Last week I wrote about four ways to draw CO2 out of the atmosphere, something we are almost certainly going to have to do to stabilise the climate in the long term. All of those techniques were natural ones, better ways of managing land to expand its capacity to lock up carbon. To my mind, those are the best starting points, as all of them have multiple benefits. There are, however, other carbon negative interventions under discussion, and it’s worth mentioning a few of them.
- Biochar – this is an ancient soil improvement technique whereby charcoal is dug into the soil, where it soaks up water and nutrients and improves soil fertility. It has been rediscovered in the age of climate change, as the carbon in charcoal remains stable for a long time. Thus the CO2 that was absorbed by plants as they grew isn’t released back into the atmosphere, but buried away underground instead. There’s a lot of research going into this at the moment, because in theory we should be able to use waste biomass, and then generate heat or energy as we burn it, which would be useful. Obviously we’d then be able to use the biochar to improve soils at the same time as reducing emissions, so all round it could be a useful technique. See the UK Biochar Research Centre for more.
- Bioenergy with CCS – we are familiar with Carbon Capture and Storage (CCS) for coal power plants and heavy industry. That aims to scrub the CO2 from burning coal, and store it. If we use the same technique with power plants burning biomass, then it becomes a carbon negative technology. Like the biochar above, the plants drew the carbon out of the atmosphere as they grew, and then CCS captures it. The challenge is, as usual, to find places to store large quantities of CO2. One relatively new option is to use the waste CO2 to make plastics, though that would only be a small percentage of the total.
- Air capture – this is the high tech end of the spectrum, where engineers devise ‘artificial trees’. There are many different design ideas for these, as you will have seen if you ever read green tech blogs. Generally speaking there are two main approaches. One is machines, where air is blown in one end and blown out the other with less CO2 in it. Demonstration models exist, but they are fairly energy intensive. One example in Canada produces a half tonne of CO2 for every tonne it captures, and the energy needs of direct air capture make it rather impractical at the moment. The second approach is ‘passive’ CO2 extraction, using a resin or other material that naturally absorbs CO2. This has a better chance of success, but like CCS, the CO2 needs to be stored somewhere. At the moment, the most promising market for captured CO2 is to sell it to oil producers to pump into the ground to force more oil out of faltering wells. That’s obviously massively counterproductive from a climate change point of view. Another possibility is to use the captured CO2 to make liquid fuels, essentially recycling CO2 rather than bringing new carbon into the atmosphere by burning fossil fuels. That’s an economically feasible idea, though wouldn’t be carbon negative any more.
Those are three more ideas for extracting CO2 from the atmosphere. We’ve looked at four land management methods, and three technological solutions. There are a couple more options to mention, and they would be better classified as geoengineering techniques. Those will be a third post.