I feel slightly smug because I am actually storing more carbon in my trees than I am releasing by burning. Sadly, this is not enough.
Even if you are carbon neutral, it is not enough. It is not enough to be taking out as much carbon as you put into the atmosphere because the net amount of carbon in the atmosphere must fall if global warming is to be kept below disaster level in the life times of our children.
Professor Jim Jones of Massey University has been looking at the possibility of whether by mixing the ancient skill of charcoal-making with a bit of modern technology he could develop a way of sequestering carbon from the atmosphere and help agriculture - all at the same time.
The idea is simple. If you cut down a tree and let it lay on the ground it will eventually rot and most of the carbon, which makes up roughly 40 per cent of its mass, will go into the atmosphere. If you burn the tree, again roughly 40 per cent of the mass will disappear into the atmosphere.
However, if you heat the wood in a process called pyrolysis, vapours are driven off which can be cooled and condensed into tar and other products.
Provided this is done with no oxygen present, the wood does not burn but remains like a carbon skeleton. This is charcoal.
If this charcoal is incorporated into the ground it will stay there for hundreds of years, meaning the carbon in the charcoal is recycled through the atmosphere at a tiny proportion of the rate caused by burning and rotting. It is effectively sequestering carbon from the atmosphere.
Using marginal land (land not used for food production) to grow trees and then putting the timber through the pyrolysis process, as much as 12 per cent of atmospheric carbon could be removed from the atmospheric cycle.
This is not a total solution to the problem but it is a step in the right direction.
The good news does not stop there. Incorporating charcoal into the soil helps to reduce acidity (in the short term at least) and its porosity improves aeration and water retention. Reduction in soil acidity means reduced need for liming of the soil and reduced generation of nitrous oxide which is between 250 and 300 times more active than carbon dioxide as a greenhouse gas.
The improved friability of the soil means it is easier to pull a plough through it, so the tractor burns less diesel.
Improved aeration allows soil to drain better reducing the production of methane and, as a way of reducing the greenhouse effect, methane may be a better target than carbon dioxide.
Although the atmospheric concentration of methane is only about 0.5 per cent of that of carbon dioxide, its greenhouse effect is about 80 times greater. Methane breaks down chemically in the atmosphere over timespans measured in decades and so a reduction in methane production in 2018 will show a benefit by 2030.
Incorporating charcoal into soil reduces the need for fertiliser.
This action has three parts. The rate of build-up of new organic matter in the soil is increased by the presence of the biochar which enhances soil quality; complex chemistry between biomass and biochar improves the availability of phosphorus in the soil further reducing the need for fertiliser; and the porous nature of the biochar means applied fertiliser stays in the soil longer and so less needs to be applied.
Having given us the good news, Professor Jones looked at how it all stacks up in the real world.
The real world is run by accountants and so we were now looking at carbon credits. His research looked at orchard prunings, logging residues and wheat straw and the different regimes that can be used with these sources of biomass.
Simply ploughing the residues back into the ground means you are putting carbon into the atmosphere by burning diesel in your tractor. Converting these residues to biochar before putting them back into the soil meant sequestering between 200kg and 300kg of carbon for every 1000kg of biomass processed, which is a very good result. The practicalities are complex.
A charcoal-producing unit can be small enough to carry on the back of a truck so the biomass is processed on site and transport costs are negligible.
If you build a large permanent unit it has the savings of economy of scale, but the biomass and charcoal need to be transported - at the cost of carbon - to and from the site.
This is where the whole thing comes crashing down. Whether you build a large or small plant, it will cost about $400 per tonne to sequester carbon. Sadly, this is way beyond what the industry will presently support and this poses a very difficult question.
Global warming caused by humans is a reality. Technological interventions such as reduction of biomass to biochar are a long way from being economically viable.
At what point, do we have to say avoiding the possible wars and societal breakdown that rapid climate change could cause is more important than turning a buck?
■Frank Gibson is a semi-retired teacher of mathematics and physics who has lived in the Whanganui region since 1989.
