A growing
desert
A 2007
paper in the journal Science forecast that the drying of the
Southwest would begin sooner rather than later. Changing
atmospheric circulation patterns and warmer air capable of sucking
up more moisture will push the Sonoran desert of northern Mexico
further north. This process will probably also occur at similar
latitudes worldwide.
The
drying may have already begun. As evidence, Allen points to recent
forest diebacks in disparate regions around the world: in
Mediterranean Europe, the Sahel of North Africa, and in Australia.
“We’re
starting to see dieback that may be above [routine] levels,
particularly in dry areas,” he says. “Increased water stress
is pushing some species over the edge.”
Richard
Seager, senior research scientist at Columbia University’s
Lamont-Doherty Earth Observatory in New York City, and lead author
on the Science paper, says, “It could be an anomaly…. But when
they start lining up, that’s when you say, ‘It looks more like
a pattern.’ ”
A
changing landscape
When
Allen first began monitoring his plot, ponderosa pine trunks
littered the ground. Located 6,600 feet high in the foothills of
the Jemez Mountains, the area was just below where the tall,
straight pine species now grows. Allen reasons that the species
had probably descended from higher elevations during the Little
Ice Age, a relatively cool, wet period that ended in the
mid-1800s. As temperatures rose again, optimal ponderosa
conditions moved back up the mountain. The trees hung on until the
drought of the 1950s.
By the
1980s, juniper and piñon, whose berries and nuts are carried far
and wide in bird bellies, had moved in. But this was not quite the
simple story of species succession it seemed. A century earlier,
livestock had begun grazing in large numbers in the Southwest. The
grazing pressure favored woody plants over grassy ones –
livestock ate grasses and left woody plants to grow.
Land
managers also began suppressing fires. Until around 1910,
wildfires had burned through Southwestern ecosystems about
every five to 15 years locally, according to tree-ring records –
apparently for millennia. Fires continually burned back
encroaching juniper and piñon trees, leaving only those trees
mature and strong enough to withstand a burning understory. Absent
the fires, young trees moved in unchecked. During the 20th
century, areas that were once predominantly grassy – or in the
case of Allen’s plot, ponderosa with a grassy understory –
became scrubby, erosion-prone piñon-juniper forest.
Then,
beginning a decade ago, a series of hot and dry years ended a
17-year run of relatively moist conditions. The stress killed some
trees outright and left many more severely weakened. The stage was
set for the final coup: Droves of pine beetles attacked the
debilitated trees. The swarms were the southernmost flank of an
infestation by several species that occurred – and in some
places is ongoing – from New Mexico up the Rockies to Alaska.
Here,
parsing the ultimate causes gets complicated. Bark beetles are
native to the area, and droughts occur periodically in the
Southwest, where weather is greatly influenced by sea-surface
temperatures in the Pacific Ocean. Indeed, in 1956, the
tail end of the last great regional drought, conditions were
actually drier in some parts than in 2002, the second-driest year
on record.
But there
was one crucial difference during the most recent drought: The
mean temperature was 1.5 degrees F. higher, part of a warming
trend over western North America. The extra heat, says Allen,
overstressed the trees and pushed large portions of the system
over a threshold.
Whereas
in the 1950s, only those trees at the lower, drier end of the piñon-juniper range died, this time trees succumbed throughout
their entire range. Around Bandelier, 95 percent of mature piñons
have died. The dramatic shift underscores a facet of climate
change that scientists are often at pains to communicate:
Ecosystems are shaped by extremes of hot and cold, wet and dry. If
those extremes are pushed even a little further away from the
mean, the effect can be drastic, especially when relatively
slow-growing trees are involved.
Case in
point: A mere 1.5 degrees F. increase has helped to wipe out
nearly all mature Bandelier piñons .
“How
big and how fast these changes can be becomes much more obvious
when you look at these dramatic shifts,” says Dave Breshears, a
professor of natural resources at the University of Arizona,
Tucson. “We’re really going to be living in a different
landscape in the future.”
Full
circle
The new
climate regime will probably affect how people use water in the
area, says Columbia’s Dr. Seager. Thanks to rivers like the
Colorado, the Southwest has a healthy water supply, especially
compared with northern Mexico, he says. But it’s hardly
unlimited. Scarcity will probably force people to rethink how
water is divvied up.
