How long does the CO₂ have to be stored in #CarbonDioxideRemoval #CDR? At least 35 years? 100 years? Longer?🤔
In a new study, Zeke Hausfather, Reto Knutti, and I find that for #netzero CO₂, anything less than 1000 years is not enough to neutralize warming. 🧵
https://lnkd.in/ekYKzCny
Firstly, this all depends on how we assume the re-release of CO₂ to happen: All at once after x years, continuously over x years, or as a result of a leak or a chemical decomposition, with an average re-release time of x years. We focus on the latter but show the other cases in the supplementary materials.
Fundamentally, CDR induces a cooling that–in a net zero framework–aims to neutralize the induced warming of a CO₂ (or greenhouse gas) emission. Because most of the warming of emitted CO₂ remains for thousands of years, the induced cooling of CDR must be maintained for as long.
If the CO₂ after its removal is stored only temporarily, the induced cooling of CDR will get lost, too. If done so as part of a net zero CO₂ emissions claim, over time, a net warming sets in similar to if there hadn't been any CDR in the first place.
Thus, if we along with strong reductions in other GHG emissions claim that global CO₂ emissions have been reduced to "net zero" because a CDR activity only needs to store the removed CO₂ for 1000 years or less, we miss the targeted climate-neutral case and continue to warm.
These results reinforce the "like-for-like" principle: credible neutralization claims using CDR in a net zero framework require balancing emissions with removals of similar atmospheric residence time and storage reservoir, e.g., geological or biogenic https://lnkd.in/e35e5W8h
However, lower-durability removals have value, too:
1⃣In the absence of specific net zero or neutralization claims
or:
2⃣If the re-release of CO₂ is treated the same as an emission of CO₂, so it must be neutralized with new CDR.
So personally, I would recommend differentiating between Carbon Dioxide Removal, with storage durations of at least 1000 years (= durable CDR, dCDR), and temporary removals (= tCDR). Similar to how we differentiate between income and a loan.
We intentionally didn't link different levels of CO₂ storage durations with different CDR methods because (i) this could change, (ii) it depends on how a particular CDR method is done, and (iii) there is already a lot of information about the permanence of different CDR methods.
We would like to thank Sabine Mathesius and Jay Fuhrman for their contribution to the peer review of this work and their valuable feedback, and Heike Langenberg for acting as an excellent editor. This work was realised as part of the UPTAKE and SPEED2ZERO projects.
Unfortunately, there is a typo in the manuscript, which will be corrected soon. With a typical 100-year storage duration, net zero CO2 emissions with 6 GtCO2 per year residual emissions result in an additional warming of 0.8 instead of 1.1°C by 2500 compared to permanent storage.