It’s easy to imagine that biomass burning such as forest fires add significantly to the amount of carbon dioxide in the atmosphere. After all, burning turns the carbon stored in solid wood into CO2, right?
Actually, things are a little more complicated, says Rowena Ball, at the Australian National University in Canberra. Biomass burning can actually store carbon, but only if the conditions are just right.
At high temperature, biomass burns to form water, carbon dioxide and other gases such as methane.
But at lower temperatures, biomass burns to form water and charcoal. When that happens, the carbon is stored and the burning process becomes a net carbon sink.
The question that Ball raises is whether, by carefully managing the way we burn wood and other vegetation, we can remove enough carbon dioxide from the atmosphere to have a significant effect on our climate.
That’s not an easy question to answer. Like almost every topic in climate science, there are numerous subtleties that make the calculations fiendishly complex.
For a start, charcoal formation is hugely exothermic. This heats the biomass, raising its temperature to the carbon dioxide forming regime. The result is that biomass burning is a kind of thermal oscillator that switches back and forth between carbon storage and release. The process that wins out depends on various other factors, such as dampness.
Even when charcoal formation wins out, that doesn’t always help. The amount of carbon stored has to be balanced against the amount released from the charcoal reservoir.
The amount of time that carbon is stored as biomass is about 100 years or so, about the lifetime of the average tree.
The amount of time carbon is stored as charcoal is much longer, thousands of years, but eventually it can be oxidised by various processes and released into the atmosphere as CO2. The rate at which this happens depends on the amount of charcoal in the reservoir. So if the reservoir becomes too big, this oxidation process eventually makes it a net source of carbon dioxide for the atmosphere.
So how much charcoal is currently in the reservoir? That’s one of the important questions that needs to be answered before we can tell whether biomass burning will help or hinder us.
Ball suggests that in the last few hundred years, the Earth has undergone a period of severe fire suppression as humans have cleared forests and removed that surface area from the cycle of fire and growth.
That means the charcoal reservoirs could be running low. If so, charcoal formation would remove carbon from the atmosphere.
That’s a big if. Ball and others will need to do a lot more work to convince anyone that this is a wise move forward. And there are plenty of other factors that need to be taken into account, such as the effect of soot in the atmosphere, which is far from well understood.
That’s not to say this is not a road worth following, just one well laid with many snares and traps of a complex science.
Ref: arxiv.org/abs/1006.4675: The CharCive Challenge: Regulation Of Global Carbon Cycles By Vegetation Fires