Hi, I’m Peter Clift, Professor of Geosciences at Louisiana State University.
In earlier videos we explored how the growth of mountains in Asia
and changes in the global climate have influenced summer rainfall, we also show how the rainfall
has controlled the rivers that supply sediment and water to the densely inhabited floodplains and
deltas of the region. Now we’re going to see how changes in the monsoon affected ancient people
as well as exploring how future climate change may affect impact modern society.
So how does the monsoon affect early societies? Their rise and collapse.
Everyone needs to eat and a functioning society requires
a reliable supply of food. Crops like rice corn, and wheat are especially important.
These and other crops depend on good growing conditions: sunlight, water,
and nutrient-rich soil. We know that the climate across Asia has become progressively colder
and drier over the last eight thousand years but the cooling has not been steady or gentle.
These changes have affected the many cultures and civilizations of India, SE Asia, and China.
The earliest civilizations of this region developed in the Indus Valley
in what is now India and Pakistan, as well as the Yellow River in northern China. Both of
these areas experienced a sharp drop in population when cities were abandoned around 4000 years ago.
Climate records show that this was a time when the monsoon rains diminished,
and archaeologists think that the drying climate was an important factor
that led these people to scatter into the countryside as farms failed to feed them.
Climatic evolution has been reconstructed from studying both stalagmites in caves
and lake sediments because these are linked to the supply of water. These records from southern China
reveal that many Chinese imperial dynasties ended at times when summer monsoon rains were weak.
Drought would have caused crop failures, food shortages and high prices.
Skyrocketing prices would have put pressure on the government especially because the Emperor
himself guaranteed the rains and the success of the harvests. A weaker, poorer government
would also have been more vulnerable to invasion and civil unrest at home.
The armies of Genghis Khan are famous for conquering all of Eurasia from
Korea to Hungary during the 13th century, forming the largest land empire in history.
The Mongols were able to sweep all before them because of their amazing cavalry, but horses need
to eat grass. New climate reconstructions based on tree rings - which grow faster when the climate is
wetter - show that the Mongol invasion occurred during one of the wettest episodes in Asia
over the last thousand years. Later, as the climate dried, grasslands contracted and the
Mongol Empire broke up, no longer able to easily communicate across the vast distances.
The Khmer Empire ruled much of SE Asia around their heartlands in modern Cambodia
from the 9th to the 15th centuries. The fabulous temple and city at Angkor Wat
was built in the 12th century by the Khmers but later abandoned in the early 15th century.
The city subsisted on water supplied from Ton Le Sap lake which is filled by monsoonal flooding
from the Mekong River. Tree ring records from this area show that the city was abandoned
following several decades of drought. Without the overflowing summer water from the Mekong
the large population of the city was unable to continue in this location.
Droughts continued to cause famine and societal unrest until the latter part of the 20th century
when the construction of dams allowed better water control. Even so,
such modern methods still have their limits, as shown by the catastrophic flooding in 2010
across Pakistan when the Indus River burst its banks and caused billions of dollars of damage and loss of life.
It is uncertain how well we will be able to respond to changing monsoon strength
in the future. A warming world mostly means stronger summer rains but these will in the
form of more destructive storms rather than the gentle rain favored by farmers.
Furthermore, higher temperatures cause more evaporation and potentially drier soils.
Areas of Pakistan, India and especially northern China are particularly projected to experience
significant drying over the 21st-century, while rising sea levels may inundate rich agricultural
land along the coasts. While some of these changes may be mitigated, this will take forward planning
as well as better modeling of how the monsoon is affected by other factors such as pollution.
I hope you have enjoyed learning about the monsoon and seeing how it affects and is influenced by
the people, the geology, and the environment around Asia. You can also check out these references to learn more,
especially this newly released book that discusses in detail
how climate and human societies changed across Asia over thousands of years. And be sure to watch the
three other videos we have made that talked about different aspects of the Monsoon!