Heat sends Southwest climate back in time

Land managers also began suppressing fires. Until around 1910, wildfires had burned through Southwestern eco­­systems about every five to 15 years locally, according to tree-ring records – apparently for millennia. Fires continually burned back encroaching juniper and pinyon trees, leaving only those trees mature and strong enough to withstand a burning understory. Absent the fires, young trees moved in unchecked. During the 20th century, areas that were once predominantly grassy – or in the case of Allen’s plot, ponderosa with a grassy understory – became scrubby, erosion-prone pinyon-juniper forest.

Then, beginning a decade ago, a series of hot and dry years ended a 17-year run of relatively moist conditions. The stress killed some trees outright and left many more severely weakened. The stage was set for the final coup: Droves of pine beetles attacked the debilitated trees. The swarms were the southernmost flank of an infestation by several species that occurred – and in some places is ongoing – from New Mexico up the Rockies to Alaska.

Here, parsing the ultimate causes gets complicated. Bark beetles are native to the area, and droughts occur periodically in the Southwest, where weather is greatly influenced by sea-surface tem­­peratures in the Pacific Ocean. In­­deed, in 1956, the tail end of the last great regional drought, conditions were actually drier in some parts than in 2002, the second-driest year on record.

But there was one crucial difference during the most recent drought: The mean temperature was 1.5 degrees F. higher, part of a warming trend over western North America. The extra heat, says Allen, overstressed the trees and pushed large portions of the system over a threshold.

Whereas in the 1950s, only those trees at the lower, drier end of the pinyon-juniper range died, this time trees succumbed throughout their entire range. Around Bandelier, 95 percent of mature pinyons have died. The dramatic shift underscores a facet of climate change that scientists are often at pains to communicate: Ecosystems are shaped by extremes of hot and cold, wet and dry. If those extremes are pushed even a little further away from the mean, the effect can be drastic, especially when relatively slow-growing trees are involved.

Case in point: A mere 1.5 degrees F. increase has helped to wipe out nearly all mature Bandelier pinyons.

“How big and how fast these changes can be becomes much more obvious when you look at these dramatic shifts,” says Dave Breshears, a professor of natural resources at the University of Arizona, Tucson. “We’re really going to be living in a different landscape in the future.”

Full circle

The new climate regime will probably affect how people use water in the area, says Columbia’s Dr. Seager. Thanks to rivers like the Colorado, the Southwest has a healthy water supply, especially compared with northern Mexico, he says. But it’s hardly unlimited. Scarcity will probably force people to rethink how water is divvied up.

According to the United States Geo­logical Survey, for example, in Arizona in 2000, agriculture used four times as much water as everything else combined. But farming contributed less than 1 percent of the state’s GDP. Maybe Arizona is not the best place to grow water-intensive crops like cotton, he says. (The state ranks ninth in the US as a cotton producer.)

Warmer temperatures may have an inordinate impact on species that inhabit the area’s mountains – essentially cool, wet islands surrounded by hot dry deserts. A moist dirt-loving salamander endemic only to the Jemez Mountains, which are only 11,500 feet at their highest, could be pushed off the mountain altogether.

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