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A STAR attraction at the is a tree. Not an elegant product of evolution, but something that looks rather like a steampunk collision of an industrial air-conditioning unit and an accordion. What researcher Klaus Lackner’s mechanical tree has in common with the natural variety, however, is that it is great at sucking carbon dioxide out of the air.

We are going to need a lot of that in the coming decades if we are to achieve net-zero carbon emissions by mid-century and so head off the worst of the climate crisis. The key word here is “net”. Even when we have wiped out all the emissions we can, intractable sources will remain, from the likes of food production, flying and heavy industry. Negative emissions technologies are intended to bridge the gap – by removing CO₂ already in the atmosphere.

This past year, individuals and companies from to and have committed significant sums to various schemes to do just that. But they are controversial. Campaigner Greta Thunberg recently derided governments for pinning their climate plans on “fantasy-scaled” versions of “barely existing” technologies. Even if they can scale up, there are concerns over whether the cure would be worse than the disease, due to potential downsides of negative emissions technology for biodiversity, water consumption, food production and energy use. Time to ask: when it comes to carbon removal, do we really know what we are doing?

As last week’s report from the Intergovernmental Panel on Climate Change (IPCC) made plainer than ever before, we are …