Extracting carbon dioxide from well mixed air is not just technically difficult, it's expensive as well.
On the roof of a large recycling centre at Hinwil, Zurich, stand 18 metal fans, stacked on top of each, each about the size of a large domestic washing machine. These fans suck in the surrounding air and chemically coated filters inside absorb the CO2. They become saturated in a few hours so, using the waste heat from the recycling facility, the filters are heated up to 100C and very pure carbon dioxide gas is then collected.
This installation, called a direct-air capture system, has been developed by a Swiss company called Climeworks. It can capture about 900 tonnes of CO2 every year. It is then pumped to a large greenhouse a few hundred metres away, where it helps grow bigger vegetables.
Right now Climeworks is selling the gas to the vegetable growers next door for $600 per tonne, which is very expensive. The firm believes that like solar and wind energy, costs will rapidly fall once production is scaled up.
One of the things about CO2 that makes it attractive for developers is that it has many uses in the world. From fish food to concrete; from car seats to toothpaste - entrepreneurs are trying to use carbon dioxide as a raw material. There's also a roaring trade in CO2 in the US, where it's being used, without irony, to boost the extraction of oil from wells. One of the most ambitious plans is to extract CO2 and turn it into fuel. Driving down the price of capturing CO2 is key to making this idea work.
Making fuel or other products out of CO2 might help but it won't achieve the type of large scale take-down from the atmosphere that many scientists now fear will be necessary over the next 20-30 years if the goals of the Paris climate agreement are to be met.