Ways to remove co2 from atmosphere

Cutting carbon dioxide emissions to “net zero” is essential if we're to put the brakes on man-made climate change. But for net zero to be reached, we have to strip far more carbon dioxide (CO₂) from the atmosphere (and lock it away) than we’re managing so far. Putting the CO₂ that’s already in the atmosphere to commercial use can make a big difference. 

Every year nearly 40 gigatonnes (Gt) of CO₂ emissions are released worldwide, adding to the 3,150 Gt already in the atmosphere. Governments are responding, and the energy system is at the beginning of transformation that could end up being faster than expected. Still, efforts to cut the amount of CO₂ being pumped out remain far too modest.

We can, however, also leverage natural processes, especially photosynthesis, which transforms CO₂ into biological products like wood or nutrient-rich soils. We can also use technology to convert the gas into chemicals, fuels or rock. Some of this CO₂ can be sequestered for centuries – which is a prime objective. Or it can be used to make biofuel, reducing the amount of new emissions we generate from fossil fuels, and potentially buying us time.

In all, our team of researchers has identified 10 different pathways to removing CO₂ from the atmosphere. Together, these could reduce the net impact of our emissions in the short term, and help us reach net zero by 2050. (These are discussed in more detail in a paper published by Nature). 

Some of these pathways are immediately economical – and indeed are already in use. Others need initial financial support, though the cost should decline as the new technologies scale up.

The path to CO2 utilisation

The routes to putting CO₂ to use broadly fall into three categories: ‘cycling’, ‘closed’ and ‘open’ pathways. A 'cycling' pathway entails taking CO₂ from the atmosphere, using it to produce fuels that are then burned, thus releasing it back into the air. By contrast, 'closed' pathways take the gas from the air and effectively lock it up forever – by, for example, incorporating it into concrete. Finally, 'open' pathways involve biological systems, in which CO₂ is naturally absorbed by plants or algae and turned into biomass and soil, but could be released back into the atmosphere at variable rates (potentially decades or longer) through natural mechanisms.

Five of these 10 processes involve photosynthesis. That, perhaps, shouldn’t be a surprise: plants have a huge impact on the carbon cycle. Some 440Gt of CO₂ flows in and out through plants every year, dwarfing the 34Gt that’s pumped into the atmosphere by human activities. Unfortunately, only 2 to 3 per cent of that photosynthetic carbon stays in soil and plants. The rest is re-emitted by respiration – photosynthesis converts sunlight and CO₂ into sugars, but these are then converted by the plant into energy, with the gas then released back into the atmosphere. If soil carbon uptake could be increased by a mere 0.4 per cent, and then the carbon were to stay there, net zero would be achieved. But that’s a huge challenge.

One possible solution is to encourage the growth of biodiverse forests, which can be maintained for their ecosystem services (flood protection, wildlife habitat, climate regulation and carbon storage) and human services (recreation and mental health), or used in clever wooden structures and increasingly tall buildings. Another is to grow more microalgae, generating biofuel or agricultural feed. Improved land management can help increase soil’s carbon content, which can also increase plant yields. And biochar – made from the carbonisation of plant material – can also be dug into the ground, enriching the soil.

And then there are the man-made pathways, such as stripping carbon dioxide out of exhaust gases to be converted into chemicals or using CO₂ as a basic building block in the creation of hydrocarbons. While in the long-term we may not want more hydrocarbons, if the carbon atoms in the fuel came from the atmosphere and were returned to the atmosphere, the impact could be carbon neutral. It is also possible to inject CO₂ into the ground to increase oil recovery; but this has the rather perverse outcome of increasing fossil fuel use. CO₂ can also be locked into concrete building materials.

Carbonomics

It’s hard to overestimate the complexity of analysing the ultimate cost and benefit of these processes. During our research, we spoke with experts around the globe and crunched the numbers in our paper to come up with a best effort at a first pass, producing a “cost curve” of the different technologies, showing their scale and impact.

Some already make economic sense – or at least do at first sight. Forestry, land management, biochar, chemical production and oil recovery are currently profitable ways of extracting CO₂ from the atmosphere – though in some cases a rethink of existing agricultural or industrial practices is needed. In other cases, like converting CO₂ into fuel or developing microalgae, substantial government support is needed to make such technologies viable; the cost is currently at a level over $100/tCO₂. But even at those levels, some state support might make sense. That’s because microalgae, which can be converted into biofuel, or feed, or even bioplastics, is anywhere from two to 10 times more efficient at capturing CO₂ than plants.

Some of these processes will demand large infrastructure investments. But once widely adopted, returns to scale can make them surprisingly economic. In other cases, the pathways will be substitutes – use land for one purpose and it’s not available for another. Some will have an unanticipated impact on other parts of the economy that, in turn, will affect total CO₂ production. Technological change will undoubtedly have an influence. There are many complexities to consider. 

Sometimes, the benefits come with side-effects that need careful management. Take making urea. Converting CO₂ into this key building block of nitrogen-based fertilisers can already be done profitably. But nitrogen-based fertilisers also carry the disadvantage of generating nitrous oxide, a greenhouse gas that’s 300 times more potent than CO₂ over a 100-year time horizon. Or take using atmospheric CO₂ to cure concrete. While this can lock CO₂ away permanently, the energy intensiveness of cement production means that it might make more sense over the longer term to substitute it with timber as a building material.

Crucially, though, it’s important to understand that putting atmospheric CO₂  to commercial use can be important in tackling climate change. In some instances, it is already happening. In others, it is just a matter of adopting different practices or bringing technologies down the learning curve. Getting this going at scale will require subsidy today. But the subsidy required is likely to be small in comparison to the damage done by each additional tonne of CO₂ we emit. None of this changes the fact that we have to cut emissions, fast. But on the quest to net zero, every little helps, and taking CO₂ out of the air is likely to be an absolutely vital part of finishing the job.