Monsoon rains supply about 25 percent of the groundwater recharge that feeds this area’s aquifer. The rains are also crucial to maintaining urban and residential greenery and triggering plant growth in the mountain highlands.

Mike Christy / Arizona Daily Star

Continued climate change will likely slash the Southwest’s summer monsoon rains, a new study says.

The strong possibility that annual monsoon rainfall totals will drop 30 to 40 percent in this region by the end of the century if major steps aren’t taken to reduce greenhouse gas emissions is the take-home message of the study, written by researchers from Princeton and a federal weather agency among others.

“If you think about our children, the next generation, the monsoon in general and where our water comes from will be fundamentally different for our children than it was for us,” said one of the study’s eight co-authors, Sarah Kapnick, a federal researcher who says that Western water issues are “what keeps me up at night.”

Southern Arizona monsoons are critical for the region’s water supply, producing more than half the Tucson area’s normal annual rainfall of about 11.5 inches. This past summer’s monsoon totaled 8.57 inches, well above the normal monsoon of about 6 inches.

If the monsoon were to be slashed 40 percent, that would cut our total annual rainfall by about 20 percent.

“It would have significant impact” on the Sonoran Desert, said Thomas Meixner, a University of Arizona hydrologist who was not involved in the study. “There would be less plant production, less food for animals, less water for them to drink.”

Monsoon rains also supply about 25 percent of the groundwater recharge that feeds this area’s aquifer. The aquifer is heavily used for drinking water in Tucson’s suburbs such as Oro Valley, Green Valley and Marana and by private well owners in unincorporated suburban areas.

Tucson relies mostly on Colorado River water for drinking. But the aquifer will serve as a backup supply for Tucson in the event of a Colorado River shortage.

Also, monsoons are critical for forests, wildlife and rangeland health in the mountain highlands and grasslands, where summer rains trigger much of the plant growth and productivity, Meixner said.

Monsoons also are important for maintaining urban and residential greenery in particular, he noted.

If you want to see how the Sonoran Desert surrounding Tucson after a century of declining monsoon rains would look, just drive west to the Gila Bend area, which currently gets 30 to 40 percent less monsoon rains than we do, said Mark Dimmitt, a retired Arizona-Sonora Desert Museum scientist.

“About half of (this) region’s plant species are adapted to respond primarily to summer rains. Densities of many species would decline significantly, and numerous others would die out here altogether,” said Dimmitt, who also wasn’t involved in the study.

Basically, the Sonoran Desert’s most arid areas, now centered in the lower Colorado River Valley, would expand. The desert’s wetter areas, such as the Tucson area’s saguaro-palo verde forests, would try to migrate to higher, wetter elevations, Dimmitt said. But there are two problems with that, he added.

First, most of these desert species aren’t tolerant of the colder winters existing at higher elevations. The greater instability triggered by climate change means that extreme conditions such as periodic, severe freezes will continue for many decades or longer, he said. So the numerous, drought-tolerant, cold-sensitive plant species that live in Organ Pipe National Monument, for instance, couldn’t move here, he said.

“But the other problem is the speed of climate change, which is occurring many times faster than earlier natural climate shifts. So many plant and animal species will not be able to migrate fast enough to keep up with their moving zone of survivable climate,” Dimmitt said.

Findings important
for water planning

Researcher Salvatore Pascale, the new study’s lead author, and co-author Kapnick said the study’s findings also are relevant for long-term water planning.

“Both for water use, and how you store water. It’s more important that you store water so you have it when it’s dry,” said Pascale, a Princeton University research scholar.

Water storage projects take years to a decade or longer to plan and build and can last decades, Kapnick said.

“They require knowledge of future climate, to ensure water supply in dry years,” said Kapnick, a National Oceanic and Atmospheric Administration research scientist in Princeton. NOAA is the parent agency of the National Weather Service.

“This research can also be useful for storm sewer planning, and for flash flood planning.

“Understanding how extreme weather may change is very important for building our resiliency,” to help society cope with the changes, Kapnick said. “When you have extreme weather, you have extreme precipitation events.”

How warmer weather reduces rains

More greenhouse gases in the air will increase temperatures, most climate scientists say. The warmer air warms the oceans and increases the amount of water vapor in the atmosphere.

That will increase the stability of the lower atmosphere. In a summer storm, the air rises, but a more stable environment makes it much harder for precipitation and thunderstorms to develop, Kapnick said.

“When you have air rising in the atmosphere, that creates rain clouds, creates rain and creates thunderstorms,” she said.

This study was published online last week, in the journal Nature Climate Change. Its conclusion differed from several  monsoon studies that have suggested that the monsoons will simply shift in time due to climate change, with decreased rains through July but increasing precipitation in September and October.

The reason for the different conclusion is that this study was able to correct and account for errors in past monsoon studies in calculating sea surface temperatures, which play a significant role in developing thunderstorms, the researchers said.

“These temperatures actually matter for regional air circulation, providing moisture for storm conditions to develop,” Kapnick said.

But the study didn’t predict how long it will take for the monsoon rains to drop up to 40 percent. The researchers simply used a computer model to project what would happen if carbon dioxide concentrations in the atmosphere doubled compared to 1990 levels.

How long that will take depends on how much emissions of CO2, a greenhouse gas, rise or fall across the planet over the coming decades, Kapnick and Pascale said.

“If we continue as we are now, it may happen by the end of the century,” Pascale said.

Current trends show that global greenhouse gas emissions continue to rise even as they are declining in the U.S. and other Western industrialized countries.

Under what climate scientists consider the worst-case scenario for increasing greenhouse gas emissions, CO2 levels could more than double by 2100. If some efforts are made to reduce emissions, CO2 levels could still double or rise by lesser amounts by then, depending on how much emissions are reduced.

A cautionary note

This study comes a few months after another study led by University of Arizona researchers found that monsoons are rumbling through parts of Arizona with more intensity but produce less total rainfall statewide than they did 60 years ago.

That study found the increased intensity is occurring in areas outside Tucson, such as the Phoenix metro area, Casa Grande, Ajo, Yuma, Bullhead City, Kingman, Gila Bend, Lukeville, as well as Luke Air Force Base, the Barry Goldwater Air Force Range and the Tohono O’odham Reservation.

The total decline in monsoon rainfall exceeded 30 percent in the Mogollon Rim area of central and northern Arizona. In the Tucson area and other parts of the state, the declines were smaller, particularly in this area.

UA professor Christopher Castro, a co-author of that study, said the new study made a worthwhile effort in trying to improve global climate models’ representation of monsoon behavior.

But he offered cautionary notes. In particular, he noted that the conclusions were based on the work of only one computer model.

Pascale agreed that this study’s findings need to be confirmed with the use of more than one computer model. Kapnick said the researchers are now working on building higher-resolution computer models to study the monsoon in more detail.

This is only the second of several studies they’re planning on monsoon behavior, she said, and the researchers said its findings aren't conclusive..

“We want to improve our understanding of weather and climate and water in this region,” Kapnick said.

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