Since 1970, Arizona has warmed at a rate 0.72 degrees per decade, the fastest among the 50 states, based on an analysis of temperature data by Climate Central, an independent organization that researches and reports on climate. Even as the temperatures have leveled off in many places around the world in the past decade, the Southwest has continued to get hotter.
“The decade of 2001 to 2010 in Arizona was the hottest in both spring and the summer,” said Gregg Garfin, a professor of climate, natural resources and policy at the University of Arizona and the executive editor of a study examining the impact of climate change on the Southwest.
Warmer winters mean less snowfall. More of the winter precipitation falls as rain, which quickly flows away in streams instead of seeping deep underground.
The soils then dry out earlier and more quickly in May and June. “It’s the most arid time of year,” Dr. Garfin said. “It’s windy as well.”
The growing season also starts earlier, so there is more to burn.
“The fire season has lengthened substantially, by two months, over the last 30 years,” said Craig D. Allen, a research ecologist at the United States Geological Survey station at Bandelier National Monument in New Mexico.
The fire potential is exacerbated by the past policy, beginning around 1900, of putting out all fires. Fires are a natural way of clearing out the underbrush. With that natural rhythm disrupted, the flammable material piled up, so when it did catch fire, it ignited a giant fire that burned hotter and wider.
This total-suppression policy began to ease as early as the 1950s, when scientists began to see fire’s role in ecosystems. It was completely abandoned nearly two decades ago.
But in the 1970s, the Southwest entered a wet period, part of a climate cycle that repeats every 20 to 30 years. “That wet period helped keep a lid on fires,” Dr. Allen said. “And it also allowed the forests to fluff up.”
Since 1996, the climate pattern, known as the Pacific decadal oscillation, has swung to the dry end of the spectrum, and the region is caught in a long-term drought.
Stephen J. Pyne, one of the nation’s leading fire historians and a professor at Arizona State University, said, “How we live on the land, what we decide we put on public and private lands, how we do things and don’t do things on the land, changes its combustibility.”
In many landscapes, he added, “you’ve enhanced the natural combustibility” by building hundreds of thousands of homes in fire-prone areas, and for years suppressing natural fires, allowing a buildup of combustible materials like the “slash” debris left behind by logging.
The article explains how this is the dry phase of the Pacific Decadal Oscillation; so we can hope the wet phase will be wet enough to suppress fires. However, the simple fact of additional effects brought on by PDO does not mean warming goes away when the PDO switches phases. Note especially the lengthening fire season over three decades; incrementally, warming is making these well-known cycles, warmer, and too often, destructive or more destructive.