One of the deadliest wildfires in a generation vastly expanded Monday to cover more than 8,000 acres, sweeping up sharp slopes through dry scrub and gnarled piñon pines a day after fickle winds and flames killed 19 firefighters.

The gusty monsoon winds where the Colorado Plateau begins to drop off into the Sonoran Desert continued to bedevil about 400 firefighters who were defending 500 homes and 200 businesses in the old gold mining villages of Yarnell and Peeples Valley.

Scientists said those blazes and 15 others that remained uncontained from New Mexico to California and Idaho were part of the new normal — an increasingly hot and dry West, resulting in more catastrophic fires.

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.”