The primary solution to avoid the worst consequences of CO2 induced climate change is to reduce the release of additional CO2 into the atmosphere. So far we have not achieved even this goal – the global release of CO2 reached a new record in 2018 at 37.1 billion tonnes. We need to reverse this trend, for CO2 emissions to actually start decreasing globally. There is debate about how quickly this has to happen to avoid specific outcomes, but it’s pretty clear that we need to significantly reduce CO2 emissions over the next 50 years. Ideally we would get to net-zero emissions – or better yet, net negative emissions. But how is that possible?

This is the idea of carbon capture – taking carbon back out of the atmosphere for long term storage. The problem is not that we can’t do this. We can. The problem is that current processes are limited. They have two main problems. The first is that they can’t be done on a meaningful scale. We mostly have laboratory proof-of-concept techniques, but without a clear way to scale them up to industrial levels. We need to be removing billions of tonnes of carbon from the atmosphere, anything less is just a drop in the bucket. The second problem is energy efficiency.

The term “carbon capture” refers to various methods. This includes just growing biomass, which naturally incorporates carbon from the atmosphere. Plant life temporarily stores carbon, until it dies and rots. Or it can be turned into biofuel, or buried underground for more permanent storage. You can also use minerals to capture carbon, or phytoplankton which then sink the ocean floor when they die. The term can also refer to the process of recouping some of the carbon released when burning fossil fuels.

What we really want, however, is direct air capture of carbon dioxide – taking carbon that is already in the air and removing it. We can do this now, but again not on a scale or efficiency that we need.

What would we do with the carbon once we take it out of the atmosphere? There are three basic approaches. The simplest is simply to bury it in a solid stable form. This will sequester the carbon long term, reversing the process of burning fossil fuels which releases previously sequestered carbon.

We could also use the carbon to make fuel. This is less useful than long term sequestration because the fuel will eventually be burned, releasing the carbon again. This results in a carbon neutral process, rather than net negative. But some of this may be useful to replace fossil fuels for things like jet fuel, where we won’t have electric options anytime soon.

The third approach is to make useful stuff out of the carbon, such as building materials. This approach may may the process more cost effective, because you get a useful product at the end. We then have to consider the lifecycle of that product – what happens to it at the end of its life? If it ultimately gets buried, that works. That’s the same as sequestration, we’re just getting a useful product out of it in the meantime. This can also offset other raw material and manufacturing processes, so there is an energy savings there as well.

This sounds nice, but unfortunately I don’t think we are close to anything like this happening. The energy efficiency problem means that it simply does not make sense while we are still releasing carbon from our energy infrastructure. For example, there are frequent research advances in this area, but we are still making baby steps. One recent one found a method for improving the capture of CO2 from the air using an electrolyzer. Again, this is a lab process that they hope can scale up, but that remains to be seen. But they boast that this increases the energy efficiency of the process to 35%.

Of course, any energy we use in the carbon capture process has to be carbon neutral itself. If we burn coal to produce electricity to run a carbon capture plant, that makes no sense. But even if we use clean energy to run the plant, that still doesn’t make sense in the current situation. We would be far better off using that clean energy to offset carbon-producing energy, rather than using it to remove released carbon at a 35% efficiency.

As an analogy, if you had a large loan at 8% interest, would you invest money at 3% return? No, you would be better off using that same money to pay off the high interest loan first. Similarly, any clean energy we make is best used to replace carbon-releasing energy – reducing the release of carbon in the first place, rather than using that same energy at great inefficiency to recapture some of that carbon.

So the real question is – what will it take for us to cross the line where direct air carbon capture makes overall sense? This depends on a number of variables. One is the penetration of clean energy into our energy infrastructure. If, for example, we get to the point where we produce all of our energy with zero carbon release, then it makes sense to use additional clean energy to remove carbon from the atmosphere. But there will likely be a point before zero carbon energy where it will make sense. We may, for example, get to limits on intermittent renewable energy sources (like wind and solar) because we lack sufficient grid storage. The grid may not be able to make use of additional renewable energy production, and so that extra capacity could be used for carbon capture. Carbon capture, in that sense, could be used as a means of energy storage for intermittent sources.

Another interesting variable is what we do with the carbon. If we are able to make it into a solid useful product, like building materials, will that be sufficient to offset the energy inefficiency of the carbon capture process? In other words, if we also add the energy necessary to produce whatever product the solid carbon is replacing, what is the net balance?

And of course the most important variable is the energy efficiency of the carbon capture process itself. This right now is the main limiting factor, and that is why each of these advances is so important. But we are not there yet.