1 Not Just Recycled Sunlight – Thomas Smith

Not just recycled sunlight: Biomass burning and its

influence on global climate change

Prescribed Forest Fire in Banff National Park, Canada (photo taken by T. Smith)

Thomas Smith

PhD Student, Environmental Monitoring and Modelling Group

Department of Geography, King’s College London

Environmental Physics Group Essay Competition – Entry 2010

2 Not Just Recycled Sunlight – Thomas Smith

Since the beginnings of the industrial revolution in the 18th century, biomass burning has remained a

significant component of humankind’s emissions to the atmosphere. Biomass burning is postulated as being

one of the main drivers of interannual variability of carbon dioxide (CO2) and methane (CH4), both

important greenhouse gases, with burning in high fire activity years (e.g. 1994/1995 and 1997/1998)

releasing enough of these gases to account for the anomalous pulses in CO2 concentration observed

globally1. Biomass burning, therefore, represents one of the most important fluxes of the carbon cycle,

forming an integral component of our planet’s tropical forest, savanna, grassland, chaparral, boreal forest

and tundra ecosystems.

On average, satellite records show that 3.5-4.5 million km2 of vegetation burns in global wildfires each

year, an area roughly equal to the total size of the European Union. In a world with an emerging carbon

economy, abating carbon emissions by improving fire management has the potential to yield significant

financial rewards, providing jobs and bringing thousands of people out of poverty. Yet questions remain as

to whether fire regions act as carbon sources or sinks, whether humans or climate are more important in

determining fire patterns, and whether greenhouse gas abatement can be improved by practicing different

land management in fire affected regions. If we are to begin answering these questions, it is important to

understand the chemistry of combustion and the physics behind the many local and global effects of both

the trace gas and smoke emissions from biomass burning.

Recycling sunlight in a burning rainforest

The rainforest is burning in Sumatra. A forty metre tall meranti tree is close to the fire line. As the flames

from the surrounding canopy rage closer to this ancient tree, water quickly evaporates from its leaves

closest to the inferno, transforming the foliage into dry fuel for the fire. When the turbulent, convective

winds within the canopy bring hot air to the immediate surroundings of the desiccated foliage, the

temperature rises above 140°C. At this temperature, combustion of the tree begins with the pyrolysis of its

leaves and stems; the long-chain cellulose molecules begin to crack, producing hydrocarbon gases and solid

char deposits. Almost immediately following pyrolysis, at sufficiently high temperatures, the plumes of

hydrocarbon gases emanating from the tree ignite, forming the flames of the fire. Slowly but surely, the

trunk, the main artery of the tree, dries out and burns. As the biomass of the tree is quickly converted into

simple molecules of carbon dioxide, nitrates and other gases, the weakened trunk succumbs to its own

weight and the tree falls in flames. As the tree is slowly consumed, the supply of hydrocarbon gases slows,

lowering the temperature of combustion. At this stage, complete oxygenation of carbon and nitrogen

becomes difficult, leading to greater emissions of carbon monoxide, methane, ammonia and other non-

oxygenated compounds. Without the supply of hydrocarbon gases, the flames subside, giving way to cooler

smouldering of the solid char deposits, releasing fine carbon particles into the buoyant atmosphere. Once

the fuel is spent, the combustion process ends, leaving a layer of potassium and silicate rich ash on the now

deforested, bare ground.

This brief, catastrophic episode at the end of this tree’s life quickly rebalances one hundred years of

organic chemistry performed by the tree. For the duration of 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. Great temperatures in excess of 600°C are needed for the

carbon and oxygen atoms to be reunited in combustion; once provided with a source of ignition, this

bonding releases a great amount of energy, sustaining the 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!”

Natural or ‘human-made’?

So there we have it, a biomass fire is simply recycled sunlight, an exchange of carbon between the

biosphere and atmosphere to balance things out. Indeed, fire is endemic to many of our planet’s natural

3 Not Just Recycled Sunlight – Thomas Smith

environments. A forest fire will clear the forest floor of leaf litter and logs, returning their nutrients to the

soil. The seeds of some species are protected by wax coated cones, requiring the heat of a fire for

releasing the seeds. Fires will open the forest canopy to sunlight, encouraging regeneration and regrowth;

the patchy mosaic of different tree species of different ages that results from this is vital to the

biodiversity of many fire-affected regions.

