Megadrought • NASA study forecasts an 80 percent chance of a 35-year drought by mid-century in the Southwest.
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The specter of drought hanging over the Southwest is already pretty dire, with forests drying out into beetle-killed tinderboxes and reservoir levels plunging. But the current dry spell may barely register in comparison with what has happened in the distant past and could happen in the near future, according to research released this month.
And we may have ourselves to blame.
If the trajectory of greenhouse-gas emissions continues, human-caused climate change would drive up the chance of the Southwest experiencing a "megadrought" lasting 30 to 35 years to 80 percent, said lead author Benjamin Cook, a climate scientist with NASA's Goddard Institute for Space Studies.
Past droughts, such as the Dust Bowl of the 1930s, were temporary changes to an otherwise-stable environment, but global climate change is creating a new normal.
"In the future, we are looking at a really significant transition to drier conditions that will be with us for a long time," Cook said. "We will still have natural variability, but happening with a much drier baseline."
If this scenario plays out, such a prolonged period of high temperatures and low snow would wreak havoc for water planners, already struggling to cope with shrinking rivers and a booming population, which is expected to double in Utah in coming decades.
Current conservation strategies may not be enough to cope with droughts that exceed 20 years.
"The real challenge," Cook said, "is adapting our strategies for water shortages to these more extreme events."
"Is there a trend here?" • Recent studies have forecast that global climate change could increase aridity in the Southwest, but the NASA-led study is the first to indicate such drying may dwarf past droughts that are measured in tree rings.
But Utah's snow-survey supervisor, Randy Julander, is skeptical.
"Whenever you have modeled results that predict not just extremes, but exceptional, unbelievably rare extremes, a lot of caution goes up immediately. Is there a chance it could happen? Absolutely. We have seen it in the paleoclimatological record," said Julander, a hydrologist with the federal Natural Resources Conservation Service.
Still, evidence of a human role in climate change continues to mount, with most of the hottest years on record occurring during this century. According to the National Oceanic and Atmospheric Administration, last year was the world's hottest on record and last January was the second warmest.
In Utah, 2014 was the state's fourth-warmest year despite the mild, damp summer. Salt Lake City saw 220 days with temperatures above normal and nine days set heat records.
This year has gotten off to a balmy, snow-free start with numerous record-high temperatures. So far in February, record-high temperatures have been tied or broken 294 times at Utah locations, while high minimum temperature records have been tied or broken 193 times, according to a NOAA database.
State climatologist Robert Gillies cautioned against reading too much into such observations.
"It's the inherent variability of the West's weather that makes life difficult for climate scientists," Gillies said. "The big question is: Is there a trend here? Is there a trend where this will happen more often?"
The answer may be yes. Records show Utah has warmed during the past 50 years and at a rate much faster than the global average, according to Gillies.
Warming usually goes along with wetter weather, Cook explained, but that doesn't appear to be the case in the Southwest, where future megadroughts could be coupled with extreme heat.
"More important is warm temperatures in the future means we get more evaporation," Cook said. "You dry the soils more than you would even if you're not changing precipitation."
Studying Utah's shifts • The new NASA study, published in the journal Science Advances, used tree-ring data and new-generation models to draw its conclusions about climate trends, testing the modeled predictions against historical observations.
Documented in the rings of ancient trees is a megadrought that parched the Southwest in the 13th century (the 1200s) and probably played a role in the disappearance of the Fremont and Ancestral Puebloans, two ancient American Indian cultures that thrived on the arid Colorado Plateau for centuries.
A new study by Utah scientists, including Gillies' colleagues at the Utah Climate Center, also tapped tree rings to characterize stream flows in the Bear River headwaters, which were periodically racked by major droughts.
"Tree rings are a powerful tool to get annual resolution. You can literally look through every year for 1,200 years. That opens a window to look into the past for how climate has fluctuated," said co-author Simon Wang, a climate scientist at Utah State University.
Researchers concluded that Utahns' historic experience of climate was far from the norm, which proved to be much drier.
Analysis of rings recorded in Utah juniper trees indicates the second half of the 20th century was the second-wettest period in the 1,200-year study period.
"Can we stay that wet forever? What would happen if we fall back to a more normal dry period? Are we prepared for that?" asked Wang, who leads USU's dendrochronology lab. He hopes his colleagues' work will help inform decisions by the region's water managers.
This week, Wang published another study that explains North America's wacky "dipole" weather, with recent drought and mild winters in the West and record cold and blizzards in the Northeast.
He attributes this situation to climate change, which has warmed the western Pacific's tropical waters between the Philippines and Hawaii, altering the ocean currents that modulate North America's climate.
For the past three winters, a high pressure-low pressure divide has persisted, a meteorological yin-yang spanning the continent. The pattern has obstructed moist weather from penetrating the West and ushered major snowstorms into the East.
"The term global warming is not accurate. The warming is not uniform, it is asymmetrical. It warms much faster in the tropical parts of the western Pacific," Wang said. "When you add heat in that part of the ocean, you get high pressure over western North America. This pattern has been consistent the last three years."
His study projects the pattern to strengthen.
"This year it shifted inland," Wang said, "and it now impacts Utah more than last year."
Weighting the dice? • Gabriel Bowen, a University of Utah geochemist who mines ancient cave and lake sediments to reconstruct climate, has also studied this phenomenon. He suspects disruptions to the air currents are playing a role, twisting the jet stream into a giant sideways S-shape snaking across the continent.
"A big thing we are doing to the climate today by burning fossil fuels is the poles are warming faster than lower latitudes. That creates a temperature gradient where the energy is more evenly disturbed in the atmosphere, leading to more wild behavior in the jet stream," Bowen said.
"When the jet stream is very wavy and S-shaped and shoots far north of us, we get sunshine and dry weather and the East gets clobbered by cold Canadian air," he added. "That's what's happening right now and during much of last winter. We believe we will see more of it in the future."
Yet Utah's climate picture will always be complex, Gillies said. Weather patterns over the state's north and south halves are governed by two different global meteorological systems, so their precipitation fates may drive in opposite directions. He pondered whether the north may get wetter, while the south gets drier.
Last year the high pressure ridge hung over California, locking that state in its worst drought in years.
NASA's megadrought study also modeled for reductions in greenhouse-gas emissions associated with fossil-fuel use. The chance of a big drought by mid-century dropped somewhat, but hardly faded to background levels.
"For the Southwestern U.S., I'm not optimistic about avoiding real megadroughts," said co-author Toby Ault, an assistant professor of Earth and atmospheric sciences at Cornell University. "As we add greenhouse gases into the atmosphere and we haven't put the brakes on stopping this we are weighting the dice for megadrought conditions."
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