Adjusting impermanent nature interventions to be equivalently permanent to geological sequestration

Prof Srinivasan Keshav (@ProfKeshav) is using computer science to move towards a sustainable future through clean energy and environmental conservation. He is a co-director of 4C - the Cambridge Centre for Carbon Credits - and the Robert Sansom Professor of Computer Science in the Department of Computer Science and Technology at the University of Cambridge.

When it comes to carbon sequestration, there are two traditional categories. Permanent geological sequestration entails CO2 being pumped down into cavities in rock to form a layer of calcium carbonate. This method is quite effective and very long-term but it is expensive. The second category is nature-based solutions, like planting a tree. When a tree is planted, it locks down carbon in its tissue – both in the above ground biomass that we can see, and in the below-ground biomass, like roots. This type of sequestration is far more cost-effective and comes with many cobenefits for humans and wildlife, but it has a key disadvantage – it is impermanent. Eventually the tree will die, and much of that captured CO2 will be released back into the atmosphere.

Does that impermanent sequestration of carbon dioxide still have value in the fight against climate change? How can we begin to measure what that value is? And how do we know how long that carbon dioxide will be sequestered for?

The framework we use for working this out rests on a concept known as the ‘social cost of carbon’. CO2 in the atmosphere lasts for 300-500 years, where it has a number of detrimental effects. The damage of those effects to society can be estimated in dollars, and that total can be divided to attribute a cost to each single tonne of carbon released in a particular year. The damage inflicted by a single tonne throughout its entire lifespan is the social cost of carbon. It is worth noting that this estimate encompasses a discount factor to allow calculation of the present value of future damage – damage occurring today is given more importance than damage occurring far in the future. The discount factor is normally given to be 2-3% per year.

This allows us to value permanent sequestration; the social cost of a ton of carbon emitted in any year is exactly the same as the valued benefit of sequestering that same ton of carbon in that year. But we can also use it to value impermanent sequestration in forests.

We can subtract the social cost of the carbon dioxide as it is lost from a forest, year by year, from the social benefit of the initial sequestration of the carbon dioxide. This can then be divided by the social benefit to give the ‘equivalent permanence’.

For example, if a forest never gets cut down after a protection scheme (i.e. it remains there in perpetuity) then there will be no CO2 emitted and no social cost, and so the equivalent permanence will have a value of one. In contrast, if the forest were to burn down in flames the instant it was created, then the social cost and benefit exactly cancel out, and the equivalent permanence is given a value of zero. In reality, all real-world protection schemes will fall somewhere between those two values.

Thanks for this summary to James Miller, student environmentalist and film-maker.