Flagstaff could see up to a 40 percent decline in summertime monsoon precipitation if carbon dioxide emissions continue to rise across the globe.
The projection comes from a new study by a team of researchers from Princeton and the National Oceanic and Atmospheric Administration that modeled how rising surface and air temperatures caused by increasing emissions would affect the monsoon in the southwestern United States and northern Mexico.
A change in these summer rains has implications for water resources as well as water-related hazards like flooding across those regions, said Sarah Kapnick, a research physical scientist with NOAA and a co-author on the paper.
So what sort of impact could Flagstaff see if that projection plays out?
With winter snowmelt being the main recharge for local water supplies, the city’s water security may not see a huge hit with a decline in monsoon precipitation. But the story is different for elements like the region’s fire season, its ponderosa pine trees and other native plant species. For them, those summer rains play a major role.
“Plants in our area definitely respond to and are dependent on the monsoon, they are adapted to our climate and that’s what our climate does,” said Lynne Nemeth, executive director of the Arboretum at Flagstaff. “They bloom with the moisture from the monsoon and that’s when they are thriving.”
WHAT THE MONSOON MEANS IN FLAGSTAFF
In overall numbers, the monsoon does provide a hefty proportion of Flagstaff’s precipitation. The city receives an average of 21.8 inches of moisture per year, with rainfall during monsoon season making up about 8.3 inches of that, or nearly 40 percent.
The summer rains help reduce fire danger across northern Arizona in the summer and, along with spring snow melt, are a crucial source of moisture for local vegetation.
“Decreasing that second precipitation event would likely have a strong impact on local vegetation,” said Kristin Haskins, director of research at the Arboretum.
When it comes to ponderosas, the monsoon has more of an indirect impact. According to research out of Northern Arizona University, medium to large-sized ponderosa pine trees around Flagstaff use almost none of the water that falls as rain during the summer.
“It may seem surprising that summer rain is not used, but it makes sense when you consider that most of the tree roots are below 20 cm depth, yet the summer rains wet only the soil surface (except in super heavy monsoon years),” George Koch, a biology professor at NAU wrote in an email. It was Koch’s doctoral student Lucy Kerhoulas, who completed the ponderosa pine research.
At the same time, Kerhoulas found a positive correlation between years with lots of summer rain and greater late-summer growth in ponderosas. Koch said they think that while the monsoon rains may not directly impact tree growth, they are accompanied by milder, more humid air that spurs the trees to ramp up photosynthesis.
SUMMER RAINS AND WATER SUPPLIES
The rainfall that comes from monsoon thunderstorms makes only a small contribution to groundwater recharge in this region, said Brad Hill, director of Flagstaff’s water services department. Much more groundwater recharge comes from snowmelt, though modeling shows that only 4 to 8 percent of total precipitation ultimately filters down into the local aquifer system, Hill said.
It’s also rare that summer rain will have any effect on the level of Upper Lake Mary, Hill said. In 2013, the monsoon brought such heavy rainfall that the lake rose by 11 percent in two weeks, but that was a big anomaly, he said.
Flagstaff citizens don’t appear to cut back on their water usage in response to the monsoon rains either, Hill said. Looking back over the past 10 years of data, there’s no strong correlation between a month of hefty rainfall and any lower water demand, he said.
“What we surmise is while people cut back when it rains, when the sun comes out they want more water,” Hill said.
STABLE OR UNSTABLE
The projected decrease in monsoon precipitation calculated by the Princeton and NOAA researchers has to do with the forces that cause the summertime thunderstorms to form. Instability in the atmosphere combined with moist air close to the surface is what creates those rain-bearing storms, said Salvatore Pascale, lead author on the paper. But rising surface temperatures have the effect of reducing moisture in air close to the surface and increasing atmospheric stability, which inhibits the updrafts that cause thunderstorms, Pascale said.
The eye-catching 40 percent decrease in precipitation that the study estimates is based on a scenario of carbon dioxide concentrations in the atmosphere doubling compared to 1990 levels of 350 parts per million.
Models show that such an increase isn’t unlikely.
In one scenario used by the Intergovernmental Panel on Climate Change, under which actions are taken by countries to reduce greenhouse gas emissions so they peak in 2040 then begin to decline, carbon dioxide concentrations would reach almost double 1990 levels by the end of this century.
STORMS MORE INTENSE, BUT NOT LATER
Past studies have shown that global warming will cause the monsoon to shift later in the season, but Pascale said his study didn’t produce that finding. In his models, the impact wasn’t a delay in summer moisture, it was an overall drying out, he said. A future study will look at how global warming might impact the intensity of monsoon thunderstorms, Pascale said.
One study published in September by University of Arizona researchers analyzed historical data and found the intensity of monsoon precipitation is already increasing in some parts of Arizona. Shifts in the thermodynamic environment have been facilitating “stronger organized monsoon convection” for at least the past 20 years, the authors found.