Over the past two million years, cyclical variations in the amount of solar energy reaching the Earth have caused it to experience a series of long glacial periods separated by short, warmer intervals known as interglacials. The last glacial period, which ended 12,000 years ago, began 80,000–70,000 years ago.
At the beginning of this period, sea levels dropped by 80 metres as large amounts of snow accumulated at high latitudes, eventually leading to the formation of the ice sheet around the North Pole. But cold temperatures are generally associated with dry weather and scarce precipitation – for snow to fall, the weather needs to be humid and the temperature only moderately low, hence the long-standing paradox.
In the present study, researchers analysed marine sediment cores collected off Galicia in Spain and from the Bay of Biscay. They used fossil pollen grains to characterise the temperatures on land and microscopic marine organisms called foraminifera to determine the temperature of the ocean.
The results indicated that during the period under study, water temperatures in the Bay of Biscay remained relatively high, whereas those in mainland Europe gradually fell. The humidity released by the resulting thermal contrast, which was presumably carried northwards by wind, caused the snowfall that formed the polar ice sheet, the researchers believe.
This story was published in the October 2013 edition of Geographical Magazine