Despite the massive wildfires that have swept the western U.S. in recent years, the region has barely begun paying down the huge fire deficit it has accumulated over the last 100 years.

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A new long-term analysis published in the Proceedings of the National Academy of Sciences clarifies just how out of balance the situation really is. Using existing records on charcoal deposits in lakebed sediments to establish a baseline of fire activity for the past 3,000 years, the study's authors conclude that humans caused fires to shift from their 1,000-year maximum to their 1,000-year minimum in less than 100 years.

That dramatic decline is due mainly to the influx of explorers and settlers since the peak in the 1800s, plus a century of U.S. national policy requiring immediate suppression of all wildfires. Worse, though, is that we humans began minimizing wildfires right at a time when they most wanted to burn, due to rising temperatures and drought.

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The new analysis drives home this point by showcasing a happy equilibrium between climate and fire that existed from at least 500 A.D. until the mid-1800s. By comparing the timing of charcoal deposits with independent fire-history data drawn from historical records and fire scars on the landscape, the 12-member team could see clearly that burning generally increased when temperatures and drought area increased, and decreased when temperatures and drought declined:

• Dry intervals, such as the "Medieval Climate Anomaly" between 1,000 and 700 years ago, had more burning.
• Cool, moist intervals, such as the "Little Ice Age" between 500 and 300 years ago, had less.
• Since the mid 1800s, the trend in fire activity has strongly diverged from the trend predicted by climate alone.

"Policies of fire suppression that do not account for this unusual environmental situation are unsustainable,” co-author Patrick J. Bartlein of the University of Oregon said in a press release.

The team points out that the U.S. Forest Service has given local forest managers more latitude to evaluate which fires to suppress in recent years. Ensuring public safety while letting fires burn is a tough job, but paying down its fire deficit in regular installments may be less painful for the American West than being forced to pay up all at once.

IMAGE:

Active flame front of the 2007 Zaca fire near Santa Barbara, Calif. (Photo courtesy U.S. Forest Service and John Newman via Wikimedia Commons)