| By Becky Ham
SCIENCE
WASHINGTON
-- Timing couldn't have been worse for the group of colonists who came ashore
on Roanoke Island in 1587, attempting to establish the first permanent English
settlement in the New World. Along with the usual hardships of starting a new
society on the edge of the wilderness, the colonists were confronted with the
region's worst drought in 700 years, which caused mass starvation and aggravated
tense relations with Native Americans. By 1590, the ill-fated settlers had vanished
with little trace. Roanoke's collapse in the face of harsh climate puts it in
distinguished company, a researcher reports in the journal Science. RECENT
CONCERNS about global climate change, especially the impact of human activity
on these changes, have sent scientists scrambling to discover how current climate
trends fit into the larger picture of climate change throughout Earth's history.
With a growing pile of paleoclimate data at their disposal, researchers are also
taking a closer look at how complex societies responded to past climate change. Understanding
how these cultures adapted to events like persistent droughts could provide valuable
perspective on how modern societies might respond to future climate change, says
Peter deMenocal, a researcher at the Lamont-Doherty Earth Observatory of Columbia
University.
THE LONG VIEW Modern
climate data, recorded by instrument, allow us to study climate phenomena that
vary on a time scale of several years, such as El Nino, deMenocal notes in Friday's
issue of Science. But that record is far too short to capture large-scale events
like the drought that wiped out Roanoke - events that we can confirm through other
detailed climate records like tree rings or lake sediments.
These climate
proxies, among others, indicate that the past 12,000 years of Earth's climate
have been a bumpy ride, punctuated by widespread cooling events and droughts that
persisted for decades and often centuries. These events are associated with changes
in ocean circulation, solar radiation and volcanism. Many of them occur on thousand-year
cycles, indicating that solar variability and interactions between the ocean and
atmosphere are especially important factors influencing these climate changes.
A
growing store of detailed and well-dated paleoclimate records has contributed
to "a transformation in the field that allows scientists to evaluate cultural
records within the context of climate," deMenocal says. Researchers are using
this new perspective to examine how past societies managed to cope Ñ or failed
to cope Ñ with prolonged and severe climate events, events without equal in modern
times.
COLLAPSE AND ABANDONMENT
This new outlook suggests,
for instance, that the Akkadian Empire in Mesopotamia may have been one of the
earliest complex societies felled by severe drought. The Akkadian Empire was established
around 2300 B.C., linking rain-fed agricultural fields in northern Mesopotamia
with irrigation agriculture in the south. The empire stretched from the present-day
Persian Gulf into Turkey before it abruptly collapsed less than 200 years later.
By
2170 B.C., archaeological records document a mass exodus from the north, with
settlements abandoned and refugees pouring into southern Mesopotamia. Excavations
at one of these settlements, Tell Leilan, show that the collapse is marked by
a thick layer of windblown dust without any artifacts. Three hundred years later,
smaller and more nomadic groups finally ventured north again.
An excavated
sample of residential occupation within the lower town of Tell Leilan in northeast
Syria during the final days of the Akkadian Empire.
Researchers found the
same telltale signature of drought in a deep-sea sediment core drilled from the
Gulf of Oman. The core documents a dramatic 300-year period of windblown dust
that could be dated to roughly the same period as the Akkadian collapse. Chemical
traces in the dust allowed the scientists to pinpoint its origins in Mesopotamia.
The severe
climate change that may have helped topple the Akkadians probably had its roots
in the far-off North Atlantic Ocean. Atlantic deep-sea cores suggest that the
start of the Mesopotamian drought coincides with widespread cooling in the northern
part of the ocean, which could have affected rainfall that typically fed Mesopotamia's
rivers. Modern records show that water supply for Mesopotamia can be cut in half
when North Atlantic sea surface temperatures are unusually cool, deMenocal notes.
Several thousand years later, on the other side of the world, the Tiwanaku
culture in South America faced a similar climate challenge. Between 300 B.C. and
A.D. 1100, the Tiwanaku built an urban complex that probably supported nearly
half a million people. They sustained this dense settlement through raised field
agriculture, a technique that improved drainage and recycled nutrients in the
poor tropical soil. The
raised fields of the Tiwanaku may be a good example of a complex society already
responding to the challenges of a marginal environment, and therefore vulnerable
to sudden climate change, says deMenocal. Around the year 1100, the cities and
fields were abruptly abandoned.
Once again, the paleoclimate record may
contain clues to the Tiwanaku collapse.
The Quelccaya ice core, drilled just
125 miles from Lake Titicaca, contains an annual record of precipitation for the
region. This ice core record shows close overlap between the time of the Tiwanaku
abandonment and the start of an increasingly dry spell. Sediment cores from Lake
Titicaca itself also chronicle the event, showing a 33-foot drop in the lake level
at the time. The drought persisted for several centuries, during which the Tiwanaku
went into a slow decline.
What drives climate change?
Small
changes make big differences Solar
input
The sun's energy, after traveling 93 million miles to get to Earth,
hits the upper atmosphere at about the intensity of three 100-watt bulbs per square
yard. A third is reflected back into space, two thirds warms the planet and drives
its weather engine.
The atmosphere
Earth gets its livable temperature
(on average 59 degrees Fahrenheit) thanks to a delicate balance of gases that
create a "greenhouse" effect by trapping heat inside the atmosphere. Greenhouse
gases -- water vapor, carbon dioxide, methane, nitrous oxide, and others -- absorb
heat energy, then re-radiate a portion of it back to the surface.
The
oceans
Covering two thirds of the planet, oceans are the key source of
moisture in the air and they store heat efficiently, transporting it thousands
of miles. The oceans and marine life also consume huge amounts of carbon dioxide.
The
water cycle
Higher air temperatures can increase water evaporation and
melting of ice. And while water vapor is the most potent greenhouse gas, clouds
also affect evaporation, creating a cooling effect.
Clouds
They
both cool Earth by reflecting solar energy and warm Earth by trapping heat being
radiated up from the surface.
Ice and snow
The whiteness of ice
and snow reflects heat out, cooling the planet. When ice melts into the sea, that
drives heat from the ocean. Northern Hemisphere snow cover has declined 10 percent
in two decades, but no significant melting of the Antarctic ice sheet has been
detected.
Land surface
Mountain ranges can block clouds, creating
'dry' shadows downwind. Sloping land allows more water runoff, leaving the land
and air drier. A tropical forest will soak up carbon dioxide, but once cleared
for cattle ranching, the same land becomes a source of methane, a greenhouse gas.
Human
influences
Humans might be magnifying warming by adding to the greenhouse
gases naturally present in the atmosphere. Fuel use is the chief cause of rising
carbon dioxide levels. On the other hand, humans create temporary, localized cooling
effects through the use of aerosols, such as smoke and sulfates from industry,
which reflect sunlight away from Earth.
'WE HAVE TO RELEARN HISTORY'
The
fate of the Akkadians and the Tiwanaku begs the question: If a centuries-long
drought descended today, how would we respond?
One of the most destructive
droughts in recent memory is the 1930s American Dust Bowl, which lasted only six
years. In this short time, it managed to become "one of the most devastating
and well-documented agricultural, economic and social disasters in the history
of the United States," according to deMenocal.
Researchers need to
gather more data on past climates before this information can be used to accurately
predict and design strategies for future large and persistent climate changes,
says deMenocal, but acknowledging the historic impact of these severe events is
an important step toward that goal.
"Right now, we address climate
change on the basis of a painfully short climate record, just the past 150 years
or so that instruments have recorded," deMenocal says. "We now have
to relearn history on a long-term scale."
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