How much carbon will we need to remove?

There is a big difference between the CDR needed to reach net zero, for neutralizing methane, and removals to bring back down temperatures after overshoot.

There are a lot of projections and plans hinging on the question of how much carbon dioxide we need to remove (CDR), but few satisfactory answers. 

The discussion is mixing up very different scenarios. There is a big difference between the need to use CDR now, to reach net zero, and to bring temperatures back down.  Let’s examine the different needs for carbon removal:

Do we need carbon removal before net zero?
The main reason to deploy carbon removal today is to build the industry, foster innovation and help lower costs. Here, the “need” for CDR is set by how much we think we will need to reach and maintain net zero. This build-out would remove carbon on the way, of course. A massive CDR ramp-up with exponential growth to 5 Gt (billions of tonnes) in annual capacity by 2050 would remove around 20 Gt cumulatively between 2023-2050. That represents 2-3 percent of the expected total emissions during that period. 

CDR before global net zero also enables companies and countries to take action and reach national or corporate net zero before we reach a global net zero scenario. That demand can be what drives part of the ramp-up of CDR capacity to meet global net zero needs. 

How much CDR do we need to reach and maintain global net zero?
Precisely determining the future optimal use of CDR in net zero is impossible. Not the least because it depends on unknown technology developments. But we must make assumptions to know what to build for. That is one of the reasons why separate targets for CDR and emission reductions are so important in the medium term; they ensure that we actually start building CDR capacity in parallel to reducing emissions. 

Very few CO₂ emissions are strictly impossible to reduce to zero. How much CDR will be used depends on the cost, resource use, side effects and availability of carbon removal and emission reductions, as well as the political choices we make. 

Some CO₂ emissions are likely cheaper to abate with CDR than with reductions. Aviation is a good example where producing synthetic fuels will likely cost more and use a lot more resources than offsetting kerosene with DACCS. (However, that could change if the way we make synthetic fuels is revolutionized). This is likely also the case for some heating needs and industrial processes. If CDR becomes very cheap, it might compete with a large range of emission reductions. However, It is essential to consider the broader societal impacts of a choice of solution. Price alone should not determine what is deployed. Of course, it is always cheapest to avoid emitting activities altogether, but that carries alternative costs.

I think the CO₂ emissions for which permanent CDR will be the most affordable and sustainable option could be between 2 and 5 Gt per year (5-13% of today's fossil CO₂ emissions), but I have low confidence in the estimate.

Do we need to use carbon removal to offset methane and nitrous oxide?
There are also non-CO₂-greenhouse gas emissions that are nearly impossible to eliminate, mainly nitrous oxide and methane from agriculture. Here, the discussion gets tricky since in contrast to CO₂, stable methane emissions do not lead to increased warming in the long term due to its short lifetime. Increases in the level of methane emissions increase warming substantially though. A small reduction in methane emissions is sufficient to stop further warming because of methane's short lifetime. Global carbon budgets also assume about half of today's methane emissions level to continue indefinitely. In other words, we would not need to offset methane to stop warming if the annual decrease in methane emissions is above a certain level. 

However, our previous methane emissions contribute over 0.5 °C to today’s warming. A greater decrease in methane emissions would lower its contribution to warming and create more headroom until we reach the agreed temperature limits. This margin will likely be eaten up by warming from CO₂ until net zero- simply because we cannot reduce our CO₂ emissions fast enough. We could decide that net zero targets should include methane to meet this potential for temperature reduction. Most countries and companies’ net zero targets include methane, but its effect on climate compared to CO₂ is poorly understood among decision-makers in my experience. Simply equating methane emissions to CO₂ emissions with GWP 100 or GWP 20, as is commonly done, is not helpful as it does not recognize that falling methane emissions lead to cooling. GWP* is a more suitable metric.¹

To offset methane, it would be enough to do so with carbon stored away temporarily.² Using permanent carbon removal to continuously offset methane would lead to progressive global cooling.

Methane emissions are expected to remain significant. In the UN's most ambitious scenario, SSP 1-1.9 anthropogenic methane emissions fall from around 300 million tonnes of CH₄ today to around 150 million tonnes by 2050³. If we use methane’s warming potential over 20 years, GWP 20⁴, (there is no GWP 12 calculated), then neutralizing the effect of 150 million tonnes of methane on temperature would require a massive 12,7 Gt of CO₂ sequestered per year.⁵ ⁶ As the carbon would not need to stay away from the atmosphere, it could be temporarily stored in soil, products or through deferred timber harvests.⁷

This is just a simple calculation using GWP 20. A detailed modelling and discussion of how methane's warming effect should be offset with short-term CO₂ storage is needed. 

