However, Lackner thinks the gas is too useful to petrify. His idea is to use the carbon dioxide to make liquid fuels for transport vehicles. Carbon dioxide can react with water to produce carbon monoxide and hydrogen – a combination known as syngas because it can be readily turned into hydrocarbon fuels such as methanol or diesel. The process requires an energy input, but this could be provided by renewable sources, such as wind energy, Lackner suggests.
We have the technology to suck carbon dioxide out of the air – and keep it out – but whether it is economically viable is a different question. Lackner says his trees would do the job for around $200 per tonne of removed carbon dioxide, dropping to $30 a tonne as the project is scaled up. At that price – which has been criticised as wildly optimistic (the American Physical Society's most optimistic calculations for direct air capture are $600 per tonne of carbon dioxide removed, although the UK's Met Office is more favourable) – it starts to make economic sense for oil companies who would pay in the region of $100 per tonne to use the gas in enhanced oil recovery.
Ultimately, we have to decide whether the cost of the technology is socially worth the price, and that social price is likely to fall as climate change brings its own mounting costs. Economically too, if the price of carbon rises, then this could lead to two effects. Investing in air capture will likely be seen as an equivalent to "avoided emissions". And then it will become a worthy investment.