|Fig. 1: A wildfire in California. (Source: Wikimedia Commons)|
Wildfires burn about a million hectares of land in the Western United States each year.  How much energy does that release? We can roughly estimate this quantity by piecing together several bits of information from various studies.
We will assume that the bulk of the biomass burned in wildfires is the wood from the trees. Burning wood releases about 2 × 107 Joules per kilogram.  With this conversion factor in mind, our goal is now to find the total mass of wood burned per year.
The foresting industry typically measures amounts of wood in units of volume—not mass.  Luckily, we can use a rough estimate for the typical density of wood, 350 kg/m3, to convert from kilograms to cubic meters.  We now seek the total volume of wood burned per year.
We know the total area burned per year, so next we would like to know the volume of wood in a typical hectare of forest. Surveys for this kind of data distinguish between forest land and timberland. Timberland is an area of forest that can support commercial levels of wood harvesting, while the more general forest land encompasses timberland and forested areas with smaller capacity. For our wildfire calculations, we want to use forest land; however, due to commercial interest, timberland has been studied in more detail. In particular, volume of wood, or growing stock, is only defined for timberland. In addition, only trees larger than 12.7 centimeters in diameter at breast height are included in the wood volume, and only around half the forest land in the Western United States is timberland.  In order to proceed, we will make the rough assumption that all forest land has equal volume of wood per hectare to that of timberland.
In the Western United States, the total area of timber land is about 45 million hectares, and its growing stock is about 9000 million cubic meters (Western United States consists of Arizona, Colorado, Idaho, Montana, Nevada, New Mexico, Utah, Wyoming, Oregon, Washington, and California).  This yields 200 cubic meters of wood per hectare.
The total energy released per year from wildfires in the Western United States is thus roughly
|E||=||106 hectares × 200 m3/hectare × 350 kg/m3 × 2 × 107 Joules/kg|
|=||1.4 × 1018 Joules|
For magnitude comparison, this is about 1% of the world's yearly energy expenditure in coal. 
© Jonathan Timcheck. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 J. S. Littell et al., "Climate and Wildfire Area Burned in Western U.S. Ecoprovinces, 1916-2003," Ecol. Appl. 19, 1003 (2009).
 N. Krajnc, "Wood Fuels Handbook," Food and Agriculture Organization of the United Nations, 2015.
 W. B. Smith, J. L. Faulkner, and D. S. Powell, "Forest Statistics of the United States, 1992, Metric Units," U.S. Department of Agriculture, General Technical Report NC-168, 1994.
 R. R. Maeglin and H. E. Wahlgren, "Western Wood Density Survey Report No. 2," U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Research Paper FPL 183, 1972.
 "BP Statistical Review of World Energy 2015," British Petroleum, June 2015.