Understanding carbon dioxide removal
To prevent dangerous warming, we need rapid and deep emissions cuts. But having delayed action for decades, reducing emissions may no longer be enough.
How we address climate change as a global problem depends both on our choices in policy and economic development and on the physical constraints of the climate system, notably the greenhouse effect itself. As long as we continue to emit carbon dioxide and other greenhouse gases into the atmosphere, we’re putting more pressure on the climate while the list of options for policy choices gets shorter. As we discussed in the [explainer on carbon budgets] (link to the carbon budgets deep dive), there is only so much we can emit before we breach the safety limits on temperature rise. And if we ‘overspend’, current and future generations may face the bill in the form of [various climate impacts] (link to the case studies deep dive).
The underlying physics of this problem we face also presents potential solutions – in other words, if putting CO2 into the atmosphere from power plants or transport can heat up the Earth, then taking it out can compensate and limit, or even reverse temperature rise. This is the idea behind carbon dioxide removal (CDR) technologies, which are now, after years of insufficient progress on climate change, being increasingly discussed as a potential solution.
Today, CDR underlies many ‘net zero’ plans and pledges, where some greenhouse gas emissions that have already happened or will happen in the future are ‘cancelled out’ by removals, often happening somewhere other than the emitter’s location. It is also used in the so-called overshoot scenarios, where the Paris Agreement temperature goals are temporarily broken because we don’t reduce emissions fast enough. In these cases, CDR helps us get back into the ‘safe zone’ later this century, turning our total CO2 emissions negative by taking out more CO2 than we emit.
Can you really pull CO2 out of the atmosphere?
CO2 extraction is possible and has, in a way, always been happening. There are various kinds of CDR being discussed now, depending on what chemical mechanisms are used to capture the greenhouse gas in question, and how the captured carbon is then stored. One of the mechanisms is photosynthesis, the natural process in which plants and some bacteria convert water, light energy and carbon dioxide into chemical energy, releasing oxygen in the process. Photosynthesis is the reason why the Earth’s atmosphere has enough oxygen to support complex life forms like humans. It is also the means by which living organisms capture and store more than half of all CO2 emissions from current human activity, making them carbon sinks.
Scientists have been accounting for these natural carbon sinks in their modelling. For photosynthesis to count as a proper CDR method, however, it has to be intentional and additional to what nature is doing on its own. In other words, we can’t simply think of all plants on Earth as one giant CDR project and rely on it. Instead, people can intentionally plant and maintain forests on land or in coastal areas, enrich soils, or grow some plants used to produce biofuel (and then capture the CO2 from burning it).
Furthermore, if plants can use chemistry to capture CO2, then people can also use chemistry to do this. For example, if certain solvents or sorbents are exposed to open air, they will bind to CO2 and absorb it, a bit like a sponge absorbs water. And, like a sponge, the CO2 can then be separated out and the initial chemicals reused. This process is called direct air capture (DAC), and the resulting CO2 is stored somewhere to make sure it is completely removed and not re-emitted to the atmosphere. There are solid and liquid materials that can be used for DAC, as well as some less developed innovative methods of scrubbing CO2 from the air.
Weathering, or gradual deterioration of rocks and minerals in contact with air, water or living organisms, is another natural process that already impacts the world around us and which we can intentionally make use of to counter climate change. Some chemical mechanisms of weathering involve CO2 from the atmosphere, and this itself can be purposefully enhanced to speed up reactions, for example, by spreading finely ground basalt on surfaces. As with plants, however, it counts as CDR only if it is intentional and the CO2 is successfully captured.
Can CDR solve our climate problem?
Even though the fundamental chemistry of various kinds of CDR looks solid on paper, making it a reality has been a challenge so far. The chemical processes of direct air capture and enhanced weathering can be energy-intensive and not very efficient. Currently, CO2 captured via these methods costs so much that it makes little economic sense to pursue these projects. Part of the reason for this is that, unlike renewable energy, these technologies are still in their infancy.
In the case of biological CDR, plants have been perfecting photosynthesis for millions of years, making that natural ‘technology’ fairly mature. But biological CDR using forests planted by humans can have significant impacts on food security, biodiversity and land rights. Furthermore, while it is less expensive than chemical CDR, the current size of our influence on the climate makes it impossible for biological CDR to compensate alone. Once again, the physical constraints are at play here: for our current level of greenhouse gas emissions, there simply is not enough land on Earth. And, of course, carbon removal by plants would not be permanent or even long-term if the trees and plants used are not carefully protected from fires or logging.
If CDR is going to be used to cover our collective ‘overspending’ of the carbon budget, there needs to be proof that enough CO2 would actually be permanently removed from the atmosphere. This would itself require reliable methods for measuring the CO2, reporting the information and then verifying the results, none of which are yet in place.
Overall, counting on CDR to solve our climate problem would be like placing a risky bet on a future outcome that is still only vague. These technological solutions are simply not certain to mature in time to prevent us from causing temperature overshoot. If we decide on policies today that rely on them and they do not work in the way we need, we will have missed the opportunity we have now to stop emissions from getting into the atmosphere in the first place – and there will be no going back in time to choose differently.
What can CDR do then?
The IPCC states that CDR can help us counterbalance so-called ‘hard-to-abate’ residual emissions from things like aviation or heavy industrial processes. Total decarbonisation in these sectors can be either prohibitively expensive or take too long because the necessary technologies are not yet available at scale. To help avoid the climate impacts from emissions in these areas, CDR can be used to compensate for their activities by removing CO2 from the atmosphere.
Carbon removal in the next few decades is likely to be limited and nowhere near capable of dealing with the amount of emissions that need to be accounted for each year to avoid exceeding the limits of warming. As such, the IPCC indicates that the limited capacity of CDR should cover only hard-to-abate sectors and cannot serve as a ‘get-out-of-jail-free’ card for everything else. The latest IPCC report actually reduced the role of CDR in its proposed economic pathways compared to its earlier scenarios for meeting the Paris Agreement goals. The message is clear: reducing emissions should be the focus of our actions.
- At an event during COP27 in Egypt, IPCC authors speak about CDR and how it is featured in the Working Group 3 report
- The first of its kind State of Carbon Dioxide Removal Report, released in early 2023, looks at the state of CDR globally
- A glossary of CDR terminology from the American University
- Another explainer on how CDR works from the American University