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Paleoclimate
A new trigger for Ice Age retreat

About 14,600 years ago, a huge pulse of freshwater drained from continental ice sheets into the world’s oceans. Over 500 years, a discharge equivalent to five Amazon Rivers raised sea level by 20 meters — marking one of the most dramatic chapters in Earth’s episodic climb out of the last Ice Age.

Traditionally, paleoclimatologists have thought that the meltwater came from the Laurentide Ice Sheet, responding to an abrupt warming of the northern hemisphere called the Bolling-Allerod.

Last year, geophysicist Peter Clark from Oregon State University and colleagues proposed that the meltwater actually came from Antarctica. The calving and melting of a massive portion of Antarctica better explained the observed pattern of sea-level rise, they argued in Science (Geotimes, June 2002).

Now Clark’s team has partnered with climate modelers from the University of Victoria to suggest that the Antarctic meltwater pulse into the Southern Ocean also changed global ocean circulation patterns. As reported in the March 14 Science, those changes influenced climate thousands of miles away and, in fact, triggered the Bolling-Allerod warming, turning on its head the traditional view that warming caused the melting.

“Everyone has been looking at the impacts of meltwater into the North Atlantic,” says Konrad Hughen, a paleoclimatologist at the Woods Hole Oceanographic Institution. “The great contribution of this study is that the authors are looking outside the box, beyond the North Atlantic, and investigating the potential scenarios of discharge into the Southern Ocean.”

The first evidence for a massive meltwater pulse came from Barbados in 1989. Dating of remnant corals in Barbados that had lived near the ocean surface prior to the meltwater deluge indicated that the pulse occurred 14,235 years ago, several hundred years after the onset of the Bolling-Allerod warming. This timing led researchers to propose that the warming caused the melting. However, recent data from the Sunda Shelf in the South China Sea suggest that the meltwater pulse began 300 years earlier than suspected — making the onset of the warming and the melting roughly synchronous and opening the possibility that the melting caused the warming.

Encouraged by this possibility, the authors of the Science study used an ocean-atmosphere model developed at the University of Victoria — the UVic Earth System Climate Model — to explore the impacts of a meltwater pulse into the Southern Ocean. “When we dump the water in a place where the Southern Ocean is sensitive, the climate responds in ways that fit the geologic records quite nicely,” says lead author and climate modeler Andrew Weaver at the University of Victoria.

Today, the North Atlantic Ocean transports heat from the equator to the northern hemisphere through an ocean heat pump. Warm equatorial water travels north, and as it travels, it cools. Eventually the water gets heavy enough (its density increases as it cools) that it sinks, and circulates back toward the equator. The sinking, called the North Atlantic Deep Water (NADW) formation, literally pulls warm surface waters northward, and thus creates the engine for a North Atlantic heat pump.

Geologic records suggest that the heat pump did not operate 14,600 years ago, and so the modelers initialized the oceans without an active pump. In the simulations, the sudden meltwater pulse lowered the density of the surface water in the Southern Ocean, which in turn raised the local sea level, Clark says. Much like water piled up at one end of a bathtub, the water then flowed northward. It cooled and sank, sparking NADW formation and turning on the heat pump. Once started, positive feedbacks sustained the pump, which in turn warmed the northern hemisphere in amplitude and timing that matched the Bolling-Allerod.

Although the study presents a coherent picture of paleoclimate, the timing of events remains an obstacle, Hughen says. Coral records from Tahiti support the Barbados record indicating that the meltwater pulse came well after the onset of warming. The authors argue that the Barbados record might underestimate the age of the pulse; an inaccurate assumption about the degree of bottom and surface water mixing could have skewed the radiocarbon dating of the corals. But the authors need to make a stronger case for why the widely-used radiocarbon assumption does not hold up in this case, Hughen says.

Another problem is that the Antarctic just could not have held enough vulnerable ice to account for the massive meltwater pulse that the authors invoke, says Dick Peltier, a paleoclimatologist from the University of Toronto. He argues that no more than half of the meltwater could have come from the Antarctic.

Clark suspects that a strong warming event in the southern hemisphere, ultimately linked to changes in Earth’s orbit, triggered the Antarctic melting. “The key is to find evidence from Antarctica itself of a meltwater pulse.” There have been strong hints, he says, but no smoking gun.

Greg Peterson


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