Bushfires can interact with the various different aspects of Earth, known collectively as ‘spheres’. These are the Atmosphere, Hydrosphere, Biosphere, Lithosphere (or Geosphere) and Anthroposphere*; which are all interrelated and connected. This is one of the major concepts in Earth Sciences. The effects of bushfires on these spheres are addressed and explained below.
Fig. 1: Earth’s Spheres. Web. June 2015.
Atmosphere – The layer of gases surrounding the Earth
The albedo effect is the fraction of incoming solar radiation that is reflected off of a surface, i.e. reflectivity . A surface with a high albedo reflects most of the incoming radiation, whereas a surface with a low albedo absorbs most of the incoming radiation. This in turn can affect the local weather in terms of humidity and pressure.
Fig 2: ABC,Bushfire In Adelaide Hills, 2015. 2015. Web. June 2015.
Bushfires lower the albedo of the ground upon which they act because the action of burning organic matter such as grasses and trees causes them to darken (ref. fig 2); thereby lowering their albedo. As the darkened area absorbs more solar radiation, the air surrounding it rises in temperature which can lead to changes in the weather . A warmer local atmosphere can encourage the formation of low pressure zones and warm fronts; thereby possibly leading to the formation of clouds and hence rainfall following a bushfire. The effects of immediate rainfall post-bushfire are discussed in the ‘Hydrosphere’ section. The burning of organic matter can lead to the liberation of sulphur into the atmosphere. The high heat associated with bushfires is sufficient to split diatomic nitrogen and; through reactions with atmospheric oxygen and water, can lead to the formation of acid rain in the form of nitrous, nitric and sulphuric acid . The low pH of acid rain in comparison to the pH of normal rain (5.6) can have drastic effects on native flora and fauna, as well as on man-made and natural limestone structures; which can increase the rate of erosion in bushfire affected areas.
Hydrosphere – All of the water on the Earth’s surface
Following a bushfire, a rainfall may ensue due to the reasons as discussed above. If an immediate rainfall with a high magnitude of precipitation does occur, runoff can occur whereby ash, clay and soil particles can be carried away from the location where the bushfire occurred and into local waterways such as streams and dams .
Fig 3: Australian Government, Diagram Of Bushfire Interactions With Hydrosphere. 2015. Web. June 2015.
Low intensity fires causes leaves to fall off of trees which, when paired with immediate post-bushfire rainfall, have their organic material and compounds leeched into the runoff water and into nearby water sources. This organic matter is broken down microbially and can cause the water to become stagnant due to the excessive consumption of oxygen from the water. This in turn causes Manganese and Sulphur levels in the water to rise, which is associated with taste and odour problems with the water. High intensity fires remove most organic matter via volatilisation; with most of the organic nutrients being passed into the soil. Heavy rainfall following a high intensity bushfire leads to runoff; a mix of ash and fine soil and clay particles containing a high concentration of phosphorous compounds, but a low concentration of nitrogenous compounds. This runoff may end up in a waterway which can pose the risk of an algal bloom due to a high nutrient concentration; which would, in turn, cause the waterways to become stagnant due to an excess consumption of oxygen from the water by the algae .
Biosphere – The region or regions of Earth populated by living organisms
Many flora and fauna inhabit the bushfire prone areas of Adelaide (discussed in the locations tab). If fires are intense enough, a considerable amount of trees can die as a result; resulting in a habitat loss for many fauna. This may lead to the fauna relocating to an already populated area, which could lead to competition for food and resources amongst the community. This is turn would lead to a decrease in certain populations of fauna as a result of natural selection and thereby an imbalance in the food web and/or ecosystem. A decrease in vegetation of certain areas would also cause a disruption in the food web as fauna would have to compete for now scarce resources if enough vegetation is eliminated from the area . Ash produced from fires is chemically basic (high pH) and can affect nearby flora and thereby fauna; as it may not be the natural pH at which the plant requires for normal growth and function. If bushfires are very common in areas, flora may be destroyed prior to reaching sexual maturity and therefore will not be able to reproduce; leading to a net decrease in the productivity of flora and oxygen production by photosynthesis in the area.
Lithosphere – The region of Earth consisting of the crust and upper mantle
Bushfires are not geohazards that affect the deep Earth, but rather the very top layer of the lithosphere. Bushfires can reach temperatures between 3000°C and 4250°C, which is sufficient enough to change the chemistry of the topsoil in a given area. This leads to a lowering of nitrogen concentrations in the topsoil, which is already at a low concentration in Australia . Rocks that may be present in bushfire affected areas are subject to heating. Most rocks that are heated to over 540°C and then cooled rapidly (e.g. immediate rainfall) are able to be cracked or crumbled; depending on the rock type. Quartzite, sandstone and slate tend to become crumbly near cracks and thus are able to be eroded at a greater rate. The erosion of rocks by rain or even acid rain can leech chemicals into the runoff and affect the other spheres in the same manner as discussed above, in terms of runoff contaminants . Bushfires pre-heat their fuel (organic matter) via convection and radiation so that it easily combusted by the time the fire reaches it. It is because of these heating methods that the slope angle of the ground affects the rate at which the fire spreads. Bushfires will accelerate up a slope and decelerate down a slope. As a rule of thumb, for every 10 degree increase in slope from flat ground (0 degrees), a fire will double in the rate at which it travels. For example, if a fire is spreading from flat ground up a 30 degree slope, the rate at which it travels will increase by a factor of six .
(refer to slope image in ‘Causes and Preventions’)
* The effects of anthroposphere are discussed under causes and prevention