Warming temperatures and a historic drought made Northern California’s Valley fire faster to spread and harder to fight. But the bigger culprit, sadly but thankfully, was something under human control: forest mismanagement.

There are five times as many trees per acre in this area today as there were 150 years ago, and the underbrush is twice as thick, according to Scott Stephens, a wildland resource scientist at the University of California at Berkeley. If smaller wildfires had been allowed to burn 20 years ago, the forest wouldn’t have grown so dense, and today’s flames wouldn’t have had the fuel to reach the canopy, consume the treetops and send embers flying, spreading the blaze and resulting in three deaths and the loss of some 600 houses.

The Valley fire, in other words, demonstrates what can happen when public and private landowners (much of the Valley fire is on private land) fail to manage their property. As long as people neglect to thin trees and reduce combustible underbrush — either mechanically or by intentionally setting small fires or letting lightning-ignited fires burn — conflagrations like those now burning in California, Oregon and Washington will continue to grow more widespread and severe. It’s already looking to be a record-breaking year for wildfires in the U.S.

The problem, according to forest experts writing in Science last week, is the widespread official reluctance to risk damage or injury in managing small fires. During the decade ending in 2008, less than half of 1 percent of natural fires were allowed to burn under observation. Extinguishing these small fires preserves fuel to make the next ones more intense.

The U.S. Forest Service — which is in charge of most fire management — also has a financial incentive to put out all fires: While the costs of clearing the underbrush and managing small fires comes from the individual forests’ limited operating budgets, national “fire suppression” is funded by Congress.

Last year, the federal government spent$3 billion putting out fires — five times as much as 20 years ago — and state expenditures have doubled since 1998 to $1.6 billion. This year, for the first time, the Forest Service will devote more than half its budget to fires. Costs are rising for insurance companies, too. The Valley fire is expected to result in the greatest losses since the $2.6 billion Oakland fire in 1991.

The White House in 2014 issued a new national fire management strategy to encourage such practices as thinning underbrush and setting controlled fires during moderate weather. Ideally, in areas close to people’s homes, thinning is done by hand and fires are put out. But farther away, prescribed fires should be set, and in remote forests, natural fires should be allowed to burn under observation.

Over the next decade, most of the 155 national forests will be required to update their plans for fire management. It’s an opportunity to get serious about preventing future Valley fires.