Humans burn plant matter for many reasons; clearing forests for agricultural land, slash-and-burn agriculture, ritual savannah burning, wildfires. Recent research by Professor Mark Jacobson at Standford University suggests that burning living matter may contribute far more to climate change than previously thought. This is because, unlike other types of emissions, burning plant matter releases carbon particles into the atmosphere which accelerate warming. These particles are also very damaging to human health, and are responsible for the deaths of 250,000 people every year.
Each year, humans pump nearly 40 billion tons of carbon dioxide into the atmosphere. We are now certain that these emissions, along with other greenhouse gases, are altering our climate and warming the planet. One major source of carbon dioxide emissions is burning plant matter, either deliberately or because of wildfires. But the contribution of fires to climate change has not previously been quantified.
Carbon emissions collectively refer to carbon that is released into the atmosphere, usually as a result of human activity. Carbon dioxide (CO2) is the principal greenhouse gas, but there are others. Methane (CH4) is another form of carbon that is a major contributor to climate change, and produced in enormous quantities by livestock such as cattle. But fires also release other forms of carbon into the air. Plant matter contains huge stores of carbon, which is released by fire in the form of carbon dioxide and carbon particles. Fires release carbon particles in the form of soot (black carbon) and other particles known as brown carbon. Fires also produce ash, known as white and grey carbon.
Black and brown carbon are, as their names suggest, dark in colour. This means they absorb heat from the sun, accelerating global warming. These dark carbon particles collect in the tiny water droplets inside clouds, and during the day they absorb heat, warming the water droplet and speeding up its evaporation. As the water droplets evaporate, the humidity inside the cloud drops and the cloud begins to shrink and disappear. The problem is, clouds are pale in colour and naturally reflect the sun’s light away from the Earth. With fewer clouds, more of the sun’s rays reach our land and oceans and begin to warm them.
Black and brown carbon particles also find their way into snow and ice, and cause similar problems there. Carbon particles darken the snow causing it to absorb more of the sun’s radiation, warming and melting it. This problem is compounded because as the snow and ice melts it reveals dark soils and oceans, which absorb more sunlight and warm the planet.
Prof Jacobson used 3D computer simulations to model the impacts of burning plant matter (biomass) on the Earth’s climate over the next 20 years. The simulation revealed that, when we take black and brown carbon into account, burning biomass is far more damaging per unit weight than other human activities. The model predicted that over the next 20 years, human greenhouse gas emissions including carbon dioxide, methane, nitrous oxide and cloroflourocarbons, as well as black and brown carbon, are set to warm our climate by 2 degrees Celsius. Burning plant matter contributes 18% of total carbon dioxide emissions – roughly 8.5 billion tons of atmospheric CO2, and causing nearly half of the warming predicted by the model. Fires have another fairly obvious warming effect on the climate. Fires release heat and this direct heat is responsible for about 7% of the total climate-warming effect of burning biomass.
Ash Ameliorates the Effects, Partially
As well as producing CO2 and black and brown carbon, fires produce ash – white and grey carbon. Again, aptly named, these particles are pale in colour and act to partially ameliorate the negative effects of the other emissions. Being pale, they reflect the sun’s light away from the Earth and reverse some of the harm done by black and brown carbon particles. Unfortunately, their positive effects aren’t sufficient to totally reverse the damage. According to Jacobson’s model, white and grey carbon will act to cool the planet by about 1 degree over the next 20 years. This leaves us with a net temperature increase of 0.9 degrees, and biomass burning is responsible for 0.4 degrees!
A Burning Health Concern
Black and brown carbon is also damaging to our health. Every year, around 8% of mortalities due to air pollution are due to biomass burning, representing the deaths of about 250,000 people. Of these, 90% of deaths are thought to be due to black and brown carbon particles, which have been linked to cardiovascular disease, asthma and respiratory illnesses, lung cancer and low birth weight.
One source of energy that is often touted as being ‘sustainable’ and ‘green’ is the use of agricultural and lumber waste for fuel. The problem with this, as Jacobson’s study shows, is that although it is a renewable source of energy, is is far from sustainable or green. Burning biomass can never be clean or climate-neutral. By releasing CO2, black and brown carbon, and by directly heating our planet, burning any kind of biomass is contributing to climate change.
We have relatively little control over wildfires, however we do have the power to curb other activities that involve burning plant matter, if we want to reduce global climatic warming over the coming decades.
Want to Know More?
- Jacobson (2014) Effects of biomass burning on climate, accounting for heat and moisture fluxes, black and brown carbon, and cloud absorption effects Journal of Geophysical Research: Atmospheres