A question has bothered climatologist Park Williams during the decade he鈥檚 been probing drought in the Southwest. Like other climate scientists, he knew from research papers and worldwide storm patterns that a warming atmosphere is thirstier and sops up more moisture from oceans and the land.
鈥淏ut, in the Southwest, we've seen the exact opposite happening,鈥� said Williams, an associate professor in the University of California, Los Angeles鈥� geography department. 鈥淔or the last 50 or so years, we've actually seen the amount of water vapor molecules in the atmosphere decline鈥� while temperatures have climbed about twice as fast as the global average.
A from UCLA researcher Karen McKinnon largely solved that puzzle by showing the hottest days in the summer months are getting dramatically drier as a result of the Southwestern spring heating up and leaving less moisture behind to cool the summer through evaporation.
The study, published in the journal, Nature Climate Change, shows a surprising new way in which heat and humidity are interrelated and comes at the beginning of a summer that is already sweltering and plagued with wildfire and drought. For some, the paper also raises concerns about future warming in southwestern states.
鈥淲hen you look at those hottest days, we're starting to see pretty large decreases in humidity,鈥� said McKinnon, who works in UCLA鈥檚 Department of Statistics and Institute of the Environment and Sustainability.
Her team analyzed spring and summer data from 28 weather stations at airports in southern California, Arizona, Colorado, Nevada, New Mexico and Utah. They zeroed in on 鈥渟pecific humidity,鈥� a measure of the molecules of moisture in the atmosphere that ignores factors like temperature that are counted in the more commonly discussed 鈥渞elative humidity.鈥�
鈥淥n these hot dry days, the source of the moisture in the air is basically coming from the soils from the land surface鈥攁nd we're seeing declines in near surface soil moisture over the summer over these past couple of decades鈥攁nd that's leading pretty clearly to these decreases in humidity,鈥� she explained. 鈥淪o, we're having lower humidity days as the soil moisture gets drier.鈥�
The researchers found that specific humidity had decreased across the region by an average of about 20 percent. And, in California and Nevada, the decreases were about one-third of the mean value.
In the midst of this Western US , I'm both pleased and depressed to share new work that finds that hot extremes have become drier in the United States Southwest, published today in Nature Climate Change. A 馃У(1/)
— Karen McKinnon (@karalimck)
The link between heat and dryness in the Southwest has been the subject of a growing body of research that鈥檚 attracted lots of attention in recent years. Last year鈥檚 record-setting wildfires in two states, the in Lake Mead, Utah鈥檚 worst drought on record and Arizona鈥檚 year for heat-related fatalities are all reminders that heat and dryness go hand in hand in the Southwest, and have scary consequences.
In the paper, McKinnon鈥檚 team pointed out that these scorching, parched days desiccate vegetation, increase wildfire probability and set the stage for fires to spread more quickly.
A Feedback Loop Brings New Drought Concerns
The pattern identified in the study is a positive feedback loop with negative implications. Increasing heat in the spring dries the soil, which, in turn, raises summer temperatures due to the lack of soil moisture, which helps cool the landscape when it evaporates.
The weekly U.S. Drought Monitor classifies more than half of the West as being in 鈥渆xtreme鈥� or 鈥渆xceptional鈥� drought, with between 77 and 98 percent of the land in Nevada, California, Arizona and Utah in the most severe drought categories. Meanwhile, observations show drought conditions have dominated the region over the past two decades, pushing the Southwest into what鈥檚 called a 鈥渕egadrought.鈥�
The drought has eased in Colorado since the beginning of the year, when the entire state was drought. The most recent drought map shows nearly 46 percent still experiencing some level of parched conditions, with nearly 30 percent of the state in extreme or exceptional drought.
Simon (Shih-Yu) Wang, a professor of climate dynamics at Utah State University, called the McKinnon team鈥檚 new statistical analysis 鈥渁 rigorous approach鈥� that reaffirms the significance of the soil moisture and temperature link that researchers have been studying in earnest for a decade.