According
to the United States Geological Survey, for example, in Arizona
in 2000, agriculture used four times as much water as everything
else combined. But farming contributed less than 1 percent of the
state’s GDP. Maybe Arizona is not the best place to grow
water-intensive crops like cotton, he says. (The state ranks ninth
in the US as a cotton producer.)
Warmer
temperatures may have an inordinate impact on species that inhabit
the area’s mountains – essentially cool, wet islands
surrounded by hot dry deserts. A moist dirt-loving salamander
endemic only to the Jemez Mountains, which are only 11,500 feet at
their highest, could be pushed off the mountain altogether.
The pika,
a high-elevation rabbitlike creature, could also disappear
locally, although they’ll probably persist in the northern
Rockies.
But the
story is not all doom and gloom, says Allen. Juniper- and piñon-dotted
mesas come to mind when we imagine the Southwest. But in many
areas, their dominance is an artifact of human interference. Now,
prompted by more intense extremes, the ecosystem has come full
circle, returning to something approximating its original state.
The new landscape – grasslands with a much lower density of
juniper and piñon – will be more resilient, and better able to
recover from future droughts, he says.
“For a
while we went through this ‘woodification’ trend,” says
Allen. “Now that excess is being squeezed back by dieback and by
fire. In a structural sense, it’s actually somewhat restorative.”
Scientists
at Bandelier are trying to speed along the restoration. Much of
the area’s nutrient-rich topsoil, itself a remnant of cooler,
wetter conditions that prevailed here during the last Ice Age,
which ended 10,000 years ago, has eroded away. But scientists have
found that simply strewing the now-dead tree branches across the
land can mitigate, and perhaps reverse, this desertification.
The pine
needles and woody matter slow down running water so that it seeps
into the ground rather than flowing off.
“You’re
sort of redistributing organic material,” says Brian Jacobs, a
botanist with the National Park Service in Bandelier. They also
shade the earth from the sun, lessening evaporation and helping
grasses to establish themselves.
“We’re
trying to hold on to the soils,” he says. “And we’re using
the least invasive methods for doing that.” Ancient lessons in
adapting to dry climate
When the
Spanish arrived in what is now the US Southwest in the late 1500s,
they found a settled, agricultural people throughout the river
valleys of the area. They dubbed these native Americans “Pueblo”
(“village”) Indians. But ruins of an earlier people – the
Anasazi – dotted the mesas and hilltops of the region. To the
arriving Spanish, the settlements already stood abandoned. What
had happened?
Archaeologists
are quick to correct several widespread misconceptions in this
story. First, the Anasazi didn’t vanish at all. The Tewa-speaking
Pueblo Indians are, in fact, their descendents. They didn’t
disappear; they simply moved into the river valleys.
Second,
don’t call them “Anasazi,” Navajo for “ancestor of our
enemies.” They’re “ancestral Puebloans.” (”Navajo,”
for that matter, derives from a Tewa word meaning “fields
adjoining an arroyo,” a reference to the tribe’s agricultural
practices. The Navajo call themselves “Diné” – “the
people.”) [Editor’s note: The original story misstated the
origin of the word “Navajo.”]
Why did
the ancestral Puebloans move? One theory: climate change. A
megadrought coincided with the abandonment of the Four Corners
region of the US in the late 1200s, where they’d lived for
perhaps 1,000 years. The drought link is less clear in settlements
like Bandelier in north-central New Mexico, abandoned by 1550. But
soil exhaustion from centuries of farming and deforestation may
have also played a role, says Rory Gauthier, archaeologist at
Bandelier National Monument in New Mexico.
As today’s
inhabitants contemplate a changing climate in the Southwest, they
could probably learn a few things from the successes and failures
of the ancestral Puebloans, Dr. Gauthier says. Foremost among
them: the art of conserving water: The Pueblo’s ancestors
developed intricate irrigation systems and terraced hillsides to
better conserve water. They designed their gardens rather like
waffles: a series of raised ridges created sunken squares that
trapped and held water for crop use.
“I’m
hopeful,” says Gauthier. “If we take the Pueblo people as a
model, they’ve survived for a long time. They’ve made social
adjustments. And they’ve been able to meet that challenge.”
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