But does all of this burning and emission of greenhouse gases have any influence on our climate? Unlike the

burning of fossil fuels, which can only be replaced over millions of years, the biomass burned in a fire is

replaced by new growth, with the new growth sequestering the carbon emitted by the burning. Yet only

10% of wildfire ignitions are caused by natural forces such as lightning, with 90% of wildland fires started by

human ignition. Human intervention has violently tipped the scales from equilibrium, with the widespread

use of slash-and-burn to clear fire-resistant forests for their replacement by relatively flammable

agricultural land2, often leading to soil erosion, and in some cases, desertification, preventing the future

return of a forested landscape and leading to a net release of carbon to the atmosphere. Furthermore, this

permanent destruction of forests by fire is often accompanied by the burning of the underground carbon-

rich peat layer, which may be up to 20 metres thick. In our Sumatran example, we see the annihilation of a

tree by a forest fire, its vaporisation to the gases from which it was made. What we don’t see is the

contribution that these gases, along with aerosols in the smoke, and changes in the albedo of the surface,

have on climate.

Greenhouse gases

Global climate models that are relied upon for our prediction of future climate change3 require an accurate

understanding of global biomass burning carbon emissions to account for their large contribution to the

global carbon cycle. To achieve this understanding, it is necessary to estimate the total mass of material

burned and released into the atmosphere:

�� � � � � � � � ���

Where the emission of trace gas species x (Ex) is equal to the multiple of the annual burnt area (A, m2 year-

1), the mass of organic matter per unit area (B, g m-2), the fraction of organic matter that actually burns

(�) and the mass of gas species x released per unit of burnt biomass (EFx, g kg-1). Due to a combination of

uncertainties associated with estimating burnt area, biomass fuel loads and quantifying the amount of

gases and aerosols, a large degree of uncertainty is usually quoted with total carbon emissions. Latest

estimates of total carbon emissions suggest a total carbon loss due to biomass burning (i.e. after

considering regrowth) of between 1-3 petagrams of carbon per year (equivalent to ~19% of all fossil fuel

emissions).

Emission of CO2 from biomass burning represents the single most important fire influence on contemporary

climate4, although biomass burning also releases other greenhouse gases (including methane and nitrous

oxide, as we see in our Sumatran example). If we only consider biomass burning for deforestation and peat

burning, the total contribution to radiative forcing by biomass burning (the change in the Earth’s energy

balance relative to preindustrial conditions at 1750 C.E.) amounts to 0.4 W m-2.

Whilst biomass burning does not lead to direct emissions of ozone, emissions of reactive oxygenated

compounds (e.g. nitrogen oxide, nitrogen dioxide, carbon monoxide, formaldehyde) promote the

production of ozone, a potent greenhouse gas. Ozone produced downwind of biomass burning amounts to

around 17% of anthropogenic ozone production in the troposphere (amounting to a positive radiative forcing

of ~0.15 W m-2)4.

Aerosols

In addition to the greenhouses gases given off by our incinerated tree, small particles of carbon and

sulphates, collectively known as aerosols are released into the atmosphere, carried aloft by the buoyancy

4 Not Just Recycled Sunlight – Thomas Smith

and turbulence caused by the fire. They may remain in the atmosphere for many weeks before settling.

These aerosols have a number of direct and indirect effects on climate both during their lifetime in the

atmosphere and once they settle. The direct effects include the scattering of light, leading to a cooling

effect. Globally, however, fires only contribute around 2% to this effect4. Fires have a more considerable

contribution to the warming effect caused by tropospheric black carbon, however, this effect may only

account for around 0.01 W m-2 radiative forcing.

