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Drought, Fire and Weeds Challenge the West

Are They Natural Events or Horsemen of a Western Apocalypse?

Scientists like Lisa Graumlich increasingly know what causes drought in the western U.S.
Drought. Even the word feels dry and hollow as it leaves your mouth. And fire. Say it so it sounds hot and windy, racing from the ground to the crown of a tree. Then utter "weeds" and hold on to the double vowels long enough to conjure "wild" and "unpredictable."

How should we think about these forces that challenge the big ideas people have about living in an arid west? Should we imagine them as three horsemen of a western apocalypse, charging recklessly over mountains and across the prairie? Or as mythological deities with fickle alliances-fire pairing with weeds; drought joining with fire? Or as natural events made worse by people living where they haven't lived before?

However you think of them, these forces have the power to change your livelihood or maybe just the view outside your window. Drought and fire have been at this a long time. Invasive weeds are the relative newcomer.

Drought is normal; so is rain

Blame drought in Montana on water and air. It starts with huge pools of warm and cool sea water-up to thousands of kilometers across-splashing against each other in the Pacific Ocean like a five-year-old sloshing in a bathtub. Down by the drain pipe it's the coast of Indonesia, where tradewinds stack up warm surface water. The other end of the tub is Ecuador, where an upwelling of deeper water brings cooler air temperatures.

This ocean-sized mixing of air and water affects land mass temperatures and precipitation in western North America. It steers a jet stream either across Montana, where it deposits enough moisture to grow decent crops and fill reservoirs, or drives it further south to Utah and Colorado, leaving Montana dry.

"Increasingly, we know what causes drought in the western U.S.," explained Lisa Graumlich, director of the Big Sky Institute at Montana State University in Bozeman.

Climate reconstructions using tree rings and other records show long, slow oscillations from wet to dry called Pacific Decadal Oscillation. Decades of relative wet or relative dry years are interspersed with short-term cycles known as El Nino.

No, Graumlich paused, drought is not a random event like flipping a coin. Four heads in a row doesn't promise a tail on the next toss. Climate cycles, not chance, are at play.

"We know that El Nino change is on the time scale of a couple of years, but imbedded in that are changes that play out in 20- or 30-year cycles," Graumlich said.

It was relatively wet, for example, when the railroads began depositing waves of farmers on the northern Great Plains. The crops were good until about 1917. It was dry during the Dust Bowl years; wet following WW II; then dry again starting in 1977.

In the 1990s, people began wondering why there were so many bad years, Graumlich said, when in reality the dryness was normal. Climatologists aren't sure when this dry cycle will end and the next wetter one will begin.

"Neither drought nor rain is 'normal' on the arid plains; both are," wrote Montana historian K. Ross Toole. "They are the outer reaches of the pendulum's swing."

A dangerous coupling

Toole also wrote that "certain elements of nature have an affinity for each other." But is the relationship between drought and wildfire as simple as, "When it's dry, it burns?"

The key is the ability for forests to burn, said Jeremy Littell, not the number of years between fires.

A good place to ask that question, where precipitation records can be clutched in one hand and fire scars from tree rings in another, is Yellowstone National Park. There, MSU graduate student Jeremy Littell looked at tree rings from Douglas fir forests. He found that the lower-elevation forests burned, on average, every 25 to 30 years. But the range was three years to 65 years in a single location.

"So a mean value is useless," Littell cautioned. "We should look at the entire range."

The key is the ability to burn, not the number of years between fires. And the ability is driven by climate. Just because fire comes one year doesn't mean it won't come again the next.

After looking at precipitation records, Littell found a relationship between wild fires and long dry spells brought on by Pacific Decadal Oscillations. But only between 1660 and 1720 and then again from 1800 to the present. For 80 years between the 18th- and 19th-centuries, the shorter climate cycle of El Nino seemed to rule.

This graph shows the long, slow sweeps from relative dry to relative wet in the northern Rockies caused by Pacific Ocean temperatures and the direction of the jet stream.

"The take-home message," Littell said, "is that the thing responsible for drought and therefore fire is not just one thing."

Fire suppression, fuel types and loads, elevation, shifts in forest growth, fire history: these all affect wildfire frequency and severity. But on the climate level, Pacific Decadal Oscillation has vied with El Nino for control of the drought system over the long term, Littell said. One promises a dry summer. The other delivers a barren winter.

Put the two together, and it's bad.

"When we get an El Nino in the midst of a [dry] Pacific Decadal Oscillation, that's when we burn," said Graumlich.

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