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.
The impact of changes in the ocean overturning circulation on climate has become a hot topic today as global temperatures rise and melting sea ice and glaciers add freshwater to the North Atlantic. A 2015 study suggested that cooling in the North Atlantic may be due to a reduction in the overturning circulation, while a 2016 study suggested there had not been enough freshwater to have an effect.
The new study explores what happened to ocean circulation when the Earth went through a series of abrupt climate changes in the past, during a time when ice covered part of North America and temperatures were colder than today. It looks at the Atlantic meridional overturning circulation, which distributes heat as it moves warmer surface water from the tropics toward Greenland and the high northern latitudes and carries colder, deeper water from the North Atlantic southward.
Using chemical tracers in sediment that builds up on the sea floor over time, Henry and his co-authors were able to document the relative speed of the Atlantic meridional overturning circulation during each abrupt climate change. The overturning circulation pushes water through the Atlantic Basin, distributing heat as it moves warmer surface water from the tropics toward Greenland and the high northern latitudes and carries colder, deeper water from the North Atlantic southward.
The chemical tracers show that the speed of the ocean overturning circulation changed first and that a change in sea surface temperature followed. That suggests that cooling may start with changes in the ocean circulation, influencing the northern sea surface and atmosphere, said co-author Jerry McManus, a professor at Lamont-Doherty Earth Observatory. Evidence from ice cores and deep-sea sediment has shown that the Northern climate also cooled before the Southern climate during these abrupt changes, creating a “bipolar seesaw,” with the North cool while the South was warm, and the South cooling as the North warmed.
The scientists stress that more work is needed to determine whether changes in ocean circulation initiated the abrupt climate changes or were and intermediary effect initially triggered by something else. “Our study supports the view that changes in ocean circulation were at least in part responsible for causing abrupt climate changes. However, what in turn caused those changes in circulation remains a mystery,” Henry said
Also unclear is why these abrupt climate shifts, also seen in previous ice ages, haven’t happened in the past 10,000 years. The instability appears to only occurs in a certain temperature ranges and when there is a large amount of land ice.
The series of abrupt climate changes studied here occurred between 60,000 and 25,000 years ago, ending as the last ice age peaked. Each followed a general pattern in the Northern Hemisphere: The cooling happening over hundreds to 1,000 years, then the frigid temperatures persisted for a few hundred years in what is known as a stadial, McManus said. Once warming started, it happened very rapidly, with a rise of 3 to 6 degrees Celsius in average sea surface temperature and larger changes over Greenland within a span of decades.
"We would all like to understand better how the Earth’s climate operates. This demonstrates the crucial role that global circulation can play. The dynamics of the deep ocean directly influence the Earth’s climate.”—Jerry McManus