Warming could imperil West Severe drought possible, global study indicates

By Mitch Tobin
ARIZONA DAILY STAR

Global warming could help spur severe drought in the Western United States,
according to a new study co-authored by a tree-ring researcher at the
University of Arizona.

Scientists who examined the width of tree rings over the past 1,200 years
found that temperatures were unusually high during "megadroughts" between
900 A.D. and 1300 A.D. The era, known as the "medieval warm period," may be
a harbinger of what's to come for the West as the planet keeps getting
hotter, the study said.

Were the modern, fast-growing West to suffer a drought like those between
900 and 1300, the results "would be disastrous," the researchers said.

The vast majority of climate experts believe the Earth will keep getting
warmer this century, at least partly due to heat-trapping greenhouse gases
from tailpipes and smokestacks.

The study, to be published in the next few weeks in the journal Science, was
reported earlier this month in the prestigious publication's online edition.

The scientists believe the synchronicity between the warm and dry periods
wasn't just a coincidence. They suspect higher temperatures made the eastern
Pacific resemble the La Niña pattern that typically makes the West drier
than normal.

"It's kind of a cautionary tale," said lead author Edward Cook of Columbia
University's Lamont-Doherty Earth Observatory. "Any warming in the future,
whether due to greenhouse gases or natural variation, would not be good for
the West.

"It certainly ought to be a bit of a worry that we could be looking,
long-term, at increasing aridity," he said. "Whether we'll see what happened
1,000 years ago is anyone's guess."

The study's authors are also quick to note there's no proof global warming
has caused the West's current dry spell.

"I think it's way too speculative to say that warming is in any way
responsible for these last four years of drought," said David Meko,
associate research professor at UA's Laboratory of Tree-Ring Research. "A
four-year drought is a little blip in the tree-ring record."

But that record also makes it clear the current dry streak is puny by
historic standards. Stacked against the megadroughts centered in 936, 1034,
1150 and 1253, "the current drought does not stand out as an extreme event
because it has not yet lasted nearly as long," the study said.

"More intense droughts of longer duration have occurred in the past and
could occur in the future," it said.

UA scientists have studied tree rings to learn about drought since the
1970s. But back then, only about 65 sites in North America were used to
detect climate patterns dating to the 17th century, Meko said. Today, the
number of sites is around 600 and scientists can look back to the year 800.

Many of the trees sampled in the West are Douglas firs and ponderosa pines,
which reveal the stress of droughts in the width of their rings.

"Their growth is reduced when they get less moisture, so if you look at a
cross-section of the tree, you see a narrowing of the rings," Meko said.

The tree-ring findings are also backed up by other indicators of drought,
such as fire scars on trees, charcoal deposits and the creation of sand
dunes.

Julio Betancourt, a climate expert with the U.S. Geological Survey in Tucson
who was not part of the study, praised the paper. He said there's a close
match between measurements of rainfall in the 20th century and the
reconstruction of climate patterns from tree rings.

"We can map the relative wetness and dryness almost on par with the
instrumental record," he said. "There's tremendous reason to trust these
mapped views of climate variability over the United States.

"This period from 900 to 1300 was obviously a big notch drier than the
period that followed, so it begs the question of exactly what was going on."

Betancourt said he's a bit skeptical of the link between warming and drought
because the period from 1976 to 1995 - when the planet was getting hotter -
was one of the West's wettest in the past 1,000 years.

"I'm not sure I would have extended that analysis as far as Ed (Cook) did,"
said Betancourt, who believes warming of the North Atlantic also plays a big
role in Western drought.

Scientists aren't sure what caused the medieval warming period, when wine
grapes could be planted in Europe 300 miles farther north than they are
today. But many have pointed to increased solar activity and a lack of
volcanic eruptions that would have shielded the Earth's atmosphere with ash.

The possible link between global warming and Western drought is
counterintuitive because it rests on the belief that sea surface
temperatures in the eastern, tropical Pacific will fall as the planet heats
up.

The researchers think warmer weather will stimulate trade winds, pushing
surface water to the west and prompting the "upswelling" of deep, cold water
off the coast of South America.

Contact reporter Mitch Tobin at 573-4185 or mtobin@azstarnet.com