Even within the climate community, permanence – the longevity of a carbon pool – is hardly a mainstream topic of conversation. Typically the province of forestry and LULUCF specialists, it occasionally receives an airing in debates about carbon capture and storage (CCS) but otherwise languishes in the back chapters of IPCC reports as a technical afterthought.
This is a shame, for two reasons. First, permanence is a truly foundational concept and one, frankly, that would benefit from some fresh thinking. And, second, with the advent of new forms of climate mitigation – the likes of bioenergy with CCS (BECCS), direct air capture (DAC) and carbon capture and utilisation (CCU) – and new initiatives such as the UNCCD’s championing of land degradation neutrality (LDN), GCF support to REDD+ results-based payments and ICAO’s ongoing development of CORSIA – permanence is set to move centre-stage in future mitigation thinking.
Some heterodox views aside, the ‘standard’ view of permanence outlined below is generally accepted.
Mitigation projects – those that avoid greenhouse gas emissions, such as the use of renewable energy to replace fossil fuels or the implementation of energy efficiency measures to reduce overall energy consumption – generally do not present any permanence concerns. The avoidance of emissions in a particular time period – by an industrial facility in 2017, say – can never be undone; emissions may rise in the facility in 2018 and that may be a source of concern, but such future emissions don’t alter the fact that emissions were successfully avoided in 2017.
Sequestration projects – those that involve the storage of carbon – are, in the standard view, conceptually different. Peatlands can – and, indeed, do – burn, releasing their stored carbon into the atmosphere. Forests can burn too, as well as flood and die off due to pests. The underground CO2 storage reservoirs of CCS facilities can, in principle at least, leak. And such reversals do exert retroactive harm: the climate benefit of a forest in 2017 can be undone by the loss of its stored carbon in 2018.
So far, so straightforward.
But consideration of the permanence – or, rather, the non-permanence – of carbon stores can quickly lead into technical discussions about the benefits of temporary storage.
Strictly speaking, over an infinite time horizon temporary carbon storage has no climate benefit. Whether carbon is released into the atmosphere today or in 200 years, its forcing effect on the climate is identical; the only difference is a delay in when the forcing commences.
Over a finite time period, however, there is some value in temporary carbon storage. Since policy actions are generally undertaken in the context of specific time-frames, it follows that locking-up carbon for the duration of that time-frame – or even just a fraction of that time-frame – can have some benefit, comparable in some way to avoiding an equivalent amount of carbon emissions.
A common, but by no means universal, approach is to treat the relevant time-frame as being 100 years. In this view, storage of carbon for 100 years or more can be considered ‘permanent’. A simple – and simplistic – approach is then to apportion carbon benefits in proportion to the time that the carbon is locked up. Thus, over a 100-year time-frame, 100 tonnes of carbon stored for 50 years could be regarded as the equivalent of avoiding 50 tonnes of carbon emissions.
Aside from the arbitrariness of the chosen time-frame, which has real implications for the risk of carbon stock reversal, another problem with this approach is that it ignores the fact that carbon in the atmosphere itself undergoes removal. In other words, if 1 tonne of CO2 is emitted into the atmosphere today, only a fraction of that tonne will remain in the atmosphere in 100 years’ time: the rest – something like half – will be removed by natural processes and transferred to the oceans and biosphere. Similarly, any leakage from a carbon pool into the atmosphere– after a forest fire, say – will itself slowly be removed from the atmosphere.
Allowing for these dynamic aspects slightly alters the ‘equivalence’ between sequestration and mitigation. A tonne of carbon sequestered for 50 years and which then escapes into the atmosphere will produce approximately 60% of the radiative forcing – over the overall 100-year time-frame – of a tonne of carbon that is released in year 0; conversely, the climate benefit of that tonne of sequestered carbon can be regarded as being equivalent to 40% of a tonne of carbon whose emission is avoided in the first place.
The equivalence between temporary carbon storage and avoided emissions. Credit: Murray and Kasibhatia (2013), using an atmospheric CO2 decay function developed by Moura Costa and Wilson (2000)
In practice, this ‘tonne-year’ approach to quantifying sequestration benefits is difficult to operationalise as it is acutely sensitive to the time-frame and decay function used.
Most carbon crediting schemes, faced with the challenge of rewarding sequestration projects for their climate benefits, have alighted upon alternative – generally more straightforward – solutions: expiring credits – tCERs and lCERs – in the case of CDM forestry projects; a buffer system augmented by government guarantee from the project’s host country or the buyer (Annex 1) country to replace ‘lost’ credits in the case of CDM CCS projects; and a buffer system for forestry projects under the VCS.
Conceptually, the distinguishing factor between interventions with permanence concerns and those without is clear: storage. If carbon is stored as part of a climate intervention, then the permanence of that intervention can be called into question.
All sequestration projects – reforestation, soils, ocean fertilization, direct air capture, mineralisation and others – involve the capture and storage of carbon in some form, and hence all suffer from permanence concerns.
Mitigation – the reduction of greenhouse gas emissions – is more complex. Indeed, some increasingly prominent interventions – such as REDD+ and carbon capture and utilisation (CCU) – positively blur the very meaning of permanence.
To be continued in Part 2.