Carbon dioxide is a major environmental problem, and it can be damaging to human health. But it can also be extremely useful. We use it to package salad leaves, fizz our soft drinks, render animals unconscious before they are slaughtered, and more.

Yet much of Europe has lately been suffering from a CO₂ shortage – ironic, given that it’s in the air. An obvious question has therefore arisen: why don’t we pull it directly out of the air? The answer is that CO₂ only makes up 0.04 per cent of our air, and is therefore difficult to extract directly. The research into carbon (dioxide) capture as a means of countering CO₂ emissions has not yet produced a technique that'll extract the quantities needed to support the gas industry. Instead, the main source of CO₂ for industry is a by-product of nitrogen-based fertiliser production. This industry is also heavily reliant on natural gas, so when the gas prices climbed this autumn, fertiliser plants shut down to save on costs. The knock-on
effect was a shortage in CO₂.

Nitrogen plays a critical role in the biochemistry of every living thing. It is also the most common gas in our atmosphere. But it is largely inert, which means plants and animals can’t extract it from the air. Consequently, a major limiting factor in agriculture has always been bio-available nitrogen.

Most nitrogen fertilisers are produced via the Haber-Bosch process. This combines nitrogen from the air with hydrogen gas to create ammonia. Hydrogen gas is less easy to source than nitrogen. There’s plenty of hydrogen about, most obviously as the H in H₂O and CH₄ (methane), but breaking the bonds between the hydrogen and oxygen in water, or carbon in methane, requires a huge amount of energy. Hydrogen gas is therefore mainly produced by methane steam reforming, where natural gas and water is heated to about 1,000. The end products are hydrogen gas and CO₂. These can then be separated for their respective uses: the hydrogen into fertiliser, and the carbon dioxide into your salad bags.

So, until carbon capture technology matures, the cost of the fizz in your cola will likely remain tied to the price of natural gas and fertilisers.

This piece is a preview from the Witness section of New Humanist winter 2021. Subscribe today.