Drought and Fire

Numerous commentators have blamed the number of acres burned in recent years on (your choice) increased fuels from past fire suppression, increased fuels from timber cutting, and environmentalist obstructions to fuel treatments. But a close look at the data reveal that the main factor responsible for fires today is drought.

The National Oceanic and Atmospheric Administration (NOAA) has data showing the percentage of the U.S. that is "severely or extremely dry" by month since 1900. These numbers can be compared with the number of acres burned in wildfires each year since 1960.

Such a comparison reveals a strong correlation between the amount of drought in July, August, and September and the number of acres burned that year. Calculating r, the correlation coefficient, reveals that drought is responsible for half the variation in acres burned from year to year. When examined on a decade-by-decade basis, drought is responsible for 98 percent of the variation in acres burned in each decade from the 1950s through the 1990s.

On the average, the number of acres burned each year (in millions) is about half the percentage of the U.S. that is under severe or extreme drought. The year 2002 was the droughtiest year on record (by this measure) since 1956, with 36 percent of the U.S. under severe or extreme drought. The second droughtiest year was 2000, at 34 percent. Thus, the surprise is not that so many acres burned those years but that so few acres burned: about 7 to 8 million instead of the 17 to 18 million that would be expected if the average of past years had been met.

Those who are interested can download a Excel data file with all available data on drought and acres burned. The following information can be found in the file:

The "correlation coefficient" (r) is the square root of r-squared, so an r-squared of .25 indicates a 50 percent correlation. That is, one of the variables explains about half of the variation in the other variable.

Here are a few points that are worthy of note or further study:

Anyone else with comments is welcome to email me as well.

The Thoreau Institute | Fire Research