鈥淲hen we don't have water in the soil, we're gonna get a lot hotter a lot faster,鈥� he said of the hot-dry pattern that鈥檚 become typical with climate change in the Southwest. 鈥淏asically, it's what you expect in a drought situation in semi-arid regions.鈥�
La Nina-El Nino patterns will eventually bring more precipitation to the region, Wang said, but that鈥檚 likely to bring only short-term relief.
鈥淲e can wait for natural variability to come and then give some water back to the land,鈥� he added. 鈥淏ut, over time, overall, the trend is that this [hot-dry] effect will increase.鈥�
Imtiaz Rangwala, a research scientist with the Western Water Assessment, called the paper a clear sign of a hotter, drier future for the Southwest.
鈥淭he hotter extremes could make it drier, and the drier extremes could make it hotter,鈥� he said. 鈥淭hey could feed on each other.鈥�
This is bad news, in particular, for the two largest reservoirs in the United States, Lake Powell in Utah and Lake Mead in Arizona. Both rely on the Colorado River, the lifeblood of the Southwest, to store and deliver water to more than 40 million people in the region. And both can expect more evaporation because of the higher temperatures. Already this year, the reservoirs are low enough that downstream users are anticipating new water cutbacks.
Mike Hobbins, senior research scientist at the University of Colorado Boulder鈥檚 Cooperative Institute for Research in Environmental Sciences, said the McKinnon study has strong implications, especially for wildfire. Trends identified in the study suggest that fire seasons could become more explosive, he said.
That鈥檚 because fire is strongly influenced by the difference between the amount of moisture the atmosphere can potentially hold and how much it actually has.
The difference between the two is known as the 鈥渧apor pressure deficit.鈥� And the bigger the deficit, the thirstier the atmosphere and the more erratic wildfire becomes. That, plus the dried out condition of vegetation means, 鈥渢he fire season will grow at both ends,鈥� he said.
The McKinnon paper noted that the future of this summertime trend remains unclear, even though it鈥檚 something that the research team considered in its analysis. Uncertainty surrounding precipitation trends in climate models for the Southwest鈥攕ome models show precipitation increasing鈥攎akes it hard to know how much vapor pressure deficit the region will face.
The Southwest ought to prepare 鈥渇or a range of possible outcomes,鈥� McKinnon said. Still, with the projected rise in temperatures and the new information about the hot-dry dynamic leaves little room for optimism about a moister future in the Southwest.
鈥淲e're certainly jumping into another summer that looks like it will be pretty tough in the western U.S.,鈥� said McKinnon, whose paper was published at the height of the deadly southwestern heat wave two weeks ago.
Williams, who had been looking into the possibility that wind patterns were driving the decline in Southwestern humidity, also has his eye on the vapor pressure deficit, which he calls 鈥渢he sponginess of the atmosphere.鈥�
鈥淭he atmosphere is very thirsty for water now, and that means that it is sucking harder than ever on the leaves of forests or on the surface of lakes Mead and Powell,鈥� he said. 鈥淭he evaporative demand of the atmosphere is skyrocketing right now.鈥�
McKinnon鈥檚 paper has him wondering now about how much the region鈥檚 current heat wave is being influenced by this heat-dryness dynamic and how many degrees the vapor pressure deficit has contributed.
2021 is locked in for a summer of extraordinary dryness across much of the West, with the exception of parts of the eastern Rockys. NM and south CO got a welcome storm in early June but a big monsoon is needed to put a dent in the Southwest's giant water deficit.
— Park Williams (@peedublya)
But Williams, part of a research team behind a paper last year showing that the man-made share of climate change can be blamed for about half of the historic Southwest drought, also thinks about how the extra warmth from the heat waves might promote enough convection to result in precipitation. He is also considering whether the hot, dry trend might lead to something as dramatic as a 鈥渟liding out of control, a tipping point.鈥�
鈥淭he ocean is giving the atmosphere more water molecules,鈥� he said. 鈥淪o for this region to have less鈥攕omething profound is happening at the earth surface.鈥�
This story was originally published by , an independent, nonprofit, nonpartisan news organization that covers energy, climate and the environment.
