November 6, 2012
New Climate Models Identify Trigger For Earth’s Last Big Freeze
April Flowers for redOrbit.com - Your Universe Online
Climate scientists have long debated whether floodwaters from melting of the Laurentide Ice Sheet flowed northwest into the Arctic first, or east via the Gulf of St. Lawrence to weaken ocean thermohaline circulation and have a frigid effect on global climate. This debate concerning the melt, which ushered in the last major cold episode about 12,900 years ago, has been raging for about 30 years.
A research team from the University of Massachusetts Amherst and the University of Alaska has used new, high-resolution global ocean circulation models to report the first conclusive evidence that this flood flowed north into the Arctic through the Mackenzie River valley. The results, published in the current issue of Proceedings of the National Academy of Sciences, also revealed that if these waters had flowed east into the St. Lawrence River valley, the climate of Earth would have remained relatively unchanged.
"This episode was the last time the Earth underwent a major cooling, so understanding exactly what caused it is very important for understanding how our modern-day climate might change in the future," says geologist Alan Condron of UMass Amherst's Climate System Research Center.
Glacial Lake Agassiz, at the southern edge of the Laurentide Ice Sheet, which covered Hudson Bay and much of the Canadian Arctic, catastrophically broke through an ice dam and rapidly dumped thousands of cubic kilometers of fresh water into the ocean before the events leading up to the sharp climate-cooling period known as the Younger Dryas. It is assumed that this massive influx of frigid fresh water that was injected over the surface of the ocean halted the sinking of very dense, saltier, colder water in the North Atlantic. This sinking drives the large-scale ocean circulation, known as the thermohaline circulation, and transports heat to Europe and North America. The flood weakening the circulation resulted in the dramatic cooling of North America and Europe.
The new high resolution, global, ocean-ice circulation model developed by Condron and Peter Winsor at the University of Alaska is 10 to 20 times more powerful than previous models. Using this powerful new tool, the team compared how meltwater from the two different drainage outlets was delivered to the sinking regions in the North Atlantic.
The results of this comparison showed that the original hypothesis proposed in 1989 by Wally Broekder of Columbia University, which suggested that Lake Aggasiz drained into the North Atlantic down the St. Lawrence River, would have weakened the thermohaline circulation by less than 15 percent.
This level of weakening, according to the team, is unlikely to have accounted for the 1,000-year cold climate event that followed the flood of meltwater. St. Lawrence river meltwater ends up almost 1,900 miles south of the deep water formation regions. This is too far south to have any significant impact on the sinking of surface waters, explaining why the impact on the thermohaline circulation is so minor.
The findings of the new model shows that when the meltwater first drains into the Arctic Ocean, narrow coastal boundaries currents can efficiently deliver it to the deep water formation regions of the sub-polar north Atlantic. This weakened the thermohaline circulation by more than 30 percent, leading the team to conclude that the St. Lawrence scenario is "more likely to have triggered the Younger Dryas cooling."
The team's new model runs on one of the world's top supercomputers at the National Energy Research Science Computing Center in Berkeley, California. Condron and Windor state, "With this higher resolution modeling, our ability to capture narrow ocean currents dramatically improves our understanding of where the fresh water may be going."
Condron adds, "The results we obtain are only possible by using a much higher computational power available with faster computers. Older models weren't powerful enough to model the different pathways because they contained too few data points to capture smaller-scale, faster-moving coastal currents."
"Our results are particularly relevant for how we model the melting of the Greenland and Antarctic Ice sheets now and in the future. "It is apparent from our results that climate scientists are artificially introducing fresh water into their models over large parts of the ocean that freshwater would never have reached. In addition, our work points to the Arctic as a primary trigger for climate change. This is especially relevant considering the rapid changes that have been occurring in this region in the last 10 years."
Image 2 (below): A new model of flood waters from melting of the Laurentide Ice Sheet and large glacial lakes along its edge that covered much of North America from the Arctic south to New England over 13,000 years ago, shows the meltwater flowed northwest into the Arctic first. This weakened deep ocean circulation and led to Earth´s last major cold period. A new model of flood waters from melting of the Laurentide Ice Sheet and large glacial lakes along its edge that covered much of North America from the Arctic south to New England over 13,000 years ago, shows the meltwater flowed northwest into the Arctic first. This weakened deep ocean circulation and led to Earth´s last major cold period. Credit: Alan Condron, UMass Amherst