Aug. 16, 2004
In California the wildfire season generally ramps up slowly, and
the largest fires usually don't arrive until fall. But this year is different,
says Riverside County fire captain Rick Vogt, surveying the aftermath of
a blaze that swept through the rural community of Sage, 80 miles from San
Diego, with unseasonal intensity late last month, blackening more than 3,500
acres. Fire fighters this time were able to contain the flames, but next
time they may not be so lucky. A five-year drought has left this always
arid region even dryer than usual, and when the hot Santa Ana winds start
to blow off the desert in September, it could take only a spark to set off
fires that will be much more difficult to control.
Already the fire season in Southern California is breaking records. Last
year was bad enough; this year is outpacing it in both the number of fires
started (2,749 vs. 2,453) and the amount of acreage consumed (69,167 vs.
38,523). And Southern California is not alone. A fast-moving wildfire exploded
in a canyon on the outskirts of Las Vegas two weeks ago, forcing the evacuation
of 75 Girl Scouts from a campground in the Spring Mountains — this on top
of a fire that threatened the capital of Nevada and another that nearly
destroyed a $200 million astronomical observatory in Arizona. Just a few
more big ones could easily turn 2004 into one of the West's worst fire years
And no one knows when the drought will end. Scientists believe
this dry spell, which has plagued a broad swath of the West since 1999,
is more typical of the region than its 60 million inhabitants would care
to admit. As Charles Ester, chief hydrologist for Arizona's Salt River Project,
a major provider of water and electricity, puts it, "What we took as a period
of normal rainfall in the past century was actually a period of abundance."
Consider, for example, the 1922 compact that determines the allocation
of water from the Colorado River. Scientists have shown, by studying tree
rings and other historical evidence, that the allocation was based on water
flows that were the highest they had been for more than 475 years. By contrast,
the flows since 1999 rank among the lowest. As a result, Lake Powell, the
giant reservoir created on the Colorado by the Glen Canyon Dam, stands
some 60% below capacity and seems destined to fall even lower. No wonder
that states like Colorado — whose rights to that water are trumped by the
rights of California, Nevada and Arizona — are anxiously bracing for a crisis.
At risk are not only natural ecosystems and agricultural enterprises but
also the multiple amenities that people living in the West have for so long
taken for granted: ski resorts and golf courses, green lawns and lush gardens,
swimming pools and hot tubs, not to mention such modern necessities as dishwashers
and flush toilets and the hydropower that keeps refrigerators and home
computers humming. Caught off guard, political leaders and water-resource
managers have been turning to scientists for help. What do researchers know
about patterns of drought in North America? What do they think occurred
in the mid-1990s when a big chunk of the West abruptly veered from wet to
dry? And do they believe that the current shortfall of precipitation is
just a temporary dry spell or an ominous realignment of the earth's climate
Secrets of Tree Rings
That the West is a semiarid region subject to episodic droughts has been
understood for some time. What's new is the detailed picture of those droughts
that is emerging from a vastly improved network of North American tree-ring
records that extend back more than 1,000 years. Those records — some 835
in all — are based on the growth rings laid down by multiple species of
long-lived trees, including blue oaks, giant sequoia and bristlecone and
ponderosa pines. Interpreting the rings takes skill, but the basics are
simple. The rings are wide when moisture is sufficient, narrow when it is
Dendrochronologists, as scientists who study tree rings
are called, are using these records to probe the patterns of drought over
space and time. Droughts like the 1930s Dust Bowl, which was focused in
the northern Rockies and Great Plains, appear to be quite rare, with only
two somewhat close analogues over the past 500 years. But 1950s-style droughts,
centered on the Southwest, are more common, recurring, on average, twice
each century. An event that scientists have dubbed the 16th century megadrought
resembled a combination of the droughts of the 1930s and 1950s.
And even the 16th century megadrought pales in comparison to earlier dry
spells that California State University geographer Scott Stine has documented
in the Sierra Nevada. Between A.D. 900 and 1350, he has established, trees
and shrubs grew in lake basins and riverbeds that are now filled with water.
