There was a period during the last ice age when temperatures in the Northern Hemisphere went on a rollercoaster ride, plummeting and then rising again every 1,500 years or so. Those abrupt climate changes wreaked havoc on ecosystems, but their cause has been something of a mystery. New evidence published this week in the leading journal Science shows for the first time that the ocean's overturning circulation slowed during every one of those temperature plunges – at times almost stopping."People have long supposed this link between overturning circulation and these abrupt climate events. This evidence implicates the ocean," said L. Gene Henry, the lead author of the study and a graduate student at Columbia University's Lamont-Doherty Earth Observatory.