The rainforest is burning in Sumatra, Indonesia. A majestic Meranti tree is close to the fire line. As flames from the surrounding canopy rage closer to this ancient tree, water quickly evaporates from its leaves, transforming the foliage into dry fuel for the fire. When turbulent, convective winds within the canopy bring hot air to the immediate surroundings of our tree, long-chain cellulose molecules in the desiccated foliage begin to crack, producing hydrocarbon gases and solid char deposits; with a flash, the gases ignite, forming the flames of the fire.
This brief, catastrophic episode at the end of this tree’s life quickly re-balances a century of organic chemistry. Throughout its life, the tree had accumulated a great amount of potential energy in the form of carbon stripped from its preferred partner oxygen thanks to energy from the sun through photosynthesis. Once provided with a source of ignition, carbon atoms in the tree and oxygen atoms in the air gain enough energy to reunite, releasing a great amount of energy, sustaining high temperatures and producing a chain reaction that is a fire. As the great physicist Richard Feynman once put it, “the light and heat that’s coming out [of a log fire], that’s the light and heat of the sun that went in. So it’s stored sun that’s coming out when you burn a log!”
So there we have it, a forest fire is simply recycled sunlight, a natural component of the carbon exchange between the biosphere and atmosphere. After all, fire is endemic to many of our planet’s natural environments, essential for regeneration, fertilisation and germination of many plant species. But does all of this biomass burning have any influence on our climate?
Unlike combustion of fossil fuels, which can only be replaced over millions of years, the biomass burned in a fire is replaced by new growth, sequestering (trapping) carbon emitted by the burning. Human intervention, however, has violently tipped the scales from this natural order, with the widespread use of slash-and-burn to clear forests for agriculture, often leading to soil erosion, which helps prevent future return to a forested landscape and leading to a net release of carbon to the atmosphere. Furthermore, in tropical environments like Sumatra, this permanent destruction of forests by fire can be accompanied by the burning of the deep and ancient underground carbon-rich peat layer.
In our Sumatran example, we see the annihilation of a tree by a forest fire and its vaporisation to the gases from which it was made. What we do not directly ‘see’ is the effect that these gases, along with aerosols in the smoke, have on climate. Latest estimates of total carbon emissions suggest deforestation fires and peat burning contribute between one to three petagrams (1,000,000,000,000 to 3,000,000,000,000 kg) of carbon per year (equivalent to 10-30% of all human-made emissions). Emissions of greenhouse gases from biomass burning, particularly carbon dioxide, methane and nitrous oxide, represent the single most important fire influence on contemporary climate, accounting for a fifth of human-induced greenhouse warming.
Small particles of carbon and sulphates, collectively known as aerosols, are also released into the atmosphere. During their journey, some aerosols absorb sunlight, warming the atmosphere, whilst others reflect sunlight back into space, leading to cooling. Aerosols can even influence the formation of clouds, acting as a seed for cloud droplet formation, leading to longer-living and brighter clouds which also reflect sunlight back into space. The overall effect of aerosols is the subject of on-going research, but is likely to be small compared with warming caused by greenhouse gases.
Better management of forested and peatland environments presents an opportunity for carbon abatement by reducing emissions from fires in these ecosystems. Indeed, the potential reduction of net carbon emissions from preventable human-caused fires exceeds the reductions required by the Kyoto Protocol. Pioneering carbon abatement economies have begun to recognise that significant greenhouse gas abatement can be achieved through fire management. In Northern Australia, the West Arnhem Land Fire Abatement programme led to an historic agreement between oil giant ConocoPhillips and the Northern Territory Government amounting to AUS$17 million to provide a fire management service in return for carbon credit, providing livelihoods for hundreds of indigenous land rangers.
Emissions from deforestation and peatland fires are often overlooked as a cause of climate change, yet these avoidable tragedies account for a sizeable chunk of the effect. Climate change may now be resulting in increasing fire activity, longer drying seasons and reduced life expectancy of trees due to drought-stress; a vicious positive feedback cycle that some experts claim will ultimately lead to the mass drying-out and incineration of continental-scale tropical ecosystems, including the Amazon rainforest. Better management of fire-prone ecosystems could provide significant income for some of the world’s poorest countries through developing carbon economies, and will hopefully prevent the permanent destruction of our increasingly threatened forests.