By dating the wood from their stumps, Stine has identified two droughts
that were separated by an extremely wet period. One of these droughts lasted
140 years, the other more than two centuries.
A prolonged dearth of precipitation during this time affected other areas
as well. Across Mexico, Guatemala and the Four Corners region of the Southwest,
unreliable rains threw the ancient Maya and Anasazi civilizations into crisis.
In the Great Plains, peat marshes dried up and sand dunes resumed their
wind-driven march. In Nevada's Great Basin, hardy bristlecone pines throttled
back their growth, waiting for a wetter clime. "If the future resembles
the past," says Stine, "then we're in for a lot of trouble."
What Causes Drought?
What could trigger droughts that persist for decades and even centuries?
Topping the list of suspects are swings in sea surface temperatures like
those associated with El Nino and La Nina. Every three to seven years, a
distinctive warming of the eastern basin of the tropical Pacific (El Nino)
influences North American weather patterns, often in ways that send winter
storms farther south than usual. As a result, California and the Southwest
tend to get quite wet. By contrast, the cooling of the sea's surface (known
as La Nina) tilts the odds toward drought in this same region. And there
is evidence that the swings between El Nino and La Nina are mirrored by
longer-term changes in the tropical Pacific that are less extreme but much
Some of this evidence comes from corals that grow
in the tropical Pacific. Just like trees, observes Georgia Tech paleoclimatologist
Kim Cobb, coral polyps lay down growth rings that encode information about
temperature and precipitation. About a year ago, Cobb and her colleagues
spliced together a 1,000-year-long climate record from corals collected
on Palmyra Atoll, 1,000 miles south of Hawaii. The record reveals a sequence
of long-term shifts in sea surface temperatures and precipitation that
seem reminiscent of El Nino and La Nina. Cobb speculates that extreme La
Nina — like conditions recorded by a 10th century coral might have been
connected to the drought that occurred in the Sierra Nevada at that time.
Scientists are still trying to ferret out the physical
mechanisms capable of causing long-term changes in the tropical Pacific.
Among the suspects are volcanic eruptions, which temporarily cool the surface
of the planet, and cyclical ups and downs in the sun's luminosity that occur
on time scales ranging from decades to centuries.
Climate models run by the University of Miami's climatologist Amy Clement
and her colleagues suggest that the key to the puzzle may lie not in the
eastern basin of the tropical Pacific — the area El Nino and La Nina so
profoundly affect — but rather in a pool of warm water positioned to the
west. In the models, when the so-called western warm pool cools off just
a bit or when it warms a tad more, the eastern basin of the tropical Pacific
tends to respond by doing the opposite. In recent years, it so happens,
the western basin of the tropical Pacific — along with the neighboring
Indian Ocean — has been the warmest it has been for at least 150 years.
And what that adds up to, says Martin Hoerling, a research meteorologist
with the National Oceanic and Atmospheric Administration, is "the perfect
ocean for drought."
But there is more to the story than just the tropical Pacific, thinks paleoclimatologist
Julio Betancourt of the U.S. Geological Survey's Desert Laboratory in Tucson,
Ariz. Along with several colleagues, Betancourt has investigated the influence
on Western drought of sea surface temperatures outside the tropics. The
North Atlantic, they have found, seems to play a central role. When the
surface of the North Atlantic warms, the stage appears to be set for both
1930s-and 1950s-style droughts. Tree-ring records taken from the Atlantic
basin suggest that one such warming occurred in the late 1500s, coinciding
with the 16th century megadrought.
What tips the balance between a 1930s-and a 1950s-style drought? Betancourt,
for one, believes the answer may lie off the West Coast of the U.S. Sea
surface temperatures in the North Pacific, an accumulating body of evidence
suggests, undergo distinctive patterns of warming and cooling every 20 to
30 years (in response, some think, to long-term changes in the tropics).
Accompanying these changes in sea surface temperatures are seesaws in atmospheric
pressure that alter storm tracks across the North American continent. When
waters off the coast of California warm in synch with those of the North
Atlantic, as happened in the 1930s, summer rainfall tends to fail, particularly
in the Plains states. But when these same waters cool, as occurred in the
1950s and again in the late 1990s, winter precipitation can falter as well.
