Freeze-free Desert

The Sonoran Desert is famous for its saguaro cactus and infamous for its scorching heat.

Saguaro cacti are the defining feature of the Sonoran Desert landscape.
Credit: Saguaro National Park

But freezing temperatures play a lesser-known, surprisingly important role in North America’s hottest desert; losing them could alter everything from the desert’s footprint to the types of plants that thrive within it—including the saguaro—according to The University of Arizona study, “Is the Sonoran Desert Losing its Cool?¹”

The study’s authors, Jeremy Weiss and Jonathan Overpeck, believe the answer is yes.

The pair collected data from weather stations throughout the Sonoran Desert, which covers southwestern Arizona, southeastern California, most of Baja California, and the western half of Sonora, Mexico. The study focused on the frequency of freezing temperatures, discussing only the weather trends throughout winter and spring.

Results showed the number of winter days with freezing minimum temperatures have decreased overall, with one Arizona station showing the freeze-free season has lengthened by about one month since the mid-1940s.

Essentially, winter is getting shortchanged.

“The time window is narrowing. We’re heading into winter later and going into spring sooner. There’s less time in which freezing temperatures can affect the environment,” Weiss said.

Freezing temperatures limit the distribution of desert vegetation, keeping cold-sensitive plants in the southern part of the desert and cold-tolerant plants in the northern section.

Winter’s warm-up is not surprising; the data line up with the results of other larger-scale studies, Weiss said.

Figure 1. Map of the Sonoran Desert.
| Enlarge This Figure |
Credit: Jeremy Weiss and Dan Ferguson, The University of Arizona

“It’s in-line with what we were expecting. It’s consistent with anthropogenic climate change,” he said, referring to the warming caused by human activities, such as burning fossil fuels.

Because “human-dominated” global warming is projected to speed up throughout the 21st century, temperatures will rise at faster rates year round, especially in the winter. As a result, the Sonoran Desert’s native vegetation will likely move into new territory that was previously too cold for comfort.

The desert’s southern plants may begin to move northward, as its northern plants expand into surrounding areas. The desert’s future overall boundary may reach higher elevations, stretch farther to the north and east, and contract in the southeast.

Because warmer temperatures also invite exotic cold-sensitive plants, southwestern deserts may be mostly replaced by grasslands and woodlands before the end of the century, according to data from the U.S. Forest Service’s Mapped-Atmosphere-Plant-Soil System study.

In southern Arizona, no non-native species is as aggressively making itself at home in the warmer temperatures as buffelgrass, said Julio Betancourt, a senior research specialist with the U.S. Geological Survey (USGS).

Figure 2. Buffelgrass in the Sonoran Desert.
| Enlarge This Figure |
Credit: Aaryn Olsson, Arizona Remote Sensing Center, The University of Arizona

“Buffelgrass is the worst environmental problem we face” in the Southwest, he said. The African grass kills native species by stealing water and soil nutrients and by using its dense roots to prevent native seeds from germinating.

But that’s not all it does. Buffelgrass brings the threat of wildfire wherever it goes—especially into stands of native saguaro cacti, which are not adapted to fire.

Average grass fires burn at 600 to 800 degrees Fahrenheit. Buffelgrass fires can reach 1,300 degrees and can consume an area the size of a football field in three minutes.

Unlike most native plants, the foreign grass can easily rebound from fire—a trait that aids its exponential spread. Buffelgrass has the potential to double its acreage every year.

“We’re getting ready to see the unhinging of a unique American ecosystem,” Betancourt said, which is why he and USGS scientist Travis Bean are heading the development of a buffelgrass control center.

Involving the local government and business community is the only way to get the “buffelgrass invasion” under control, Betancourt said.

“We need a big, community-wide initiative,” he said. Without it, the desert and its saguaros will disappear.