The mysterious fall of the largest of the world's earliest urban
civilizations nearly 4,000 years ago in what is now India, Pakistan,
Nepal and Bangladesh now appears to have a key culprit — ancient
climate change, researchers say.
Ancient Egypt and Mesopotamia
may be the best known of the first great urban cultures, but the
largest was the Indus or Harappan civilization. This culture once
extended over more than 386,000 square miles (1 million square
kilometers) across the plains of the Indus River from the Arabian Sea
to the Ganges, and at its peak may have accounted for 10 percent of the
world population. The civilization developed about 5,200 years ago, and
slowly disintegrated between 3,900 and 3,000 years ago — populations
largely abandoned cities, migrating toward the east.
"Antiquity knew about Egypt and
Mesopotamia, but the Indus civilization, which was bigger than these
two, was completely forgotten until the 1920s," said researcher Liviu
Giosan, a geologist at Woods Hole Oceanographic Institution in
Massachusetts.
Nearly a century ago, researchers began discovering numerous remains of
Harappan settlements along the Indus River and its tributaries, as well
as in a vast desert region at the border of India and Pakistan.
Evidence was uncovered for sophisticated cities, sea links with
Mesopotamia, internal trade routes, arts and crafts, and as-yet
undeciphered writing.
"They had cities ordered into
grids, with exquisite plumbing, which was not encountered again until
the Romans," Giosan told LiveScience. "They seem to have been a more
democratic society than Mesopotamia and Egypt — no large structures
were built for important personalitiess like kings or pharaohs."
Like their contemporaries in Egypt and Mesopotamia, the Harappans, who were named after one of their largest cities, lived next to rivers.
"Until now, speculations
abounded about the links between this mysterious ancient culture and
its life-giving mighty rivers," Giosan said.
Now Giosan and his colleagues have reconstructed the landscape of the plain and rivers where this long-forgotten civilizationdeveloped. Their findings now shed light on the enigmatic fate of this culture.
"Our research provides one of the clearest examples of climate change leading to the collapse of an entire civilization," Giosan said. [How Weather Changed History]
The researchers first analyzed
satellite data of the landscape influenced by the Indus and neighboring
rivers. From 2003 to 2008, the researchers then collected samples of
sediment from the coast of the Arabian Sea into the fertile irrigated
valleys of Punjab and the northern Thar Desert to determine the origins
and ages of those sediments and develop a timeline of landscape changes.
"It was challenging working in
the desert — temperatures were over 110 degrees Fahrenheit all day long
(43 degrees C)," Giosan recalled.
After collecting data on
geological history, "we could reexamine what we know about settlements,
what crops people were planting and when, and how both agriculture and
settlement patterns changed," said researcher Dorian Fuller, an
archaeologist with University College London. "This brought new
insights into the process of eastward population shift, the change
towards many more small farming communities, and the decline of cities
during late Harappan times."
Some had suggested that the
Harappan heartland received its waters from a large glacier-fed
Himalayan river, thought by some to be the Sarasvati, a sacred river of
Hindu mythology. However, the researchers found that only rivers fed by monsoon rains flowed through the region.
Previous studies suggest the
Ghaggar, an intermittent river that flows only during strong monsoons,
may best approximate the location of the Sarasvati. Archaeological
evidence suggested the river, which dissipates into the desert along
the dried course of Hakra valley, was home to intensive settlement
during Harappan times.
"We think we settled a long controversy about the mythic Sarasvati River," Giosan said.
Initially, the monsoon-drenched
rivers the researchers identified were prone to devastating floods.
Over time, monsoons weakened, enabling agriculture and civilization to
flourish along flood-fed riverbanks for nearly 2,000 years.
"The insolation — the solar
energy received by the Earth from the sun — varies in cycles, which can
impact monsoons," Giosan said. "In the last 10,000 years, the Northern
Hemisphere had the highest insolation from 7,000 to 5,000 years ago,
and since then insolation there decreased. All climate on Earth is
driven by the sun, and so the monsoons were affected by the lower
insolation, decreasing in force. This meant less rain got into
continental regions affected by monsoons over time."
Eventually, these monsoon-based rivers held too little water and dried, making them unfavorable for civilization.
"The Harappans were an
enterprising people taking advantage of a window of opportunity — a
kind of "Goldilocks civilization," Giosan said.
Eventually, over the course of
centuries, Harappans apparently fled along an escape route to the east
toward the Ganges basin, where monsoon rains remained reliable.
"We can envision that this
eastern shift involved a change to more localized forms of economy —
smaller communities supported by local rain-fed farming and dwindling
streams," Fuller said. "This may have produced smaller surpluses, and
would not have supported large cities, but would have been reliable."
This change would have spelled
disaster for the cities of the Indus, which were built on the large
surpluses seen during the earlier, wetter era. The dispersal of the
population to the east would have meant there was no longer a
concentrated workforce to support urbanism.
"Cities collapsed, but smaller
agricultural communities were sustainable and flourished," Fuller said.
"Many of the urban arts, such as writing, faded away, but agriculture
continued and actually diversified."
These findings could help guide future archaeological explorations of the Indus civilization.
Researchers can now better guess which settlements might have been more
significant, based on their relationships with rivers, Giosan said.
It remains uncertain how monsoons will react to modern climate change.
"If we take the devastating floods that caused the largest humanitarian
disaster in Pakistan's history as a sign of increased monsoon activity,
than this doesn't bode well for the region," Giosan said. "The region
has the largest irrigation scheme in the world, and all those dams and
channels would become obsolete in the face of the large floods an
increased monsoon would bring."