A team of scientists is unraveling the intricacies of the kind of abrupt climate shifts that may occur in the future.
By accurately modeling Earth's last major global warming and answering pressing questions about its causes, they are unraveling the intricacies.
"We want to know what will happen in the future, especially if the climate will change abruptly," said Zhengyu Liu, a UW-Madison professor of atmospheric and oceanic sciences and director of the Center for Climatic Research in the Nelson Institute for Environmental Studies.
Starting with the last glacial maximum about 21,000 years ago, Liu's team simulated atmospheric and oceanic conditions through what scientists call the Bplling-Allerod warming, the Earth's last major temperature hike, which occurred about 14,500 years ago.
The simulation fell in close agreement with conditions, temperatures, sea levels and glacial coverage, collected from fossil and geologic records.
The climate dominoes began to fall during that period after glaciers reached their maximum coverage, blanketing most of North America, according to the model.
As glaciers melted, massive quantities of water poured into the North Atlantic, lowering the ocean salinity that helps power a major convection current that acts like a conveyor belt to carry warm tropical surface water north and cooler, heavier subsurface water south.
As a result, according to the model, ocean circulation stopped.
Without warm tropical water streaming north, the North Atlantic cooled and heat backed up in southern waters.
Subsequently, glacial melt slowed or stopped as well, and eventually restarted the overturning current, which had a much larger reserve of heat to haul north.
"All that stored heat is released like a volcano, and poured out over decades," Liu explained. "That warmed up Greenland and melted (arctic) sea ice," he said.
The model showed a 15-degree Celsius increase in average temperatures in Greenland and a 5-meter increase in sea level over just a few centuries, findings that squared neatly with the climate of the period as represented in the physical record.
"This is an important step toward better understanding how the world's climate could change abruptly over the coming centuries with increasing melting of the ice caps," said Bette Otto-Bliesner, an atmospheric scientist and climate modeler at National Center for Atmospheric Research (NCAR).
With another 4 million processor hours to go, the simulations being conducted by the Wisconsin group will eventually run up to the present and 200 years into the future.