The idea of burying charcoal produced from microwaved wood to tackle global warming is still beset with scientific uncertainties, says the UK government's first report on "biochar".
The warning comes as a separate international study published this week said that as much as 12% of global greenhouse gas emissions could be offset by biochar.
Biochar involves burying cooked charcoal so that the carbon dioxide absorbed during the tree's growth remains safely locked away for thousands of years. The technique could remove billions of tonnes of CO2 from the atmosphere every year. But it has divided environmentalists, with backing from Gaia theorist James Lovelock and Nasa scientist James Hansen, but opposition from critics who say there is not enough to land carry out biochar on a large scale.
The report, commissioned by the government, looks at the stability and potential benefits of biochar in soil, as well as at the risks, including the danger of contaminants getting into soil. In addition to economic and logistical challenges, the report noted there are many scientific uncertainties about the role biochar might play. It is not clear how long the carbon would stay trapped in the soil or whether it enhances the quality of the soil as supporters claim, said Saran Sohi, leader of the University of Edinburgh's UK Biochar research centre and one of the report's authors.
However, the report concludes "biochar has a high carbon abatement efficiency" and calls for more pilot schemes that could potentially "advance rapidly the science, engineering, regulation and socioeconomic evaluation of biochar systems in the UK context".
The other study, Sustainable biochar to mitigate global climate change led by scientists in the UK, US and Australia, paints a more positive picture. Biochar could offset 1.8bn tonnes of carbon emissions annually in its most successful scenario, it said, without endangering food security, habitat or soil conservation.
"These calculations show that biochar can play a significant role in the solution for the planet's climate change challenge," said the study's co-author Jim Amonette, a soil chemist at the Department of Energy's Pacific Northwest National Laboratory. "Biochar offers one of the few ways we can create power while decreasing carbon dioxide levels in the atmosphere. And it improves food production in the world's poorest regions by increasing soil fertility. It's an amazing tool."
Separately, biochar experts said last week that global sustainability efforts are being hampered by an emphasis on English language research.
At a seminar in Tokyo, Professor Shinogi Yoshiyuki from Kyushu university, said researchers into sustainable technologies around the world needed to share information.
"This is a global issue and we need to create a global network [of information]," he commented. Yoshiyuki is vice-president of the Japan Biochar Association, which was set up last year to highlight the results of 30 years' research into biochar technology in Japan.
Sohi, who was at the seminar along with sustainability experts from the UK's Department of Energy and Climate Change, said his team's visit to Japan was "a real opportunity to collaborate and link up research".
He acknowledged that his report was based largely on English-language biochar research, but said there has been a willingness in both Japan and the UK to collaborate in researching the technology further.
• Jane Dudman's travel expenses in Japan were paid for by the Council of Local Authorities for International Relations. They had no say in the content of this article.
• This article was amended on 17 and 18 August. The original referred to a separate US study published this week. This has been corrected.
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