Uncertainties associated with the indirect effects of fire aerosol emissions are far greater. Aerosols

originating from fires are believed to significantly influence cloud formation, lifetime and albedo (the

proportion of solar radiation reflected by a surface). Fire aerosols can act as cloud condensation nuclei,

promoting the formation of clouds, decreasing the droplet size and thus increasing their persistence and

albedo, leading to cooling5. Conversely, the indirect effects of black carbon near fires can lead to a

decrease in cloud cover due to the evaporation of clouds due to localised warming caused by the absorption

of sunlight by the black carbon particles6. To date, no global estimate of the fire aerosol effect on clouds

exists. Reducing the uncertainty associated with the indirect effects of all aerosol emissions, natural and

anthropogenic, is one of the biggest challenges to current climate science3.

Surface change and albedo

Biomass burning influences the albedo of a surface through a number of short- and long-term effects. On

shorter timescales, the blackened surface following a fire will lead to some short-lived warming of the

surface due to its lower albedo7. At higher latitudes, fires can increase the exposure of snow (by removing

the canopy cover) and thus increase albedo, causing cooling. The long-term effect of biomass burning on

albedo is related to the widespread replacement of low albedo forests with croplands and grasslands that

have a higher albedo (imagine the dark rainforest in our example, and its replacement by lighter green

palm oil plantations). It is estimated that biomass burning is responsible for 0.15 W m-2 cooling due to this

change in albedo8.

In areas removed from biomass burning activity, the effects of aerosols settling from the atmosphere can

have profound albedo effects if the black carbon particles fall to coat a very high albedo surface, such as

snow. The total radiative forcing from black carbon on snow is estimated to amount to around 0.1 W m-2, of

which around 20% is attributed to fires9. Whilst the uncertainties associated with these albedo changes are

broad and relate to both radiative uncertainties, and uncertainties regarding fire histories. The negative

forcing by land use change is greater than the positive forcing by black carbon on snow, amounting to a net

negative albedo forcing4 of ~0.1 W m-2.

Towards a fire economy?

The net effect of these various influences on climate is clearly a significant positive radiative forcing

(Figure 1), although a great deal of uncertainty is tied into these numbers, particularly the effects of

aerosols. Given these uncertainties, it becomes difficult to put a price on carbon abatement by fire

management. Nevertheless, pioneering carbon abatement economies, such as the West Arnhem Land Fire

Abatement (WALFA) programme in Northern Australia10, have begun to recognise the significant greenhouse

gas abatement that can be achieved through fire management. WALFA has led to an agreement between

the oil giant ConocoPhillips and the Northern Territory Government amounting to AUS$1 million a year for

17 years to provide a fire management service in west Arnhem Land, providing livelihoods for hundreds of

indigenous land rangers.

Climate change associated with greenhouse warming may now be resulting in net increases in fire activity

due to higher temperatures, longer drying seasons and reduced life expectancy of trees due to drought

stress. The expansion of carbon abatement through fire management could provide significant income for

some of the world’s poorest countries, whilst also preventing the permanent destruction of our increasingly

threatened forests.

5 Not Just Recycled Sunlight – Thomas Smith

Figure 1. Contribution of deforestation fires to climate change4. The shaded inner bar (blue indicates cooling; red, warming) is

the estimated fire contribution to the total radiative forcing of various components of climate change identified by the IPCC5

(unshaded outer bar). CO2 emissions, formation of ozone and the direct effects of aerosols lead to warming, whilst the change in

surface albedo is responsible for cooling.

Endnotes

1 Schimel and Baker, Nature 420, 2002. 2 Mouillot and Field, Global Change Biology 11, 2005. 3 Forster et al., Climate Change 2007, IPCC, 2007. 4 Bowman et al., Science 324, 2009. 5 Andreae et al., Science 303, 2004. 6 Kaufman and Koren, Science 321, 2006. 7 Jin and Roy, Geophysical Research Letters 32(13), 2005. 8 van der Werf et al., Atmospheric Chemistry and Physics 6, 2006. 9 Flanner et al., Journal of Geophysical Research 112, 2007. 10 http://www.nailsma.org.au/projects/walfa.html