BISBEE, Ariz. — Boots dusty, lungs heaving, Dr. John Wiens searched the boulders of a desolate Arizona mountaintop for the last survivors of a 3-million-year-old lizard population — then said the words that both confirmed his life's work and broke his heart. 

"They're not there," he said. "It seems like the species is now extinct." 

The loss of plant and animal species on Earth is happening at a speed never seen in human history, according to the United Nations. That includes the likely extinction of the lizards Wiens has studied for 10 years — the population of Yarrow's spiny lizards found in the Mule Mountains of southern Arizona. 

• See more stories about how scientists are trying to protect life on Earth

"There's a lot of species on Earth, and we're going to lose a lot of them because of climate change," said Weins, an evolutionary biologist at the University of Arizona. "It's catastrophic."

Riding the elevator to extinction

Over the last 3 million years — a million years longer than humans have been around — the Yarrow's spiny lizards in the Mules adapted to live in cool mountain climates called sky islands.  

Because the desert floor below is too hot, the lizards were essentially marooned at higher elevations, as if on an island, and cut off from other Yarrow's populations in southern Arizona and northern Mexico. 

These lizards were also easy to find in the wild, unlike many other species. They would often sun themselves on large rock outcroppings. That behavior allowed Wiens and his colleagues to regularly count their population to see how they were affected by a warming climate. 

In 2014, the team could not find any lizards below 5,700 feet. Up to that elevation the temperature in the mountains had gotten too hot. In 2021-22, they returned to the Mules to count lizards in the same spot. They were gone.  

"They're dying at lower elevations," he said.  

At that point, the lizards could only be found living much higher, at 7,100 feet, a cooler elevation. In a scientific paper, Wiens and his colleagues calculated the rate at which the lizards were dying, concluding that it is among the fastest rates ever recorded. 

But because the highest peak in the Mules is 7,700 feet, the Yarrow's spiny lizards were quickly running out of elevations with cooler air. Based on its calculated rate of decline, and with nowhere else to go, Wiens projected the lizards would go extinct here by 2025 — a phenomenon that scientists call riding the "elevator to extinction." 

In March of this year, a survey trip into the mountains with CBS News proved his hypothesis correct, one year ahead of schedule. Wiens could no longer find any lizards, though it will take several more trips before rendering a conclusion. 

"It seems like the species is now extinct, this distinct lineage that's been separated for about 3 million years," he said. "This is what the future is going to look like. This is climate-related extinction."  

According to Krista Kemppinen, a senior scientist with the Center for Biological Diversity, who did not participate in Wiens' research, the implications are dire for other species in the Sonoran Desert, where the Mules are located, as they may already be at the upper threshold of how much heat they can tolerate. 

"The region is really like a ticking time bomb when it comes to climate change," she said. 

Humans "have some responsibility for this" 

According to an exhaustive 2019 U.N. report, 1 million plant and animal species are threatened with extinction around the globe. 

Wiens concluded the number is likely far higher in a more recent research paper, he published in Global Change Biology. He estimates that 3 to 6 million species will be threatened with extinction in the next 50 years, driven heavily by climate change, which will make it too warm for many species to survive. 

"As human beings, in the developed world, we all sort of have some responsibility for this," Wiens said. 

While the distinct 3-million-year-old population of Yarrow's lizard species is presumed extinct in the Mule Mountains, its distant relatives still exist in other mountainous locations in Arizona and Mexico — though many are also in decline. 

Still, across the country, 1,700 plants and animals are listed as threatened or endangered under the Endangered Species Act, which provides resources to help protect the species and their habitat. 

The act is widely viewed as an environmental success story. Some high-profile species on the list include: 

• The California condor, the largest flying bird in the U.S., with about 90 adults remaining in the wild. 

• The iconic Florida panther, with about 200 animals remaining. 

• The massive North Atlantic right whale, which roams the Atlantic Ocean; all that's left are 250 individuals. 

Still, the Endangered Species Act only covers a fraction of the species at risk, in part because the process of listing a species can be long, bureaucratic and political.  

"It can take on average 12 years, when legally it should only take two," Kemppinen said.  

Not enough time for the Yarrow's spiny lizards of the Mule Mountains. 

In:
• Endangered Species Act
• Climate Change
• Endangered Species
• Arizona

David Schechter

David Schechter is a national environmental correspondent and the host of "On the Dot with David Schechter," a guided journey to explore how we're changing the earth and earth is changing us.