Moisture Shortfall, Heat Threaten Southwestern Forests

Pinecone-littered forests draped over tens of millions of acres of mountaintops through the American Southwest are in danger of being scorched out of existence by global warming.

It’s not just rising heat that threatens to put a meteorological flamethrower to lush montane swaths of Arizona and New Mexico better known for low-altitude cacti and desert plains.  A fire-wielding threat also comes from a rise in vapor pressure deficit, or VPD — a parching force linked to climate change that rises as heat increases or as humidity decreases.

“My guess is that, yes, Southwestern forests will survive climate change,” Lamont Doherty Earth Observatory bioclimatologist Park Williams said.  “But not in a way that would be recognizable to somebody who knows Southwestern forests today.”

Scientists, including Williams, studied tree rings and climate models, reporting in Nature Climate Change in 2013 that vapor trends and rainfall and snowfall changes threatened to expose the forests of the Southwest’s sky islands — a whimsical monicker given to the mountain peaks — by the 2050s to drought stresses not seen in the past 1,000 years. That could cripple and kill the forests, they warned.

More recently, an international team of scientists that included Williams studied climate data, investigating the atmospheric factors that fueled record-breaking forest fires in eastern Arizona and northern New Mexico in 2011.  Those fires left behind great barren patches of charred earth that the trees are expected to struggle to repopulate, triggering substantial erosion that damaged farms.

The researchers discovered that the sweeping blazes were worsened by a happenstance combination of meteorological forces.  Those forces produced incendiary atmospheric conditions that are expected to become commonplace in the region’s climate-changed future.  (The fires were also stoked by the buildup of trees, branches and leaves, thickened by decades of firefighting and fire suppression.)

As climate change increases temperatures, reduces snowpacks and pushes vapor pressure deficits upward in the American West, most of the already fire-prone region is suffering worsening wildfire seasons.  A Climate Central analysis in 2012 found that, since the 1970s, the average number of large fires every year nearly quadrupled in Arizona and Idaho.  In California, Colorado, Montana, New Mexico, Nevada, Oregon, Utah and Wyoming, it more than doubled.

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