Shifting tectonic plates,
not atmospheric carbon dioxide levels, controlled the strength of the powerful East
Asian monsoon throughout its history, scientists say.

The monsoon is a seasonal system of winds that brings heavy rains
to a vast swath of Asia, from India to Taiwan, each summer. The rains are a
vitally important source of water for agriculture. Some previous research has
suggested that past eras known to have had high atmospheric CO₂ levels and
warmer temperatures might also have been times of fluctuating monsoon intensity.
The implication that monsoons are far more sensitive to climate change than
once thought is alarming in a warming world: Dramatic change in monsoon
intensity in the near future would threaten food security for over a billion

Yet the new study offers
some potentially good news on that front: Even during very warm periods in
Earth’s past, such as the Eocene Epoch that lasted from 56 million to 34
million years ago, the monsoon’s intensity wasn’t much different than it is

Alexander Farnsworth, a
paleoclimatologist at the University of Bristol in England, and colleagues combined
plate tectonic reconstructions with paleotemperature “proxies” that provide
clues to past climatic conditions. Such proxies, found in and near the Tibetan
Plateau, include ancient
fossils and pollen, as well as sedimentary deposits. Using these data, the team
reconstructed the evolution of the monsoon going back 150 million years. What
really exerted control over changes in the monsoon’s intensity were Earth’s
slowly but constantly shifting landmasses, the team reports October 30 in Science

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The study also suggests that
the monsoon is far older than once thought. “The traditional model is that the
monsoon itself has only existed for the last 23 million years,” Farnsworth
says. But new plant fossil data from the region have suggested that at least parts of
the Tibetan Plateau were very wet much further back in time (SN: 3/11/19).

Monsoon conditions existed
as far back as the Early Cretaceous Period, about 136 million years ago, the
study finds. But by 120 million years ago, the monsoon was gone, and for the
rest of the Cretaceous, East Asia remained arid. Then, around 60 million years
ago, the monsoon reappeared and began to intensify over the next 20 million
years. It remained strong and stable until about 13 million years ago, when it
kicked into high gear — a time that the scientists call the mid-Miocene “super-monsoon.”
About 3.5 million years ago, it weakened again to an intensity similar to

scientists collecting sediment samples
In China’s Yunnan Province, which includes part of the Tibetan Plateau, scientists collect sediment samples and leaf fossils dating to between 34 million and 23 million years ago. Such samples help reconstruct the ancient environment — and therefore, can reveal changes in monsoon intensity through time.A. Farnsworth

That pattern, the
researchers found, coincides with broad shifts in continental landmasses, which
can alter atmospheric circulation patterns. For example, the westward movement
of the Asian continent during the Late Cretaceous weakened the flow of trade
winds from the Pacific, reducing the supply of moisture to the region. Then,
the rise of the Himalayan-Tibetan region beginning around 50 million years ago
began to block the flow of cold, dry air down from Asia; that allowed the
warmer, moister air blowing north from the Indian Ocean to become dominant,
intensifying the rains.

Other, even more distant,
tectonic shifts may have played a role in the monsoon’s evolving strength,
Farnsworth says, such as the uplift of the Iranian Plateau beginning sometime
around 15 million years ago as the Arabian Plate collided with the Eurasian
Plate. Determining how these other shifts impacted the monsoon will be the
subject of ongoing work, he says.

Previous studies also have suggested
that the East Asian monsoon has been around longer than once thought. For
example, a 2012 study in
the Journal of Asian Earth Sciences led by paleoclimatologist Matthew
Huber of Purdue University in West Lafayette, Ind., simulated past climate
conditions 40 million years ago. That study also found that monsoon conditions existed
during the Eocene Epoch. However, Huber’s study linked those conditions to
elevated atmospheric CO2 at the time.  

But such a “time-slice”
approach, which examines conditions during a small window of time, makes it
difficult to see the how monsoon intensity varies against the big-picture
backdrop of both geology and climate. “It’s robust and meaningful that they
have these clear geologic signals through time,” says Huber, who was not
involved in the new study. In that context, “the strong suggestion is that the
monsoon in the region is much more impacted by changes in building mountain
ranges than it is by changes in CO2.”

Farnsworth notes that there
is no perfect past analog to present conditions. Even when the past climate resembled today’s, such as
during the Eocene, the tectonic landscape was vastly different. “What this
research shows is that we have to be cautious in how we interpret the past for
what will happen in the future.”

And rising CO2 isn’t the only result of human activity, Farnsworth says. “There are all these other anthropogenic effects: land-use changes, aerosols.” Whether and how these factors affect the monsoon is still an open question.


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