This is important because in the West most precipitation falls as snow
at higher elevations. Thus, a city like Reno, Nevada, gets, on average,
just over 7 in. of precipitation a year, vs. some 70 in. at the top of nearby
Mount Rose. During the 1950s drought, for example, a very large portion
of the West, along with a big chunk of the Southeast and Great Plains, experienced
long-term shortfalls of both winter snows and summer rains. "This is the
kind of drought we worry about a lot," says Betancourt — and it's the kind
of drought that the present configuration of sea surface temperatures in
the North Pacific and North Atlantic seems primed to produce.
What Happens Next?
That question gnaws at Douglas Kenney, a professor of natural resources
law at the University of Colorado in Boulder. As he sees it, "Everyone's
pretty clear that the earth's getting warmer, but it's unclear just what
that means. It might mean a wetter future or a dryer future. It might even
mean a wetter future with no net gain." How is that possible? The answer
lies in the impact rising temperatures are likely to have on the vast reservoirs
of water locked up in the mountains in the form of snow and ice.
Scientists have documented a troubling shrinkage of the snowpack across
the West, owing at least in part to the fact that rising temperatures are
inexorably forcing the snow line higher. They have also found that the snowmelt
is starting earlier in spring, as many as four weeks earlier in the Sierra
Nevada and the Cascades. And that likewise poses a problem. Why? Snow and
ice are like natural dams that hold water back during the winter months,
when the risk of flooding is highest, and then melt and release it during
the dry months of summer when moisture is at a premium.
"Drought is more than a precipitation deficit," observes University of
Washington climatologist Philip Mote. The real problem, he says, "is that
you don't have as much water as you'd like at a given point in time." And
that goes for plants as well as people. For accompanying an earlier snowmelt,
scientists note, is an earlier start to the growing season, which means
that the demand for water by forests, marshes and grasslands — not to mention
agricultural crops, lawns and putting greens — is bound to rise. In this
context, a "normal" amount of precipitation may not be sufficient; and when
precipitation drops below normal, as it has in recent years, the stress
on trees and plants is all the more extreme.
Consider, for example, the massive forest diebacks occurring across the
West. "People tend to think of forests as pretty slow changing," says Craig
Allen, an ecologist with the U.S. Geological Survey. "But once certain thresholds
are exceeded, very rapid changes can occur." In some cases, thirsting trees
perish because their circulatory systems — the long tubular columns in
the trunk that transport water from the roots to the crown — collapse. In
other cases, the trees become so weak they can no longer fend off insects
Indeed, the bark-beetle infestations that are killing trees across the
West are attributed to drought (complicated by decades of fire suppression
that have resulted in an overgrowth of trees). And nowhere is the beetle
infestation worse than in the mountains of Southern California, whose stressed-out
forests harbor hundreds of thousands of beetle-killed trees. These trees,
some with rust-colored needles still hanging from their limbs, serve as
standing fuel for fires, and an effort is under way to remove as many as
possible along the roads inhabitants in the San Jacinto, San Bernardino
and San Gabriel mountains must take when fire comes to call.
The connection between drought and wildfires is strong,
says Thomas Swetnam, head of the University of Arizona's Laboratory of
Tree-Ring Research. And the most dangerous fires, he says, occur when droughts
follow years that are unusually wet. That's because generous rains encourage
trees, shrubs and grasses to grow, providing the fuel that stokes forest
fires. This pattern of wet preceding dry, Swetnam thinks, helped feed the
intense blazes that raged through the Southwest shortly after 1850, taking
out huge stands of conifers. So, if a new El Nino materializes later this
year, as some experts expect, it may bring rains that temporarily ease the
fire danger only to increase it later.
The past is an imperfect lens through which to peer into the future, but
looking backward provides a glimpse, at least, of the sorts of extended
dry spells that those who live in this drought-prone region today should
be prepared to endure. The West, observed writer Marc Reisner, has a "desert
heart," and we ignore it at our peril.