A new theory on the cause of ice age says that the joining of North and South America changed the salinity of the Pacific Ocean and caused the growth of major ice sheets across the Northern Hemisphere 2.6 million years ago.
The change in salinity encouraged sea ice to form which, in turn, created a change in wind patterns, leading to intensified monsoons.
These provided moisture that caused an increase in snowfall and the growth of major ice sheets, some of which reached three km thick.
A team of researchers analysed deposits of wind-blown dust called red clay that accumulated between six million and 2.5 million years ago in north-central China, adjacent to the Tibetan plateau, and used them to reconstruct changing monsoon precipitation and temperature.
"Until now, the cause of the Quaternary ice age had been a hotly debated topic", said Thomas Stevens from Royal Holloway, University of London.
"Our findings suggest a significant link between ice sheet growth, the monsoon and the closing of the Panama Seaway, as North and South America drifted closer together," he said.
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"This provides us with a major new theory on the origins of the ice age, and ultimately our current climate system," he noted.
Surprisingly, the researchers found there was a strengthening of the monsoon during global cooling, instead of the intense rainfall normally associated with warmer climates.
"This led us to discover a previously unknown interaction between plate tectonic movements in the Americas and dramatic changes in global temperature.
"The intensified monsoons created a positive feedback cycle, promoting more global cooling, more sea ice and even stronger precipitation, culminating in the spread of huge glaciers across the Northern Hemisphere," Stevens added.
The study appeared in the journal Nature Scientific Reports.