Why the West Is Burning

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The connection between drought and wildfires is strong, says Thomas Swetnam, head of the University of Arizona's Laboratory of Tree-Ring Research. And the most dangerous fires, he says, occur when droughts follow years that are unusually wet. That's because generous rains encourage trees, shrubs and grasses to grow, providing the fuel that stokes forest fires. This pattern of wet preceding dry, Swetnam thinks, helped feed the intense blazes that raged through the Southwest shortly after 1850, taking out huge stands of conifers. So, if a new El Nino materializes later this year, as some experts expect, it may bring rains that temporarily ease the fire danger only to increase it later.

The past is an imperfect lens through which to peer into the future, but looking backward provides a glimpse, at least, of the sorts of extended dry spells that those who live in this drought-prone region today should be prepared to endure. The West, observed writer Marc Reisner, has a "desert heart," and we ignore it at our peril.