Tucson and Phoenix were the third- and fourth-fastest-warming cities in the United States over the past 48 years, a new analysis of weather data shows.

They were among 10 cities whose average annual temperatures rose more than 4 degrees over that period.

Also, Arizona’s 3.23 degree rise in annual average temperature made it the nation’s third-fastest-warming state from 1970 to 2018, the analysis by Climate Central finds. The nonprofit organization is comprised of scientists and journalists who study and report on changing climate trends.

The two fastest-warming cities in the U.S. from 1970 to 2018 were Las Vegas and El Paso, Climate Central found. Las Vegas’ average annual temperature rose 5.76 degrees.

Overall, cities and states in the Southwest ranked at or near the top when it comes to warming temperatures since the first Earth Day was celebrated in April 1970.

In interviews, several experts not affiliated with the report cited many factors to explain Tucson and the Southwest’s rapidly rising temperatures.

While global climate change impacts are everywhere, in the Southwest they’ve triggered increasingly arid weather that can boost temperatures.

Experts also cited the tendency of residents in this region to cluster in large cities, where massive amounts of pavement and other hard surfaces contribute to what’s known as the urban heat island effect.

Southwest cities dominate the list

Climate Central drew on data from the National Centers for Environmental Information to determine average temperature changes in 242 cities and every state but Hawaii.

Six of the top 10 and nine of the top 20 fastest-warming cities are in the Southwestern states of Arizona, New Mexico, Nevada, California and Texas.

New Mexico was the second-fastest warming state, behind Alaska.

The Arctic area, covering part of Alaska, is warming about twice as fast as the rest of the world. That’s because the continuing loss of sea ice there causes sunlight that would have been reflected by the ice to be absorbed by open water and released into the air, warming both.

The biggest factor in the Southwestern increases is probably the urban heat island effect, said Gregg Garfin, deputy director of the University of Arizona’s Institute for the Environment.

Given that, it’s not surprising that Tucson, for one, is warming faster than the state or the nation, he said.

Buildings, asphalt streets, concrete sidewalks and other impervious surfaces absorb the sun’s heat in the daytime and radiate it outward at night. That props up nighttime temperatures, making summer nights extremely uncomfortable here and in Phoenix.

It’s known from other studies comparing rural and urban temperature changes, including in Arizona, that urban temperature increases are generally larger than in rural areas, Garfin said.

Climate change and the heat island effect make it less comfortable and less safe for outdoor work, he said.

“There are effects on evaporation and evapotranspiration, effects on energy bills for summer cooling, and additional wear and tear to the built environment materials that are affected by heat, such as roadways,” he said.

Statistics bear out the greater impact of the heat island effect, said Arizona state climatologist Nancy Selover.

When comparing the Phoenix area’s average annual low temperature from the period 1941-70 to that of the period 1991-2018, she found a difference in the range of 7 to 9 degrees, Selover said. The difference in average annual high temperatures between those two periods was 2.3 degrees, by contrast.

Since minimum temperatures typically reflect heat island impacts, that’s why she believes the heat island effect was more significant, Selover said. She doesn’t have similar comparisons for Tucson but believes the two cities have similar patterns.

“If you go out at 11 o’clock at night in the late spring or into the summer, for sure, you’ll walk past a block wall and you will feel that heat radiating off that block wall,” Selover said. “It’ll be 2 o’clock in the morning and you’ll still be looking at temperatures in the upper 90s or the low 100s because it takes so long for that (surface) to cool.”

Greenhouse gas emissions also at work

But at the same time, rising temperature trends are being seen all over Arizona, even outside urban areas, said Professor Michael Crimmins, a UA climate science extension specialist.

Trends in temperature increases due to greenhouse gas emissions are “at work all of the time,” he said.

Also, the period from 1970 to 2018 has had some important temperature variability between various decades that’s related to El Niño-driven cool and wet winters and springs, and warmer, drier periods ushered in during La Niña periods, Crimmins said.

Cooler, wetter weather prevailed from the mid-1970s well into the ’80s, followed by warmer, drier La Niña winters in the 1990s, he said.

Overall, there’s “a mess of weather, climate and climate change at work here in the Southwest,” Crimmins said.

Garfin singled out other factors contributing to Southwestern cities’ fast-growing temperatures:

  • The four fastest-warming cities are all in more inland and arid locations, which should contribute to greater warming than in other parts of the country.
  • The cites are fast growing, aggravating the heat island effect with more pavement.
  • The fastest-warming cities also lie in basins inside of mountain ranges, which concentrate heat.
  • El Paso, in particular, adjoins Ciudad Juarez, Mexico, population about 2 million, further aggravating El Paso’s urban heat island effect.
  • Climate change contributes to an increase in high pressure atmospheric conditions. More high pressure leads to less rainfall and other precipitation, and more evaporation of open water and evapotranspiration of moisture from plants.
  • More drying makes it easier for the land surface to heat up, Garfin speculated.

The Climate Central report is important because it takes a conversation that often revolves around national or global temperature change and brings it down to the local level, said John Fleming, a scientist for the Tucson-based Center for Biological Diversity.

Selover said Arizona city governments, including Tucson and Phoenix, are doing a number of things to combat the temperature changes. Those steps include encouraging planting of shade trees and artificial shade structures to keep heat-susceptible surfaces from getting so hot, and encouraging the use of new building materials that do not retain heat as well.

Selover said she believes Arizona residents will rise to the climate challenges their cities face.

“People have said, ‘You know, in so many years we will be uninhabitable.’ I don’t tend to buy into that argument,” she said.

“Because that would assume we are not doing anything to try and mitigate the heat. We are using this opportunity to try and learn some lessons and try some things to find some strategies that will work.”

Contact reporter Tony Davis at tdavis@tucson.com or 806-7746. On Twitter@tonydavis987.