Geology, not CO₂, controlled monsoon intensity in Asia’s ancient past

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 people.

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 today.

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 Advances.

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 today’s.

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.