The sunset in Phoenix on Oct. 13, 2020, the 143rd day over 100 degrees in 2020. This tied a record from 1989. (Photo: Rob Schumacher/The Republic)
The Southwest has grown hotter and drier during the past decade, and new climate data from the federal government shows these changes have been dramatic, shifting the long-term averages that represent the region’s “normal.”
The country’s updated climate “normals” were released Tuesday by the National Centers for Environmental Information, encompassing weather data for the 30-year period from 1991 through 2020. The federal government releases these long-term averages every 10 years as an up-to-date benchmark for comparing with the weather on a daily basis.
Compared with the 30 years that ended in 2010, the new averages show temperatures have gotten warmer nearly everywhere in the county. But the warming has been most pronounced in the Southwest, with average temperatures generally rising between 0.5 degrees F and 1 degree F, an increase that federal meteorologists said clearly reflects the influence of global warming.
The updated averages also show large decreases in precipitation across the Southwest during the past 10 years, with many areas seeing average precipitation diminish by more than 10%.
“That’s been rather remarkable,” said Mark O’Malley, a meteorologist with the National Weather Service in Phoenix. “The Southwest has over the past 10 years experienced a drying trend. Whether that continues or not remains to be seen.”
The new averages removed the 1980s, which was a relatively wet decade in the West, and replaced it with a decade that brought higher temperatures and some of the most severe drought years on record.
The shifts have far-reaching implications in arid Western states, factoring into the availability of water, wildfire conditions and utilities’ forecasts of energy use, among other things.
This map produced by the federal government shows changes in annual precipitation from 1991-2020 as compared to 1981-2010 (Photo: NOAA/National Centers for Environmental Information)
“Regardless of what happens with the precipitation amounts, the temperature increases that we’re experiencing over the past decade, and that we’ll probably see over the next decade, are really affecting wildfire and drought. Because if you think about it, you’re adding heat,” O’Malley said.
“It dries out the trees and the shrubs and the grasses,” he said. “And so even if we get the same amount of precipitation, because we’re warmer, we’re creating an environment that’s more ripe for wildfires and more susceptible to droughts, because it’s just our temperatures are so much warmer now.”
In a summary of the new climate normals, the National Weather Service said “the influence of long-term global warming is obvious” in the rising temperatures, as shown in a series of maps depicting 30-year periods since 1901.
Areas with increases in average temperatures are depicted in red blotches on the maps, appearing starkly starting in the 1970s and spreading across most of the map during the last update, with the deepest shade of red covering much of the West and other regions, such as parts of the Northeast and Florida.
This series of climate maps released by the federal government shows changes in average annual temperatures for 30-year periods between 1901 and 2020. Human-caused climate change has been pushing average temperatures higher across the country, and in the past decade the most pronounced warming has occurred in the Southwest. (Photo: NOAA/National Centers for Environmental Information)
“For the Southwest, the warming trend has been steady and persistent,” the National Weather Service said. Changes in average rain and snow have been somewhat more varied, it said, “but in general a wetter climate has developed over the northern and eastern United States while a drier climate is being observed in the western parts of the country.”
The National Weather Service also said, “it’s not clear among climate models whether this is a more permanent shift or trend in precipitation amounts, or just a temporary decadal signal.”
Weather stations in places across the Southwest recorded substantial changes in the latest 30-year averages as compared with the normals a decade ago.
In Phoenix, for example, the average annual temperature increased 0.6 degrees, while annual rainfall decreased from about 8 inches to 7.2 inches.
Scottsdale saw temperatures rise by 0.8 degrees and precipitation decline by 1.5 inches.
Drier areas of the Sonoran Desert also experienced shifts. Yuma’s average temperature rose 0.2 degrees while average precipitation decreased about 0.3 inches. In California, the weather station in Imperial recorded averages temperatures of 0.7 degrees higher and a decrease in annual rainfall from about 3.4 inches to 2.4 inches.
Across the Southwest, the largest decreases in rainfall occurred during transition months in the spring and autumn. Summer monsoon rains also decreased in many areas.
“I think the drying is probably more pronounced than people expected it to be. And this last year, of course, was a major contributor,” said Kathy Jacobs, director of the University of Arizona’s Center for Climate Adaptation Science and Solutions.
The largest reservoirs on the Colorado River have declined as warmer, drier conditions have taken hold in much of the West over the past two decades — more than 20 years that take up much of the 30-year time frame that was analyzed.
The drought has intensified over the past year, influenced by extreme heat, and has pushed the levels of Lake Mead to the threshold of a shortage, which is expected to trigger water cutbacks next year in Arizona, Nevada and Mexico.
The drought is also worsening in California, where the snowpack in Sierra Nevada now stands at just 15% of the average for this time of year.
Lake Mead near the Arizona/Nevada border March 18, 2019. A high-water mark or “bathtub ring” is visible on the shoreline; Lake Mead is down 139 vertical feet. (Photo: Mark Henle/The Republic)
The American West has always cycled through natural swings between wet and dry spells, as exemplified by a severe drought in the 1950s that shows up in precipitation maps as brown splotches covering large portions of the West. But scientists have recently found that about half the trend of decreasing runoff in the Colorado River’s Upper Basin since 2000 was caused by unprecedented warming. With hotter temperatures, more moisture evaporates off the landscape and plants take up more water, in turn shrinking the flow of runoff into streams and contributing to drought.
In one recent study, researchers concluded that much of the West is now in a “megadrought” that’s being worsened by rising temperatures with climate change. In another study, researchers found that rainstorms have been happening much less frequently in the desert Southwest during the past 45 years.
With extreme heat on the rise, cities have been sweltering. The urban “heat-island” effect concentrates warming as exposed pavement and roofs soak up heat from the sun and push temperatures higher.
In Arizona, where people endure some of the country’s most extreme heat, state health officials reported 520 heat-related deaths last year.
During the past two decades, hotter temperatures and drought have also contributed to some of the largest and most destructive wildfires on record in the West. In parched forests, brush and grasslands, dry vegetation has been primed to burn, and has gone up in flames.
The new long-term averages compiled by the National Oceanic and Atmospheric Administration show the big-picture trends of warming and drying in the Southwest as well as detailed variations by area and from month to month.
“This is excellent communication, in my view,” Jacobs said. “Because things are changing as rapidly as they are, I think acknowledging that what we call ‘normal’ is evolving quickly is a critical point to be made to the public.”
This map produced by the federal government shows changes in average annual temperatures during the period 1991-2020 as compared to 1981-2010. (Photo: NOAA/National Centers for Environmental Information)
Connie Woodhouse, a professor at the University of Arizona’s School of Geography and Development, said the new temperature normals reflect warming that is largely due to climate change and “don’t tell us anything we didn’t already know.”
It’s also no surprise to see that it’s been drier over the past 30 years as compared to 1981-2010, Woodhouse said, because the 1980s were quite wet and the 2010s were dry.
“Time will tell if we shift into a wetter phase in the next decade or two,” she said.
Ian James covers water, climate change and the environment for The Arizona Republic. Send him story tips, comments and questions at firstname.lastname@example.org and follow him on Twitter at @ByIanJames.
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