|Sydney skyline with smoke. Photo is from cnn.com here.|
Photographer is Gregg Wood/Getty Images
Fires are a natural part of ecosystems like those in the American Southwest and Australia. For a hundred years, the U.S. Forest Service had a policy of fire suppression that resulted in the accumulation of a large amount of brush. I'm not sure about the Australian fire suppression policies. However, because Australia sees cycles of droughts and floods, large fires have been a prominent part of the ecosystem evolution.
In the Southwest, up until about 20 years ago forests had as many as 50 times the natural density of trees--typically, Ponderosa Pines (the ones whose bark smells like vanilla on a sunny day!). The forests resembled a thicket of giant toothpicks with mops of green hair on top. This was a result of the fire suppression policies. The Forest Service realized the problem that it had created and began to try to thin the trees by letting naturally started fires (e.g., those started by lightning strikes) burn when structures weren't endangered. They also started prescribed burns on days when wind conditions permitted this to be done safely. Such fires clear out the undergrowth. However, people don't like smoke from such fires, and when the occasional one got out of control, the Forest Service was subjected to law suits and so they burns were cut back.
Firefighters are now being quoted in the press as saying that the megafires are something new, that they've fought big fires before but nothing like these. According to Thomas Swetnam, a tree ring expert at the University of Arizona, old trees show scars of fires that burned them, but didn't kill them. Back in the 1600's such fires occurred about every 5-10 years--small grass and shrub fires that left the big Ponderosas and Doug firs alive. But then "around 1890-1900 the record stops--the "Smokey Bear effect."What happened? The Civil War had ended, Reconstruction was nearly finished, and the Manifest Destiny doctrine resulted in the westward expansion. Settlers brought livestock that ate the grass, so fires had little fuel. Then the U.S. Forest Service was formed and, as Swetnam says, "its marching orders were 'no fires.'"Expert wisdom was wrong. (You can see a time-laps series over 88 years showing changes in a forest landscape here.)
The result was a Southwest with forests packed with trees, shrubs and grass--fuel. When fires start in forests with these conditions, the immense heat that they generate actually precooks fuel in front of an advancing fire by drying it out.
There are, broadly, three types of fires (this classification comes from the U.S. Forest Service and is specifically for conifer forests, but general enough to apply here): ground or subsurface fires; surface fires; and crown fires. Ground or subsurface fires spread slowly without visible flames. Surface fires can spread with the wind or upslope (so-called "heading surface fires") or into the wind or downslope (so-called "backing surface fires.") Crown fires advance both through the tree tops and through over the surface. The transition from a surface fire to a crown fire is a significant escalation in the fire intensity, particularly because convection increases allowing embers to be spread far away from the initial fire. Three types of fires are shown in the figure below (taken from the USFS report referenced above).
Forest fire fighters have developed various ways to try to anticipate fire conditions. Van Wagner (see the USFS report mentioned above) hypothesized that the type of crown fire to be expected in a conifer forest on any given day depended on three properties of the canopy fuel layer and two basic fire behavior characteristics. The Fire characteristics are the initial surface fire intensity and the rate of fire spread after the onset of crown combustion. The three properties of the canopy are the foliar moisture content, the canopy base height, and the canopy bulk density. The initial surface fire intensity, the foliar moisture content and the canopy base height determine whether or not the fire will ignite the foliage of the conifer, and the canopy bulk density and the rate of fire spread after crown combustion determine whether the fire can be sustained in the canopy. The initial surface fire intensity and rate of fire spread in turn depend on windspeed, slope steepness, fuel dryness, air temperature, relative humidity and fuel characteristics. Models such as these are put together in various graphs to illustrate fire potential.
|Van Wagner's diagram for fire classification|