This graph, using the FaIR climate model, shows the temperature effect of a one-time CH₄ emission offset with CDR with a CO₂ storage lasting 20 years using GWP 20. Thank you Cyril Brunner at ETH Zurich for creating the graph. 

Nitrous oxide emissions last over 100 years in the atmosphere and need to be offset with carbon stored away for as long as to halt temperature increase. 0.8-1.9 Gt CO₂e per year of N2O emissions from agriculture will likely continue indefinitely, and time-matched carbon sequestration and storage would be needed to offset them. A great candidate for this would be forestation. 

To summarize. At net zero, we may need:  

• 2-5 Gt/yr of permanent carbon removal to offset continuous residual CO₂ emissions and halt temperature increase.

• Multiple Gt/yr of short-lived CO₂ storage to offset continuous methane emissions in order to lower temperatures (12,7 Gt/yr in my GWP 20 example).

• 1-2 Gt/yr of 100-year CO₂ storage to offset continuous nitrous oxide emissions and halt temperature increase. 

How much carbon removal do we need to bring back temperatures after overshoot? 

The world will almost certainly miss the 1.5 °C limit. If all long-term national pledges are met, warming increases to about 1.8 °C. Lowering temperatures from 1.8 °C to 1.5 °C would mean that, other things being equal, an additional 666 Gt CO₂ would have to be removed, leading to so-called net negative emissions.

We “need” carbon removal to bring us back to safe temperatures in the same way as we need to solve other global problems such as extreme poverty, hunger or air pollution. It is desired, but it is not happening at a large scale enough, and there is no agreement on who should pay for it. 

When it comes to reaching and maintaining net zero we have some clarity on who should be ensuring the CDR is deployed, but determining who should pay for the net negative emissions to lower temperatures is going to be very tricky. The concept of taking responsibility for historical emissions is not widely recognized by countries, at least not the ones with large carbon debts. 

In any case, we should not count on that a lot of carbon will be removed to reverse a temperature overshoot, and cannot use future CDR as an excuse to take it easy on emission reductions today. Future generations will decide if they want to use their resources to reduce temperatures, but we should do all we can to make their problem as small as possible.

Conclusion
We need to build CDR to the size it needs to be to reach net zero, as well as enable net negative emissions. That likely means aiming for several gigatonnes of annual permanent removals by midcentury, a couple of gigatonnes CO₂ stored in forests, and multiple gigatonnes of short-term removals if we decide methane emissions should be neutralized too. 

Mixing the need for CDR to reach net zero with the need to bring temperatures back down and conflating the need to offset CO₂ with the potential need to neutralize methane is not helpful and does not provide a good foundation for making informed climate action plans.

Thank you Tito at Airminers for prompting me to write about the topic, and to Cyril Brunner for comments on this text.

1.  For a better understanding of non-CO₂ greenhouse gasses' effect on temperature at and the problems with GWP 100 and 20, see  “Demonstrating GWP*: a means of reporting warming-equivalent emissions that captures the contrasting impacts of short- and long-lived climate pollutants” https://iopscience.iop.org/article/10.1088/1748-9326/ab6d7e 
and “Net Zero: Science, Origins, and Implications” https://www.annualreviews.org/doi/full/10.1146/annurev-environ-112320-105050#_i29

2. I reserve the term carbon removal for solutions that remove CO₂ from the short carbon cycle and durably store it away from the atmosphere.

3. Global carbon budgets, how much CO₂ we can emit to stay under a temperature target, also make assumptions on how methane emissions evolve. The budget for 1.5C is 250 Gt of CO₂ but assumes that methane emissions are halved by 2050. If methane emissions do not decrease, there would basically be no CO₂ budget left. 

4. Here, GWP 20 is used to determine how many tonnes of CO₂ would need to be stored away temporarily, which could be a fruitful use of the metric.

5. Ongoing methane emissions could also be offset by a one-time CDR pulse with permanent storage. https://www.sciencedirect.com/science/article/pii/S1750583612003064)

6. GWP 20 for methane is between 84-87, meaning that 1 ton of methane requires 84-87 tons of CO₂ stored for 20 years. (150 million tonnes CH4 * 85 = 12,7 Gt CO₂) This is just an example; a more sophisticated calculation is needed to determine the offset needs.

7. Since the storage would be renewed yearly, a stock of 254 Gt CO₂ would be built up if the CO₂ storage lasted for 20 years, theoretically representing a temperature decrease of about 0.11 °C.