Click the link to read the article on the Big Pivots website (Allen Best):
November 14, 2024
New study says greenhouse gases, not natural drought, have been swinging a heavier bat in Western states since 2000
Yet another scientific study finds that rising heat has been playing an outsized role in the spreading and lengthening drought in the American West during the 21st century. The question remains how exactly we will act on this information.
The latest study was published in Science Advances under the title of “Anthropogenic warming has ushered in an era of temperature-dominated droughts in the western United States.”
“For generations, drought has been associated with drier than normal weather,”
said Veva Deheza, a study co-author and director of the National Oceanic and Atmospheric Administration’s National Integrated Drought Information System .
“This study further confirms we’ve entered a new paradigm where rising temperatures are leading to intense droughts with precipitation as a secondary factor.”
Previous studies had demonstrated the influence of precipitation and evaporative demand on drought. The researchers in this new study explicitly sought to disentangle the influences of natural variability from human causes, i.e. warming produced by the addition of greenhouse gases to the atmosphere.
They concluded that around the year 2000 that human-caused warming became the dominant factor in the severity of drought in the 11 Western states and also expanded areas affected by drought conditions.
They attributed 61% of the severity of the drought from 2022 to 2022 to high temperatures and only 31% to reduced precipitation.
“This study has uncovered that rising temperatures and the resulting high evaporative demand have surpassed precipitation deficit as the dominant drought driver around the year 2000 in the (Western United Sates),” wrote Rong Fu, a UCLA climate researcher, a lead author of the study.
“This change cannot be explained by natural climate variability and is mainly caused by warming due to anthropogenic forcing. This drought regime shift has led to increased drought severity and coverage since the turn of the 21st century, marking the beginning of a new era where (western United States) droughts are increasingly driven by evaporative demand rather than precipitation deficits.”
This study builds upon the work of Brad Udall of Colorado State University and other studies. Udall and Jonathon Overpeck, then of the University of Arizona, completed a study in 2017 that found a third to a half of the nearly 50% in reduced flows in the Colorado River from 2004 to 2014 could be attributed to warming. Their study was called “The twenty-first century Colorado River hot drought and implications for the future.”
That study helped spread the use of the word “aridification” for what is occurring. Unlike drought, it is not necessarily something that will end.
Flows in the Colorado River have declined 20%, a process often called aridification. The river and its tributaries provide water from Albuquerque to Denver to Cheyenne, as well as to most of southern California, plus most places between.
Udall last week told the Los Angeles Times that this new study came to the same conclusion of his and other studies.
“They found, just like all these other studies, that higher temperatures have been, and are going to be, a cause of more severe droughts as it warms in the 21st century,” he said. “That means that we need to plan for a hotter and drier future.”
Russ Schumacher, the Colorado climatologist, said temperatures can be expected to increase an average of between 1 and 4 degrees Fahrenheit unless emissions can be abated, creating a climate in Denver more akin to that now found in Albuquerque or El Paso and likely further reducing flows of the Colorado River that have averaged 12.5 million acre-feet this century to possibly 9.5 million acre-feet.
The seven-basin states that share the river are already struggling to come to grips with this existing hotter and drier reality than the compact that was struck a century ago and assumed more than 17.5 million-acre feet.
This, in turn, will force shifts in what crops we grow on farms as well as the choices made in urban landscaping.
Others areas in the 11 Western states don’t rely upon the Colorado River but have parallel problems. For example, portions of eastern Colorado overlie the Ogallala Aquifer, a body of water deposited up to two million years ago that is being drawn down rapidly. Most of the water goes to creating feedstock for livestock.
In his comments to the LA Times, Udall also expressed dismay at the policies that many expect Donald Trump will institute as president, rolling back efforts to hurry the energy transition.
“We know how to solve this problem,” Udall said. “Much of this will now be sidelined to pursue an anti-science agenda that will further enrich the gigantic companies that created this problem in the future place.”
Another way of understanding the study findings is that the higher temperatures caused primarily by burning fossil fuels have made ordinary droughts into exceptional droughts.
A release posted on the NOAA website explained that droughts-induced by natural fluctuations in rainfall still exist, but there’s more heat to suck moisture from bodies of water, plants, and soil.
“A warmer atmosphere can hold more water vapor before the air mass becomes saturated, and precipitation can form. This creates a cycle in which the warmer the planet gets, the more water can evaporate from the landscape and remain stored in the atmosphere longer before it returns to earth as rain or snow.”
Droughts can form even if precipitation patterns remain within a normal range as higher temperatures and evaporation remove water from soils. They can last longer, cover wider areas, and be even drier with every little bit that the planet warms.
To tease out the effects of higher temperatures on drought, the researchers have separated “natural” droughts due to changing weather patterns from droughts due to human caused climate change in the observational data over a 70-year period. Previous studies have used climate models to conclude that rising temperatures contribute to drought. But without observational data about real weather patterns, they could not pinpoint the role played by evaporative demand due to naturally varying weather patterns.
