One hundred years of carbon offsetting?

An acre of land can absorb 40-800 tons of CO2 over the course of a century. Today’s note ranks different options for CO2 sequestration over 100-year timeframes. Active management techniques and blue carbon eco-systems are most effective.

Nature based solutions to climate change are among the largest and lowest cost opportunities in the energy transition, with potential to absorb well over 20GTpa of CO2e for costs around $3-50/ton. We argue they will disrupt the entire new energies industry and will increasingly be adopted in the decarbonization strategies of climate-conscious organizations (research below).

One of the question marks over nature-based solutions is their impact over long time-frames. Another is the CO2 that can be offset per unit of land (note below)

Hence the purpose of this short research note is to quantify how much CO2 is most likely to be removed from the atmosphere and sequestered for different negative-emissions technologies, over the course of a century. Our answers are laid out below and we will run through the options in order.

Peatlands are most likely to absorb around 40 tons of CO2 emissions over the course of a century. This is the lowest volume shown in our chart, as an acre of peatland tends to absorb around 0.4T of CO2 per acre per year. Where peatland stands out, however, is that they continue accumulating CO2 at this rate for millennia. Hence an acre of peatland typically contains over 1,600 tons of CO2-equivalents, which is 4-8x more than a terrestrial forest and more than other blue carbon ecosystems (data below). This means that preserving pre-existing peat bogs is debatably more important than establishing new ones.

Restoring soil carbon in agriculture is next in our chart. It can absorb 75 tons of CO2 emissions over the course of a century. This number remains lower than other CO2-offsetting technologies. But it has the advantage of being compatible with crop-based agriculture and could therefore be implemented across a vast 4bn acres of croplands, where soil carbon has likely fallen from 4% in pre-industrial times to 1% today, due to mechanized agriculture, and explaining 20-30% of all anthropogenic CO2 emissions. We are seeing exciting evidence that CO2 markets could incentivize farmers to change their practices (note below).

Carbon capture and storage technologies, including direct air capture, are next on our list and are expected to sequester around 100 tons of CO2 per acre in our base case. In turn, this is derived from technical papers in our model below. However the range is broad, spanning from 5 tons to 1,000 tons of CO2 per acre, depending on reservoir quality. Injectivity is also expected to follow a decline curve, over a c50-year sequestration project. Total CCS or DAC costs will likely range from $70-200/ton, which is generally higher than nature based solutions.

Simple reforestation comes next, expected to sequester 200 tons of CO2 per acre over the course of a century. This has all been achieved after c40-years, as a typical forest’s growth follows a sigmoidal trajectory. After c40-years, the rate of trees’ growth has approximately halved (data below) while the rate of biomass decomposition will cancel out new carbon uptake. We note our numbers here are on the conservative side, as forest biomass is generally estimated between 200-400 tons of CO2e per acre in technical papers. To be clear, for reforestation projects to absorb CO2, you must start with an area that is not forested (e.g., sourced from the world’s 2.5bn hectares of degraded lands), re-forest them, then the forests must remain in tact.

Active forestry can almost double the net CO2 absorption from forests over a 100-year timeframe. Specifically, our recent research considers the opportunity turning forest products into carbon-negative construction materials, such as cross-laminated timber, locking up the carbon in the wood products for centuries (note below). Our numbers assume that one-third of the forest’s carbon is lost due to harvesting and in processing forest products. Then a new cycle of reforestation can begin immediately. This explains the “saw-tooth” profile for sustainable forestry in our chart above.

Most of the carbon-absorbing systems considered above tend to mature and slow down their rates of CO2 sequestration. But three further examples in our chart continue their rate of CO2 absorption almost unabated.

Seaweed aquaculture has been the focus in our recent research, underpinning a vast carbon sink, forming 20 tons of dry biomass per acre per year, of which c10% tends to detach and sink into the deep ocean, where it is thought to be effectively sequestered for millennia. Seaweed and kelp biomass turns over around 10 times per year. Hence the flywheel of ocean carbon sequestration keeps spinning indefinitely. Over a century, seaweeds should sequester around 230 tons of CO2 per acre of seaweed cultivation per year.

Mangrove forests may absorb 450 tons of CO2-equivalents over the course of a century, helped by two factors. First, they are fast growing plants, absorbing around 9 tons of CO2 per acre per year, compared with our base case of 5 tons of CO2 per acre per year for terrestrial forests. Second, they shed material into swamp-like blue carbon eco-systems that lie among their thick roots. Material continues accumulating in this eco-system, which actually turns shallow waters into swamps, then from swamps into land. Again our numbers may be conservative, as some technical papers estimate that mature mangrove forests contain around 1,000 tons of CO2e per acre. Mangrove restoration costs $3-130/ton, depending on the location and the hurdle rate (note below).

Biomass burial can likely sequester the most CO2 per acre of any option in our chart. This involves fast-growing crops, which absorb 10-20 tons of CO2 per acre per year, then harvesting this biomass, and burying it 10-meters underground, so that the carbon is effectively trapped. We estimate that two-thirds of the fixed CO2 could be buried, the buried material will decompose at a rate of c1% per annum, and with a $15-50/ton CO2 price the practice could sequester around 8x more CO2 than converting the crops into biofuels (note below).

Biochar? While adoption of this biomass burial practice is currently negligible, a similar feat can be accomplished via biochar. This is already a $1-2bn per year market, accelerating at 10-30% per annum. Instead of burying the biomass, it is pyrolysed into an inert material, which in turn can be scattered onto soils, as a one-off, or year after year.

Our conclusion is that vast amounts of carbon can be removed from the atmosphere, in natural ecosystems, over the course of the next century. Each acre of land can absorb 40-800 tons of CO2e. Active management techniques such as biomass burial and sustainable forestry are most effective, while blue carbon eco-systems can also yield rapid and sustained CO2 uptake. Please contact us for any questions on nature-based carbon offsets.