Unless you renew them, temporary carbon removal credits do not help solve the climate crisis, and they are likely not a cost-efficient way to reduce warming-induced harm today.
There are ways to store carbon that are very temporary. Storing carbon in soil where contracts are short and the risk of reversal high is one way. Paying someone to push their timber harvest forward with one or a few years is another.
Several companies are selling temporary carbon removal credits, storing the CO2 for as little as one year. NCX (formerly Silvia Terra) pays forest owners to wait with cutting down their trees, using what is known as ton-year accounting. NCX argues* that buying several 1-year credits can make up for the release of 1 tonne of CO2 that stays in the atmosphere for hundreds of years. Nori that sells 10-year credits from soil carbon projects has put forward similar reasoning.
But carbon that is removed and stored for a very short time (1–10 years) does not help solve the climate crisis. It does temporarily create a slightly less warm world which can in theory avoid some current harm from global heating. The effect of this is however likely very low and there are arguably much more cost-effective ways of reducing current harm from warming.
Temporarily stored carbon does not help to solve the climate crisis.
Temporary carbon removal does nearly nothing to affect temperatures after the carbon is re-released again. Therefore it does not help to keep warming below 1.5C (which primarily happens after 2030). If there were very strong feedback loops at work today, even temporary removal would have value but this does not seem to be the case. And since the carbon is released again in just a few years, temporary carbon removal does not affect the carbon budget positively or help stop future tipping points either.
Temporarily stored carbon is likely not a cost-efficient way of reducing current harm from global heating compared to the alternatives.
Very importantly, the harm caused by global heating is non-linear. For example, going from 2 to 2.5C warming almost certainly creates more harm than going from 1 to 1.5C. In models, excess deaths from warming happen predominantly at the end of the century. This paper calculates that 1 million tonnes of CO2 released in 2020 would cause 5 deaths between 2046 and 2050, but 40 deaths between 2096 and 2100 (assuming warming rises to 4 degrees). Based on this, temporary removal of 1 million tonnes between 2021 and 2031 could end up saving 0 lives. Temporarily avoiding a temperature rise now is not equal to avoiding it when temperatures are higher.
Temporarily removing and storing carbon today could help alleviate some other, mainly non-fatal damages of climate change, such as material costs from extreme weather, but there are likely much more cost-efficient ways of doing so. Climate adaptation measures such as storm protecting, early warning systems and more resilient crops will almost surely create more impact per dollar spent. If you are just looking to alleviate harm in general, measures such as disease prevention in low-income countries are very likely even more cost-effective.
This is not to say that temporary credits such as those that NCX or Nori* sell are worthless. A different approach from overbuying 1-year credits would be to renew the carbon removed each year or decade. Instead of purchasing 17 credits with 1-year storage to offset your 1 ton CO2 emission as NCX suggests, you would purchase a 1-year storage credit every year to offset your original emission. This is a much more sound approach in theory, but in practice, it’s obvious that it would be very difficult to guarantee that an actor will continue renewing carbon credits each year for a century.
A more feasible approach could be to pay for temporary storage for a few years or a decade and then buy permanent carbon removal, such as with DACCS once the credit expires. This could be an option for when the supply of permanent credits is low, or for a cash-strapped company that wants to make a net-zero claim today. But that would also require the buyer to keep track of old commitments. Will a company in 2031 remember that previous employees bought 10-year temporary credits in 2021 to offset 2020’s emissions? Carbon plan has developed a calculator to assess the cost of renewing temporary credits.
Another option is to use temporary credits to offset short-lived emissions that match the storage time. Soil carbon with contracts for 10 years could perhaps be used to partially offset methane emissions with a half-life of 9 years. But the global warming potential of methane would then need to be calculated to match the temporary storage of CO2.
Generally, I believe that the best approach for individuals and companies that want to counter the damages their emissions are doing is to find and fund highly effective climate projects that help solve the climate crisis (without necessarily making any offset or net-zero claims). That could for example be funding new nascent carbon removal methods, fund decarbonisation efforts that have a high green premium, or funding advocacy to help decision-makers get off fossil fuels and protect forests. This is the approach that we are taking with the Milkywire climate transformation portfolio. As global emissions are reduced and permanent carbon removal solutions are scaled up this can be transitioned into a carbon removal-only strategy.
(Making a commitment to permanently remove all historic emissions at a future date is yet another option but doesn’t include temporary storage. For example, saying that by 2040 company X will have permanently removed all carbon emitted 2000–2040.)
Temporary credits can have their place if used right, but that's not what we are seeing right now.
Update 14 Aug 2021: A clarification was made that the concept of ton-years is not created by NCX. NCX also provided a reply, follow the discussion between us here.
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