Scientists had thought the the monsoon – the biggest weather system in the world – had begun 22-25 million years ago as a result of the uplift of the Tibetan Plateau and the Himalaya Mountains. The new study, though, has found the system already existed 40 million years ago.
The research by Licht and his colleagues shows the earlier start of the monsoon occurred at a time when atmospheric CO2 was three to four times greater than it is now. The monsoon then weakened 34 million years ago when atmospheric CO2 then decreased by 50 percent and an ice age occurred.
Lead author Alexis Licht, now a research associate in the University of Arizona department of geosciences, said the study is the first to show the rise of the monsoon is as much a result of global climate as it is a result of topography.
The team’s paper was published in Nature at the weekend.
Licht didn’t set out to study the origin of the monsoon.
His research, when at the University of Poitiers, was focused on understanding the environments early primates inhabited. He chose a study site in Burma because the area was rich in fossils of some of the earliest ancestors of modern monkeys and apes.
To learn about the past environment, Licht analysed 40-million-year-old freshwater snail shells and teeth of mammals to see what types of oxygen they contained. The ratio of two different forms of oxygen, oxygen-18 and oxygen-16, shows whether the animal lived in a relatively wet climate or an arid one – a seasonal pattern very much like the current monsoon, with dry winters and very rainy summers.
“The early primates of Myanmar lived under intense seasonal stress – aridity and then monsoons,” he said. “That was completely unexpected.”
Licht will now go on to investigate how increases of atmospheric carbon dioxide affected the monsoon’s behaviour 40 million years ago.