it is more difficult to process and subject to varying yields and absorption volumes".
Egypt is pioneering an experiment in desert farming, using sewage water after basic treatment to produce wood, woody biomass and biofuel crops, such as casuarina, African mahogany, jojoba and neem, in addition to jatropha.
"In Egypt, there are 15,000 acres planted with trees of good quality but so far they have not been sold to create economic value," Hany El Kateb, a professor at the Technical University of Munich in Germany, tells SciDev.Net.
According to El Kateb, Egypt produces more than 6.3 billion cubic metres of sewage water a year, and 5.5 billion cubic metres of this would be sufficient to afforest more than 650,000 hectares of desert lands and store more than 25 million tonnes of carbon dioxide annually in new forests.
El Kateb points out that Egypt has an advantage over European countries that are leaders in forestry, such as Germany, because the same trees grow more than 4.5 times faster in Egypt where the sun shine most of the year.
But Mosaad Kotb Hassanein, director of the Central Laboratory for Agricultural Climate in Egypt, says: "One of the big challenges of planting forests in arid areas is the lack of experience, expertise and technical personnel involved in the establishment and management of forest plantations.
"The project in Egypt was lucky to have technical assistance and support establishing a forest administration from the German Academic Exchange Service."
Additional reporting by Nehal Lasheen.
Link to full paper in Earth System Dynamics