Study Shows Threat Of ‘Megadroughts’

Rising temperatures may end monsoons


Payson may have locked down its supply of Blue Ridge water just in time.

That’s because a climate shift already under way will likely result in a sharp increase in droughts lasting for decades and maybe even dry up Arizona’s summer monsoons, according to a study by researchers from Northern Arizona University published in the journal Nature.

That means competition for water in the region will likely intensify in coming decades suggests the detailed study of layers of sediment in a lakebed in New Mexico.

The reconstruction of rainfall and temperature patterns during a 200,000-year period demonstrated how frail are the patterns that produce the summer monsoons that keep streams flowing in Rim Country and make the Sonoran Desert among the world’s most lush. Typically, the region gets almost half its rainfall in the summer.

NAU paleoecology and environmental sciences professor Scott Anderson concluded that the Southwest is headed toward a decades-long megadrought that will affect the monsoons and significantly reduce the region’s water supply.

Payson is already working on building a $30 million pipeline to bring water from the Blue Ridge Reservoir atop the Mogollon Rim to Payson. The water will most likely arrive in about 2014, effectively doubling the town’s long-term water supply. That will make Payson one of the few communities in Arizona with enough water to supply all its projected growth needs.

Currently, Payson depends on well water and those well levels can decline significantly during droughts.

The climate shifts Anderson documented in layers of lake bottom sediments suggests that the projected temperature increase of 2 to 11 degrees F in this century as a result of the buildup of pollutants like carbon dioxide will likely dry up the monsoons, drain the reservoirs on the Colorado River, spawn a devastating increase in major forest fires and force widespread restrictions on water use.

Anderson based his conclusions on a sophisticated analysis of the chemistry and fossil pollen found in layers of sediment laid down through a succession of Ice Ages as a lake rose, fell, disappeared and returned repeatedly in the Valles Caldera in northern New Mexico.

Soil chemistry changes depending on whether rainfall soaks in and trickles down to the water table or evaporates off the surface.

Tree ring studies and other climate reconstructions have already documented the region’s potential for severe droughts lasting decades and dry periods lasting for centuries, even without the threat of global warming caused by the release of heat-trapping pollutants into the atmosphere.

Ironically, Anderson’s climate reconstruction suggested that without the warming trend triggered by air pollutants, the region would instead likely be entering a cooler period — with average temperature dropping 3 to 4 degrees F.

Anderson reconstructed past rainfall and temperature patterns by using the distinctive chemistry of the minerals that form in such lake-bottom environments — especially as the lake dries up and then returns.

Moreover, his analysis of ancient pollen documented changes in the plant communities over time — from oak woodlands to high desert plants specially adapted to drought conditions.

Anderson’s study found repeated periods when average regional temperatures rose by up to 12 degrees F, consistent with the worst-case scenarios for projected warming by the end of the 21st century.

During those periods, he found that the region’s summer monsoons often vanished due to shifts in the currents in the upper atmosphere that steer storms into the Southwest in July and August.

The summer monsoons largely determine the difference between the lush Sonoran Desert with its giant saguaros and California’s much more barren Mojave Desert.

Anderson tracked the climate shifts through a succession of three Ice Ages, matching rainfall patterns to regional temperatures.

Climate scientists still don’t fully understand the dramatic climate shifts that took place during the Ice Ages and the 13, warmer “interglacial” periods. The last Ice Age ended 10,000 years ago. The cold periods followed a rough pattern that probably had to do with irregularities in the earth’s orbit and tilt combined with variations in the sun’s output.

Other factors include the positions of the continents, the composition of the atmosphere, shifts in ocean currents and even carbon dioxide put into the air by volcanoes.

“The Southwestern U.S. shows significant natural variability including major historical droughts,” Anderson concluded. “Models of climate response predict future dust bowl-like conditions that will last much longer than historical droughts and have a different underlying cause — a poleward expansion of the subtropical dry zones.”

The layers of sediment in a carefully extracted, 246-foot-long core spanned a period between 552,000 and 368,000 years ago and captured two Ice Ages and the warmer periods that separated them. The cores revealed repeated periods when the temperature rose by perhaps 12 degrees F. The researchers had expected to find increased summer rainfall during those warm periods, but instead found a dramatic drop. Moreover, the full-out droughts with rainfall less than half the long-term average become more frequent and lasted much longer.

All told, the patterns that set in as average temperatures rise resembled the “dust bowl” conditions that forced the abandonment of huge swaths of the Midwest and West in the early 20th century.

“Our results strongly indicate that interglacial climates in the southwestern U.S. can experience prolonged periods of aridity lasting centuries to millennia, with profound effects on water availability and ecosystem composition. The risk of prolonged aridity is likely to be heightened by” global warming caused by the buildup of pollutants in the atmosphere.


A new study of ancient climate patterns by NAU researchers suggests that a predicted rise in global temperatures could shut off Arizona’s summer monsoons, resulting in centuries of drought and major water shortages in the region.


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