'To overcome this, we studied isotope variations in three stalagmites collected from an Italian cave, and found that these variations relate to ocean temperature changes recorded in sediment cores from the nearby sea floor,' Drysdale said.

'Stalagmites from limestone caves can be very precisely dated using trace amounts of uranium incorporated within their structure,' Drysdale said.

'We applied the accurate time scale of the stalagmite record to the sea floor sediment data. A key property of sea-floor sediments is that they detect the growth and decay of ice sheets.'

'So we have effectively provided an accurate time scale for the collapse of the ice sheets that ended the penultimate ice age. This collapse started around 141,000 years ago,' he added.

'This is as much as 8,000 years earlier than previously thought -- too early to be caused by stronger northern hemisphere summers alone, which is the prevailing theory.'

The study was published in the journal Science.