Plants and trees continually draw down carbon dioxide through photosynthesis

Across the centuries, nature has provided us with a way to keep carbon levels in balance. Unfortunately, as we have dramatically accelerated our carbon output, we have simultaneously run down our natural carbon sinks - forests and soils.

2020 has seen a massive reawakening of interest in nature-based solutions - the carbon storage capacity of trees, soil and algae. The potential is clearly enormous but the apparent simplicity of this solution may also be our undoing: it is easy to use it as an excuse for not doing enough in other areas and it is easy to be sloppy in our thinking about the contribution of these technologies - what really needs to happen for them to fulfil their potential. There is very little bad about trees themselves or about finding better ways of nurturing our soils - we just need to be cautious about how much we can realistically achieve in this space.

Another critical consideration in this space is the duration of carbon storage. Short growth crops and algae store carbon for a short amount of time. Trees take a long time to grow but then store carbon for much longer. Options for extending the duration of storage include: using more wood in construction to lengthen its lifetime; making bio-derived carbon fibre for use as a light, high strength material; and converting biomass into long-lasting biochar which is preserved underground.

As we think about carbon reduction and removal, we think of a baseline of now, early 2020.
For trees to be carbon negative they have to be additional (i.e. not just replacing existing trees). They also have to live for a long time as when they die, the carbon they store is typically released back into the atmosphere (their carbon storage life can be prolonged if they are used in construction).

The challenges, when it comes to tree planting, include:

  • the amount of productive land that must be switched to forestry (although agroforestry offers promising options of combining trees with agriculture).
  • the need to nurture trees for several years after planting - many are just planted and left in which case the dieback rates and destruction rates can be very high;
  • the countervailing effects of tree planting which can include lower surface reflectivity (lower albedo when dark trees replace white snow) and changes in water runoff/distribution, locally.

Other considerations include that:

  • broadleaf trees are better at sequestering carbon than conifers, meaning that northern forests are relatively less efficient carbon sinks than those planted closer to the equator - however conflict over land can be higher in such places;
  • local populations need to be supportive of tress planting and to have a stake in the maintenance and economic benefits - otherwise planted trees are unlikely to survive;
  • many carbon calculations are based on the sequestering capability of trees from about 30-40 years - very young trees store much less carbon as do very much older ones;
  • in order to drawn down increasing volumes of carbon we would need to increase our forest cover every year - we have to recognise that we will be fighting a very hard battle here with the ground constantly shifting under our feet as rising temperatures kill many existing trees, or at least reduce their carbon drawdown capacity.

Other types of plants and soils can also store very significant amounts of carbon (though debate rages as to how much - some argue that we could 10x carbon storage in soils while others hold that this is overly optimistic) - which is one reason why there is a good deal of interest in plants as biofuels.

The advantage of plants is that they typically store a larger proportion of their carbon underground, in their root systems, meaning that this is not automatically released when the plant dies or burns, as is the case with trees. Another way to preserve carbon for longer is to turn it into carbon fibre (previously produced from fossil fuels); research is underway into creating this from algae oil.

There is a good deal of work going on around carbon storage in soils with companies such as Nori and Indigo working with farmers to find the best ways to change their agricultural practices to better store carbon and then selling this stored carbon in the offset markets. The additional benefit here us that there is reduced need for fossil fuel based agricultural fertilisers and as the structure of the soil changes, so crop yields can actually rise, making this a potential win-win solution.

One very promising soil amendment is biochar which is produced through pyrolysis of other forms of biomass (wood, waste). In addition to the direct creation of energy, this process produces a form of carbon that is stable over a very long period of time. In effect, the carbon that would otherwise be released as the biomass feedstock decays is captured and stored underground - a form of biological carbon capture and storage.