Weatherwatch: Hotter, drier summers may mean more forest fires

The Pechanga fire burns down a mountain runoff near Temecula, California. Photograph: Steven K Doi/AP

This year has been a bad year for forest fires in the US, with wildfires burning about eight hundred thousand hectares by the end of June. This is higher than average, in spite of the relatively low number of fires. But there may be worse to come.

The Wildland Fire Assessment System maps the 'Dead Fuel Moisture Level' in the US. This indicates how much flammable wood there is on the ground, a critical factor in how fire spreads. There are several grades of fuel involved, ranging from "one-hour" twigs, which dry out or absorb moisture rapidly, to "thousand-hour" logs which only burn after a long dry spell. As drought deepens, the amount of dry fuel on the ground is growing, just waiting to ignite.

A recent study of tree-ring scarring looking back 1,400 years at Southern Methodist University, Dallas, suggests the current spate of forest fires is unusual. The pattern has remained similar, with wet years of growth followed by dry years of increased fires, but the scale of fires has changed.

In previous centuries minor fires were common, burning grasses but leaving trees standing. Twentieth-century forestry management prevented small fires, so brushwood built up, leading to fewer but more intense fires which destroy trees. Modern forestry policy encourages a natural ecosystem by allowing some fires, which is slowly improving matters. Meanwhile the series of hotter, drier summers may produce more and bigger forest fires than ever.