To get back to earth, the three spacemen had to deal with a ton of surprises and solve a lot of problems. One of the most serious was getting the system working for removing carbon dioxide from the air in the lunar module, which was their lifeboat back home. We have a similar but much bigger problem down on earth now. The CO₂ building up here is not coming from all 7.5 billion of us breathing, though. It is coming from the use of fuel that we use to keep ourselves warm, to travel and build all the things we need. Scientific authorities the world over have recognised that the excess of CO₂ in the atmosphere is the main cause of climate change. That is why research centres worldwide are developing systems for trapping it. We have already described the carbon capture and storage (CCS) technology that allows us to trap CO₂ in one place permanently. And we have written about the technology known as carbon capture and utilisation (CCU) that transforms CO₂ into useful material. But some people want to capture this greenhouse gas directly from the atmosphere.

n method is to take advantage of CO₂’s acidic properties and make it react with a base. Molecules of CO₂ can be made to react with lithium hydroxide (LiOH) to create lithium carbonate (Li₂CO₃), as they were on the Apollo 13 mission. CO₂ is thus fixed chemically, producing a solid salt that remains trapped in the filter. It can be released by using heat treatment to regenerate the filter. This is what the astronauts did to remove it from their spacecraft.

The same reaction mechanism for filtering space shuttles is still used on the Russian Soyuz vessels, but with metal oxides and potassium hydroxide (KOH). Lots of research centres are currently working on how to reduce the amount of CO₂ in the atmosphere. The main methods can be divided into two major classes. The first is physical absorption, which involves temporarily trapping CO₂ molecules in porous structures like zeolites, activated carbon and microscopic organometallic sponges. The second is chemical absorption, in which the CO₂ actually bonds with a suitable substrate. In both cases, heat treatment is used to gather concentrated CO₂, restoring the active matrix for a new capturing process. Although there is too much CO₂ in the atmosphere, the actual amount is very little, or rather it is extremely diluted. The current ratio of 400 parts per million means that just 0.04% of air is CO₂. And a good thing too, because if it went over 0.2% we would have trouble breathing.

Unfortunately, this low concentration is the main obstacle to capturing CO₂ directly from the air. In order to get one litre of CO₂ through chemical or physical filters, you need to put 2,500 litres of air through the filter. This is why no proper industrial plants have been set up for the purpose and research still takes the form of laboratory experiments on a very small scale. In 2011 the American Physical Society estimated in a study that the costs of removing just one tonne of CO₂ from the air would be 530 euros. According to projects currently in the assessment phase, these costs will start at 900 euros per tonne of CO₂ trapped before hopefully settling down to 90 euros. They are not in the same league as the costs to capture CO₂ directly from chimneys and exhaust pipes, where it makes up 15% of fumes. Water makes up about the same proportion and the remaining 71% is nitrogen, which passes through engines without getting involved in reactions.

A lot of research into decarbonization therefore focuses not on directly trapping CO₂ from air, but on a combined approach. There is no magic formula for stopping climate change. We have to fight it on multiple fronts. First of all, we must promote research, development and the spread of renewable sources. In our transition to a future with clean-energy technologies, we have to exploit fossil fuels in a more efficient way, favouring those that generate less CO₂ at parity of energy production, in particular natural gas. At the same time we have to extend the useful life of products, by designing them so that the materials they are made of is easy to recover, recycle and reuse. Finally, we have to protect forests and green areas, because right now the cheapest and most efficient means we have for directly capturing CO₂ from the air are something called trees.