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Human cultures have faced
climate changes many times before, sometimes responding with
ingenuity, other times succumbing to ecological devastation.
The current change in climate brings a special challenge.
Research by Washington State University scientists shows that
the high levels of carbon dioxide that are boosting the earth’s
temperature are also changing the way ecosystems work. When it
comes to carbon, plants and the soil organisms they depend on
have their limits—and those limits may soon affect us.
Even for seasoned archaeologists, it was an unsettling find. At
the site in Belize of a sizeable Maya town, in the layer of
artifacts left just before the town was abandoned, John G. Jones
and his colleagues from around the United States came upon a pit
full of skulls.
“In this pit there were 29 healthy human heads,” recalls the
Washington State University archaeologist. “Well, they were healthy
up to the point when they were removed.”
The skulls had belonged to men, women, and children. Their teeth
were in excellent condition, a good sign that they came from
members of the ruling elite. During the decades before the heads
were separated from their bodies, the Maya had been struggling with
political degeneration and increasingly bloody wars. Then the
climate changed. The rains stopped. Crops failed for several years
running, and the soil of their barren fields lay open to erosion by
wind and every short cloudburst.
“We’re having a very discontent population,” says Jones, who
tends to speak of ancient people and events in the present tense.
“One of the reasons they’re discontent is because of climate
change. Their rulers’ job—their only job—is to keep the rains
coming. It was a cushy job for a couple thousand years. But all of
a sudden what we’re seeing is the rain stops coming, and the
environment changes. People are starving.
“What could they do? They could yell at the rulers, saying, ‘You
promised you’d give us rain.’ They could blame it on the gods, or
ask the gods to intercede. They could move.”
In the end, he says, the people rose up, dispatched their
failing rulers, and dispersed. The skull pit marks the end of
habitation at that site. Similar events may have occurred in other
Maya centers, as the great civilization collapsed under the stress
of political unrest and ecological devastation.
“They had a good run,” says Jones. “They had 2,500 years or so.
Most cultures don’t last that long.”
Jones traces changes in climate, and how human societies
responded to those changes, by examining pollen grains trapped in lake and
estuary sediments. Pollen is tiny but incredibly durable; as
long as it stays completely dry or completely wet, he says, it
lasts just about forever. He has easily identified pollen grains
more than 300 million years old.
On field expeditions, Jones and his graduate students use a
five-horsepower motor to drive a coring tube deep into sediment
near village sites being investigated by other archaeologists. The
team they work with includes experts on soils, cultural artifacts,
radiocarbon dating, and tiny plant crystals called phytoliths, but
Jones goes first, using pollen to sketch the major event horizons
in the core. “We’ll say, ‘Hey look, something’s happening at 180
centimeters below the surface, and another one at 400 centimeters.
Let’s get radiocarbon dates on these right now, because that takes
a little time.’” After he lays out the basic framework, he and the
others go back and fill in the details.
His three-inch-wide, 11- to 15-yard-deep cores record a vast
span of environmental and human history. They go deep enough to
reach material laid down before people settled the area. Jones and
his colleagues recently showed that agriculture in the New World
began around 5200 BC, significantly earlier than previously
thought. It was driven in part by population growth and the need
for a reliable food supply, and in part by a change in climate that
made cultivation more likely to succeed.
Jones reads these events in the small but distinct tracings in
the cores. A layer of charcoal from prolonged, large-scale burning
shows when people first cleared the forests. Then tree pollen
disappears from the cores, and pollen from maize and squash appears
in large numbers. The wild ancestors of those plants grew in very
different habitats, so finding them together in a place where
neither would grow on its own means human cultivators brought them
there. Weed pollen shows up along with the crops; even the first
farmers had to contend with unwelcome visitors. By the time the
Maya came into Belize between 1800 and 1500 B.C., the land was
already shorn of trees and under cultivation.
Then, after centuries of continuous farming, come signs of the
ecological disaster that precipitated the downfall of the Maya: a
sharp decline in crop and weed pollen, and a layer of eroded soil
so common throughout the region that it’s been labeled “Maya
clay”.
“I can’t think of one core that doesn’t have it,” says Jones.
“It’s not just in Belize, it’s in Guatemala, it’s in Mexico;
everywhere we look, we see the same kind of thing.
“This is a change in climate on a horribly denuded landscape,
and all of the soils are just washing away. . . It’s a sobering
picture.”
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