The long-debated link between global warming and drought is growing much stronger, says a chapter of the new, international scholarly report on climate change to which a University of Arizona professor was a major contributor.
The Intergovernmental Panel on Climate Change's latest report found that there is now a medium to high confidence that recent droughts and drying trends can be attributed to humans in some regions, particularly Western North America and the Mediterranean area.
In fact, the report predicted that if global temperatures rise by 2 degrees Celsius, or 3.6 degrees Fahrenheit by 2100, the likelihood of extreme droughts in Western North America and the Mediterranean would rise by 100%. The planet is a little more than 1 degree warmer Celsius or 1.8 degrees Fahrenheit today than it was in pre-industrial times. The goal of many climate scientists and the 2015 Paris Climate Accord is to hold temperature increases to 1.5 degrees Celsius or 2.7 degrees Fahrenheit.
But in a high greenhouse gas emissions scenario that boosts global temperatures 5.4 to 9 degrees Fahrenheit, the likelihood of severe droughts in those regions would rise 200% to 300%, said UA Geosciences Professor Jessica Tierney. She's one of 13 lead authors of the new IPPC report's chapter on the water cycle.
"A 100% increase means that a 1-in-10-year drought would occur every five years instead," Tierney said. "A 300% increase means it would occur every three years.
"So climate change makes formerly severe, rare events occur more often," she said.
The conclusions in the latest IPCC report, released Aug. 9, represent a major change from the last IPCC report before that, released in 2013. Then, IPCC found only a low confidence that changes in global drought patterns could be attributed to human influence.
But since 2013, climate change including global warming has gotten worse, "and we've actually started to see extreme events like droughts, floods and heat waves" far more often, Tierney said.
Also, an increasing number of studies have concluded that climate change is connected with drought severity, she said. Scientists also have developed a technique of attribution studies in which they can figure out how much did global warming affect a particular extreme climactic event, she said.
The technique has been used for detecting human influences not just on droughts but on heat waves and on heavy precipitation events, she said.
The ability to attribute individual events to long-term climate change has also benefited from increased use of what's known as paleoclimate information that uses rocks, tree rings, leaf margins, certain types of minerals, warm water animal occurrences and stable isotopes of oxygen.
UA's Tierney's own specialty is paleoclimate studies in which she uses mostly sediments, mud collected from ocean bottoms, and sometimes rock outcroppings, to study past climates.
Many of our most recent droughts, including the big California drought that drastically depleted water supplies in the middle 2010s, are being driven by what is now being called "hot drought," Tierney said.
"You have to have low precipitation for drought, but what's making droughts really bad now is that it is hotter due to global warming," Tierney said in a UA news release about the IPCC report.
"When it's hot, the atmosphere has a higher demand for moisture. To meet that demand, it evaporates moisture from the soil or through plants. And when you lose all moisture in the land surface, it only makes the drought worse."
The U.S. Southwest and California, in particular, offer a case study of the global warming-drought connection, Tierney said.
"For example, the 2012-2014 California drought has been studied intensely by climate scientists who determined that it was the worst one in 1,200 years," Tierney said. "Anywhere from about 10% to 30% of that drought was caused by humans."
The drought in the Colorado River Basin also has a climate change footprint, researchers have found. Former UA climatologist Jonathan Overpeck and Brad Udall of Colorado State University in 2017 wrote a study blaming climate change for one-sixth to one-half the sharp reduction in the river's runoff since 2000. Another study in 2020 by U.S. Geological Survey researchers concluded that half of the decline in the river's average annual flow from 1913 to 2017 was due to warmer weather.
"But we also observe hot drought in the Mediterranean," Tierney said. "These are two hot spots that we've seen really bad droughts that we can attribute to humans."
A houseboat rests in a cove at Lake Powell Friday, July 30, 2021, near Page. This summer, the water levels hit a historic low amid a climate change-fueled megadrought engulfing the U.S. West.Β
The Western United State has been gripped by what some scientists have termed a "megadrought" since about 2000. In 2020, a group of nine scholars from Columbia University and other colleges wrote that the period from 2000 to 2018 in the West ranked second in drought severity only to a megadrought in the late 1500s.
An article published last month by three of those researchers concluded that the Western drought, now 22 years old, "will likely be the West's driest 22-year-period in at least 1,200 years." It was published in the Hill, based in Washington, D.C.
"In other words, 2021 will probably be remembered as a fork in the road for Western drought, when an already long and severe drought had a big growth spurt and entered legitimate megadrought territory," said the article by Professors Park Williams of UCLA and Jason Smerdon and Benjamin Cook of Columbia.
The 2020 megadrought study was one of several that the new IPCC report cited as evidence of links between warming weather and drought.
Another study used tree ring data, computer model simulations and on the ground observations to conclude that the dominant trend in aridity in North and Central America and the Mediterranean can be attributable to external human forces from 1900 to 1949 and again from 1981 to the present, after weakening in intervening years. That study was led by scientists from NASA's Goddard Institute of Space Studies and Columbia University.
The IPCC report also cited four other studies, all but one written since 2012, that it said show human-caused warming is amplifying short-term drought and long term drying trends in Western North America by increasing evaporation and the subsequent water loss to the atmosphere.
The IPCC report also noted that springtime snowpack in California's Sierra Nevada range hit possibly its lowest level in 2015 in the last 500 years. From 2011 to 2015, human-caused warming alone reduced the Sierra snowpack by 25%, said IPCC, citing a 2017 study.
The biggest indicator of these drying trends is not rainfall or other forms of precipitation, however. A more important indicator is the drying of soil moisture as warmer weather causes it to evaporate.
"The link between rainfall and human influence hasnβt really emerged" in climate studies, Tierney said. "Increased drought is coming from warm temperatures for a region like ours. If you look at projections for precipitation in our region, they're still all over the place.
"The droughts weβve seen in California and the Mediterranean, itβs the soil moisture change thatβs unusual."
The prospects of more droughts doesn't mean that global warming will mean no rainfall. In fact, the IPCC report concluded that a warmer climate increases the transport of moisture into weather systems. On average, that makes wet seasons and wet events wetter β something that's occurred this summer during Southern Arizona's near-record monsoon season.
For every 1.8 degree Fahrenheit temperature increase, the atmosphere can hold 7% more water, which explains a similar intensification of heavy precipitation events that increases the severity of flood hazards, the IPCC said.
"The severity of very wet and very dry events increases in a warming climate (high confidence), but changes in atmospheric circulation patterns alter where and how often these extremes occur, with substantial regional differences and seasonal contrasts," the IPCC report said.
As gloomy as the IPCC's drought forecasts sound, it's not a guaranteed doomsday scenario, Tierney added.
"Thereβs definitely still hope. If we cut emissions now and take a big bite out of them, we could basically preserve a world like we see today," she said. "It's not ideal. We are still going to have extreme droughts in the Southwest. We already have warmed the planet over a degree.
"In some ways we are prepared. We know itβs coming. We know weβll get less water from the Colorado. We still have our ability as humans to manage our resources and change our policies for a world where there is less water."
