Pretty good news.
Really bad news.
That’s how it’s going lately in the high-stakes effort to predict the future of Arizona’s vital monsoon.
So here’s the short version of the good news: We’re still hoping for a near-normal monsoon this year — which should dampen one of the most dangerous fire seasons in decades sometime in July. Last year, the “nonsoon” ranked as the second driest monsoon ever recorded. This year, there’s a better than 50/50 chance we’ll have a normal monsoon, based on forecasts relying on sea-surface temperatures in the Pacific Ocean.
Moreover, those monsoons do a lot more to recharge our stressed water tables than we thought. That’s a relief as the state heads into water rationing on a major scale. The federal government has warned that Arizona faces a cutoff of its water from the drained reservoirs on the Colorado River, prompting an increased reliance on already-dropping water tables.
So that’s all good news, right?
Savor it a moment.
Wish we could stop there when it comes to summarizing the latest research on the future of Arizona’s monsoon season — which sustains Arizona’s fire-adapted ponderosa pine forests and spells the difference between Arizona’s lush Sonoran Desert and California’s bleak Mojave Desert.
Alas, the long-term studies on the likely impact of rising global temperatures on the Southwest’s unique monsoon season offer nothing but bad news.
For instance, wet, reliable monsoons will likely give way to fickle, violent monsoons in coming decades.
And that means more massive wildfires, dying forests, depleted water tables and the urgent need to adapt Arizona communities to a hotter, drier climate and more major wildfires, according to a slew of recent research.
Previously, most climate and fire experts assumed wet winters largely determined the severity of the fire season — not to mention groundwater recharge. But recent studies show the two-month Arizona monsoon season plays a much larger role than we thought.
Taken together, the research suggests the state and local communities can’t count on a “normal” monsoon in coming decades.
That means the state will have to invest in more infrastructure to store runoff in both water tables and reservoirs.
Meanwhile, local governments will have to adjust to much longer, more dangerous wildfire seasons with fire-adapted codes. This includes both brush clearing to avoid town-destroying ember storms building codes that fire-harden homes and businesses.
So here’s a summary of recent studies on the fate of the monsoon in the face of rising global temperatures.
Monsoons play key role in controlling wildfires.
Tree ring studies show that in the past 400 years the worst fire seasons result from the combination of a dry winter and a weak monsoon, according to the study published in the peer-reviewed International Journal of Wildland Fire.
“It’s well-known that what really gets us through May and June in Arizona is winter rainfall and snowpack. It adds moisture to the system,” said lead-study author Alexis Arizpe, who was a University of Arizona research specialist and graduate student in the Laboratory of Tree-Ring Research when he did the research, according to a summary of the study on the website Science Daily.
“The start of the fire season is controlled by winter rain, but the monsoon controls the end of it. The physical rainfall limits fire with deceased temperatures, increased humidity and soil moisture.”
In the past four centuries, Arizona never had giant fires in a year with both a wet winter and a wet monsoon. Sometimes, dry winters resulted in big fires no matter how much rain the region got in the monsoon. But the biggest fires came when both the region’s rainfall seasons failed.
Interestingly, Mexico in recent decades hasn’t suffered nearly as much from megafires even with a dry winter and a dry monsoon. The researchers noted that Mexico generally follows more traditional land management practices — with only seasonal cattle grazing and much more frequent use of prescribed fires.
By contrast, the US Forest Service has relied mostly on all-out fire suppression for the past 100 years — allowing dangerous levels of fuels to build up in the forests.
Monsoon storms help recharge dwindling groundwater
Roughly a quarter of the rain that falls during the monsoon ends up soaking into the underground water table, according to a study by researchers from Arizona State University published in the peer-reviewed journal Water Resources Research.
Previously, researchers had assumed that the winter storms and snow accounted for most of the groundwater. They reasoned that the monsoon rain fell on thirsty soils and ran off so quickly that not much would end up in the water table.
Not so, concluded the comprehensive study between 2010 and 2018 that combined field measures, unmanned drones, a hydrological model developed by ASU and colleges from the Jornda Long Term Ecological Research Program of the National Science Foundation.
Turns out, even though monsoon rain quickly runs off the dry slopes it readily soaks in as it runs down the network of small, normally dry arroyos and streambeds.
“The results of this study show that monsoon storms serve an important role in recharging groundwater aquifers near the point of runoff generation,” says ASU hydrologist Enrique Vivoni of the School of Earth and Space Exploration and the School of Sustainable Engineering and Built Environment.
“This is an essential process that banks renewable surface water for future use as a ground water resource in the arid Southwest and throughout the world.”
Interestingly, the research lends new urgency to the controversial federal proposal to extend Clean Water Act protections to these mostly dry washes and arroyos.
The research suggests that chemicals or pollutants dumped in these dry washes can readily end up in the water table.
Global warming drying up monsoon
The recent decline in average monsoon rain will likely get worse in coming decades, according to a study published in the journal Nature Climate Change.
Previous studies have suggested the warming trend will likely delay the normal, early-July onset of the monsoon — pushing the rainfall into September and October.
That was bad enough — since it suggested the most dangerous phase of the fire season would last for an extra month. However, the recent research suggests the monsoon will get both later and weaker, according to the scientists from Princeton and the National Oceanic and Atmospheric Administration.
The new prediction stems from an attempt to correct decades of ocean temperature measurements based on soundings by thousands of ships at sea and other sources. Climate researchers have long fretted about a persistent “sea surface temperature” measuring bias between the well-measured northern hemisphere and the less intensively measured southern hemisphere. The previous predictions rising temperatures will merely delay the monsoon did not correct for that measuring bias. The most recent study did.
Changes in the atmosphere above 5 miles up combined with weakened atmospheric convection will likely combine to both delay and reduce the number and duration of the summer thunderstorms.
“Monsoon rains are critical for the southwest US and northwest Mexico, yet the fate of the North American monsoon is quite uncertain,” said Salvatore Pascale, the lead author on the paper. “The future of the monsoon will have direct impacts on agriculture, on livelihoods.”