If countries are to meet the Paris Agreement goal of holding “the increase in the global average temperature to well below 2°C above pre-industrial levels” and pursing efforts “to limit the temperature increase to 1.5°C above pre-industrial levels”, we’re now told that reducing greenhouse gas emissions alone will be insufficient. Given our energy needs and the time it will take to transition to fully renewable sources of energy, Carbon Dioxide Removal (CDR) will also be needed, on a large scale.
But there is considerable scepticism about CDR. In May, power company EnergyAustralia apologised to its customers after settling a Federal Court case launched by advocacy group Parents for Climate. In a statement published as part of the settlement, the company said: “Burning fossil fuels creates greenhouse gas emissions that are not prevented or undone by carbon offsets.”
There are several reasons why that might be true. One that critics frequently cite comes from the fact that the removals certified by carbon offsets can’t be guaranteed to last as long as the emissions they are supposed to offset. Is this a good reason for dismissing CDR?
CO₂ removal methods and the risk of reversal
Broadly speaking, there are two types of CDR methods. “Nature-based methods” use natural processes — like photosynthesis — to trap CO₂ in ecosystems such as forests, wetlands and farmlands. “Engineered” methods, on the other hand, typically use advanced technology to capture CO₂ directly from the atmosphere or industrial sites.
Both of these methods have drawn criticism. Some argue against investing in new carbon capture methods due to their high costs and technological uncertainties. Others argue that the benefits of nature-based solutions are profoundly limited, not least because of the short time horizon over which forests and other natural sinks can store carbon.
The critics of nature-based methods are on to something. If the core idea of net zero emissions is balancing greenhouse gas additions and removals, we need the removals to last as long as the additions. However, the CO₂ we release today can persist in the atmosphere for centuries or even millennia. In contrast, many nature-based methods, like planting trees, might only store carbon for a few decades. This criticism highlights a genuine concern: merely planting a tree cannot be considered a valid offset if it eventually releases its absorbed CO₂ back into the atmosphere when it dies. This carries a “reversal risk” — a risk that CO₂, once stored, will be re-released.
However, while reversal risk is undoubtedly important, this doesn’t mean that nature-based methods should be dismissed — instead, it means that they need to be managed well. Individual trees die, but provided a forest is properly maintained and managed over the long term, it can still act as a carbon sink. It’s the continuous, deliberate maintenance of forests that ensures carbon is consistently captured, even if individual trees within the ecosystem die and are replaced.
Additionally, reversal risk is not exclusive to nature-based methods. Engineered carbon removal methods and novel storage technologies also carry their own reversal risks. Storage facilities could fail, or novel technologies might prove less effective or reliable than initially expected.
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Investing all our resources in engineered CDR is problematic for another reason. Keeping within the 2°C carbon budget requires increasing the use of CDR now — and these technologies are not, even on an optimistic picture, going to be available at the scale required soon enough.
Rather than being taken as grounds for dismissing these different CDR methods, we think these criticisms support a different conclusion. Each method on its own faces a serious problem — but they can complement each other, when used together. We must combine them strategically, using the strengths of each to offset the weaknesses of the other.
Nature-based methods, if employed sensibly, offer the rapid, large-scale deployment that is needed now to help reduce peak global temperatures and slow warming trends. Engineered solutions, coming on stream later, have the potential for more secure long-term removals. These technologies, once fully developed, offer the prospect of more stable CO₂ storage options, significantly reducing the risk of reversal.
What climate mitigation requires
A number of companies recently announced they are leaving the Australian government’s Climate Active carbon credit scheme amid concerns about its integrity. Some critics of carbon credit markets suggest that they operate simply as a way of allowing companies to buy the illusion of climate action, while continuing with business as usual.
However, if the Intergovernmental Panel on Climate Change (IPCC) is right, we will need emission reductions to be accompanied by CDR into the foreseeable future, and we will need well-functioning carbon markets to deliver it. Stabilising the consequences of human activity on the climate will require reducing emissions — but alongside this, it will also require both nature-based and engineered methods of CDR, situated within a well-governed carbon credit market.
Christian Barry is Director of the Research School of Social Sciences at the Australian National University.
Garrett Cullity is Professor of Philosophy and Director of the Centre for Moral, Social and Political Theory at the Australian National University
Together with a team of international climate scientists and policymakers, they are authors of a new paper discussing these themes at greater length, “Considering Durability in Carbon Dioxide Removal Strategies for Climate Change Mitigation”, forthcoming in Climate Policy.
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