The Age of Perpetual Drought - The Ripple Effect: The Fate of Freshwater in the Twenty-First Century - page 112

University of Arizona have been able to track the region's long history of droughts, some
of which lasted for twenty to thirty years.
About a thousand years ago, the Hohokam Indians settled in the Salt River Valley,
just outside present-day Phoenix. Using stone hoes, they scraped an ingenious irriga-
tion system out of the desert—185 miles of canals that watered crops on two hundred
thousand acres. As Jared Diamond showed in his topic Collapse,irrigation entails a
number of risks; most important, if a society becomes reliant on irrigated agriculture,
it will suffer disproportionately when hit by drought. The Hohokam tribe's population
grew to an estimated 250,000 at its peak in the 1400s. But by the 1500s, archaeologic-
al evidence shows that the Hohokam began to quit their large settlements and scatter.
Scientists conjecture that it was a lack of water, perhaps a “megadrought,” or a combin-
ation of drought and flood, that led to their demise.
In the 1580s, tree rings show, a severe drought extended from California to the Caro-
linas. More recently, a Southwestern drought that began in the late 1940s ran until
1957. President Eisenhower declared New Mexico a disaster area, Arizona suffered from
forest fires, trees began to die of, and ranchers were forced out of business.
Signs indicate that Arizona forests, stressed by rising temperatures, are dying again.
During a drought in 2002, the Rodeo-Chediski wildires —the first started by an arson-
ist, the second by a stranded motorist—combined in central Arizona to scorch 467,000
acres, an area the size of Phoenix. Fed by high winds and tinderbox-dry woodlands,
it was the worst forest fire in the state's history, and it set off a soul-searching debate.
Scientists were concerned about the health of Arizona's ecosystem, politicians and en-
vironmentalists battled over logging, and the fire helped usher in controversial policies,
such as the Healthy Forests Restoration Act, which President George W. Bush signed
into law in 2003.
In the aftermath of the fires, insects moved in, causing further damage to the trees.
Some experts fear that if the trend continues, the forest ecosystem of Arizona could
reach a tipping point and collapse.
Studies show a steady decline in precipitation across the Southwest in recent decades.
Climate models for the region have predicted a deepening strain on water supplies in
coming years. Some say the change could lead to aridity akin to that experienced during
the Dust Bowl.
In 2007, Dr. Richard Seagar , of the Lamont-Doherty Observatory at Columbia
University, analyzed what nineteen global climate models projected for the future of
the Southwest. The models showed the region will become more arid this century as
a consequence of rising greenhouse gases. Seagar also studied historical records and
discovered that Southwestern droughts usually resulted from the cyclical variations in
tropical Pacific Ocean temperature. During El Niño events (a periodic warming of the