The study suggests that the joining of North and South America changed the salinity of the Pacific Ocean, which led to major ice sheet growth across the Northern Hemisphere.
The theory was based on an analysis of deposits of wind-blown dust, known as red clay, that accumulated between six million and two and a half million years ago in north-central China, adjacent to the Tibetan plateau. This allowed the researchers to reconstruct the regional history of monsoon precipitation and temperature.
They suggest that the change in salinity encouraged sea ice to form. This, in turn, altered the wind patterns, leading to an intensification of the monsoon, which provided moisture that fell as snowfall, causing ice sheets to grow to depths of up to three kilometres. They found that instead of intense rainfall being associated with warmer climates, global cooling caused the monsoon to strengthen.
‘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,’ said Thomas Stevens of Royal Holloway, University of London.
This story was published in the August 2014 edition of Geographical Magazine