The central measure of stopping human-induced global warming is net zero CO2 emissions. Under net zero, remaining hard-to-avoid emissions get neutralized by simultaneously removing an equivalent amount of CO2 from the atmosphere. Consequently, the Intergovernmental Panel on Climate Change (IPCC) concluded that Carbon Dioxide Removal (CDR) is necessary to reach net zero emissions and, subsequently, to stop global warming.

In other words, CDR–alongside substantial emission reductions–is essential to an effective climate action plan. Most global assessments range between 7 Gt and 11 Gt tons of CDR by 2050. Thus, getting to the required scale poses one of the major challenges society faces. Following various net zero emissions targets, more and more governments and companies have started to recognize the role of CDR. However, we still lack a unified definition of when an action qualifies as CDR, and definitions are often vague. 

This is especially true for how long the removed CO2 needs to remain locked away from re-entering the atmosphere after its removal. IPCC’s definition simply says “durably”. But what does that mean? For the U.S. Department of Energy, the threshold sits at 100 years. In the EU, CDR qualifies as permanent removals if its durability lasts “several centuries”. And the advanced market mechanism Frontier sets it at 1,000 years. A new study, co-authored by Cyril, sheds some light on the climate impact that such varying definitions of durability will have.

Fundamentally, the study concluded that to fully neutralize a ton of fossil CO2 emissions, a ton of CO2 has to be removed from the atmosphere and stored either permanently or, if it is stored only temporarily, an equal amount of CDR must balance every re-release of the previously stored CO2. In contrast, if CO2 is stored for only 100 years and we ignore its re-release, with a couple of decades delay, the same degree of warming kicks in as if there hadn’t been any CDR in the first place and residual CO2 emissions remained not neutralised.

^{Depending on the definition of storage duration as part of CDR, the global temperature outcome of a scenario where CO2 emissions are reduced to net zero and then maintained varies considerably. For simplicity, we show a different scenario than in the new study: net zero CO2 emissions are reached by 2050 (instead of 2076), and remaining non-CO2 emissions are kept constant after 2050 (instead of further reduced).}   

Storing CO2 permanently, i.e., indefinitely, is a very high standard only achieved by a few storage types, e.g., mineralisation in basalts. However, in the study, we also see that with storage durations of thousands of years or longer, we get very similar long-term global warming as with permanent storage. Therefore, it would make sense to choose “durable” over permanent storage in practice and define its required storage duration accordingly.

For biogenic CO2 emissions, a similar effect to fossil CO2 emissions sets in, depending on whether we assume, e.g., forests to regrow after deforestation or not. In contrast, for greenhouse gases that warm for a shorter period, also a correspondingly shorter CO2 storage duration is sufficient to neutralise remaining emissions with CDR. This is consistent with previous studies that introduced the so-called like-for-like principle. It argues that in a net zero framework, any greenhouse gas emission needs to be balanced with removals that store an equivalent amount of CO2 for as long as the greenhouse gas warms the atmosphere and store the CO2 in a similar storage pool like the greenhouse gas originated from (e.g., geological or biogenic). 

Another study by Damon Matthews and colleagues has shown that temporary CDR has value when used in addition to permanent CDR. In a (likely) scenario where set global warming limits are exceeded, additional temporary CDR can reduce peak warming, thereby crucially relieving some of the pressure on, e.g., ecosystems, or reducing the number and severity of climate extremes and tipping elements. In such a scenario, every ton of CO2 not in the atmosphere is a good ton, irrespective of whether it has never been emitted or whether it has been emitted but removed–both temporarily or durably. Certainly, in most cases, it will be simplest to stop emitting than to emit and remove.

Now let’s turn to policy. Ever since the 2016 Paris Agreement, the world has been rallying around a unified net zero target. What differs seems to be only the level of ambition as to when this target shall be reached, ranging from Finland in 2035 on the one hand to India in 2070 on the other, with most converging around 2050.

As we all know, CDR will be essential to achieve these targets. However, once we look below the surface, we realise that both governments and key institutions in the voluntary carbon market (VCM) do not agree on what constitutes the right durability for CDR. In fact, as discussed above, we see a wide variety of thresholds for durable CDR used, ranging from vague definitions, to 100 years, up to 1,000 years.

Why does this matter? The lack of a unified definition–as illustrated by the new study–can lead to completely different global warming impacts, even when net zero is “achieved”, thus jeopardising the targets agreed in Paris.

As such, we are calling for a clear, unified definition across governments and VCM stakeholders that reflects the like-for-like principle: only durable CDR shall be used for net zero fossil CO2 emissions (and other long-lived greenhouse gases), while temporary removals can be used to neutralise short-lived greenhouse gas emissions, such as methane. 

Analogous to the financial system, where we differ between money that we can keep (income) vs. money we have to pay back (loan), we recommend introducing separate terms, e.g., temporary CDR (tCDR) and durable CDR (dCDR). High impact policies, such as integrating CDR into compliance frameworks and emission trading systems, should only be done in full alignment with the like-for-like principle. Not doing so could lead to failures in halting global warming with net zero targets.

To be clear: this does not mean that we should give up on temporary CDR, e.g., soil carbon sequestration, afforestation, improved forest management with or without construction with wood and other biomass. Much the contrary. They fulfil a key role in A) buying us time, B) offsetting greenhouse gases such as methane, and C) can even be considered durable enough to offset CO2 when maintained sustainably and with a commitment to very long time frames.

When temporary CDR is used to offset CO2, we need to ensure that any purchase or procurement is coupled with a clear commitment to either purchasing more temporary future removals, or, ideally, to replace them with durable CDR.

To conclude, setting robust boundaries early on is of fundamental essence as we scale this new global sector, which will play an irreplaceable part in effectively mitigating climate change. Changing an established system later has shown to be challenging to a point where global systems suffer great pressure. How long CO2 is stored after its atmospheric removal as part of neutralisation claims is a central criterion and should be included with scientifically sound values in standards and legal frameworks. We recommend policies to differentiate between temporary and durable CDR, defining clear time periods for the CO2 storage duration and using the like-for-like principle as a guideline on how to neutralise the emissions of the different greenhouse gases.

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