Archives for July 2008

Two myths about climate change and bushfire management are often repeated in the media:

1. Because of global warming, Australia will be increasingly subject to uncontrollable holocaust-like “megafires”; and

2. Fuel reduction by prescribed burning must cease because it releases carbon dioxide into the atmosphere, thus exacerbating global warming and the occurrence of megafires.

Both statements are incorrect. However they represent the sort of plausible-sounding assertions which, if repeated often enough, can take on a life of their own and lead eventually to damaging policy change.

I’m paraphrasing from an important new report entitled ‘Bushfires, Prescribed Burning and Global Warming’ by Roger Underwood, Chairman of the Bushfire Front, David Packham, Senior Research Fellow at the School of Geography and Environmental Science, Monash University, and Phil Cheney, Honorary Research Fellow, CSIRO, Canberra.

The authors consider in detail at the carbon balance in relation to fire in the three most typical Australian ecosystems: tropical grasslands, tropical/subtropical savannahs and tall forests and conclude:

1. Although the situation is almost carbon-neutral, all fires in tropical grasslands actually sequester some carbon in the form of “black carbon” which is incorporated into the soil;

2. Over time, the carbon balance of fires in tropical and subtropical savannahs is also just about neutral. In some years more CO2 is emitted to the atmosphere from fires than is absorbed by post-fire regrowth, while in other years more carbon is taken up by regrowth than is lost to the atmosphere from fire (including prescribed burning and wildfires).

The management approach that will optimise storage of carbon in Australian savannahs is one of low-intensity, early dry-season burning under mild weather conditions. This protects the overstorey trees and woody shrubs which are consumed by hot late-season fires.

3. Tall forests store carbon in tree trunks, bark, branches and roots, in woody shrubs and mid-storey vegetation and in the litter and accumulated organic debris on the ground. Eventually all old trees begin to decay from within, and in the absence of fire, the accumulated litter on the forest floor begins to rot away. At this point, the rate of release of carbon through decay exceeds the rate of storage of carbon by new growth. Thus Australia’s “old growth” eucalypt forests eventually stop being a carbon sink and become a source of CO2.

Fuel reduction by prescribed burning employs low-intensity fires lit under mild weather conditions at a time when there is still some moisture in the fuel. This ensures that the flames are generally less than a metre high and the fire is confined to the surface layer of fine fuel and the green material in the low shrubs. A properly managed prescribed fire will be conducted at a time when organic matter (including charcoal) in the soil will not burn. The ideal prescribed burn consumes only the surface fuels, leaving behind a layer of ash protecting the soil and the heavy logs.

The amount of CO2 released by a low-intensity fire is small and the store of carbon on the forest floor is rapidly replaced as the fine fuels re-accumulate and the low shrubs regrow. By comparison, a hot summer bushfire burning under drought conditions will consume all of the surface fuels, including large logs and organic matter in the soil which may have accumulated carbon for thousands of years. An intense summer bushfire will even consume the canopies of the tallest trees. The amount of CO2 produced by a fire is directly proportional to the total amount of fuel consumed in the fire. Thus a hot summer bushfire [in Australia’s tall forests] will release massive amounts of carbon.

The authors conclude that from the point of view of carbon storage in grasslands, savannahs and tall forests, the best management approach is one in which large high-intensity wildfires are minimised by periodic prescribed burns carried out under mild weather conditions.

The authors also examine the alarmist concept that “global warming will lead to unstoppable megafires”. They observe that if the current climate change models are correct, there will only be an increase in average annual temperatures of between 2 and 4 degrees over the next 100 years. The effect of this on bushfire behaviour, by itself, will be trivial. Fire intensity is far more significantly affected by fuel quantity, fuel dryness and wind strength, than it is by temperature.

Some climate change computer models also suggest a significant reduction in rainfall, leading to increased fuel drying and increased fuel availability at lower temperatures. This is the same effect as that of drought, a phenomenon which is common in Australia. Drought does result in more intense fires…..but only if nothing is done to reduce fuels before the fire occurs.

The factor which “doomsday” commentators ignore is the opportunity for land managers to get in first, and reduce fuels before a potential megafire starts. In other words, the potential megafire can be forestalled, simply by the adoption of a program of fuel reduction prescribed burning under mild weather conditions.

Finally, the authors advocate that the Precautionary Principle must apply: this means playing safe while the research is being done. The safe approach is not to ban prescribed burning because of an unsupported assertion that it may increase atmospheric CO2 levels, but to promote prescribed burning because it reduces the size and intensity of wildfires.