In the spectacular collapse of ice sheets as the last ice age ended about 18,000 years ago scientists hope to find clues for what regions may grow drier from human caused global warming. In a talk Thursday at the American Geophysical Union’s annual meeting, Aaron Putnam, a postdoctoral scholar at Columbia University’s Lamont-Doherty Earth Observatory, painted a picture of earth’s dramatic transformation as seen in climate records extracted from ancient cave formations, ice cores, lake shorelines and glacial moraines.
Earth came out of the last ice age in two phases, triggered paradoxically by the cooling of waters in the North Atlantic Ocean, said Putnam. In phase one, the stratification of North Atlantic waters pushed Earth’s wind and rain belts south. The winds caused carbon dioxide to out-gas from the Southern Ocean, rapidly heating the Southern Hemisphere by 16,000 years ago. In phase two, with the evening of temperatures in the polar oceans, the wind and rain belts returned north. By 14,700 years ago, the Northern Hemisphere begins to rapidly warm, bringing the planet as a whole out of the ice age.
The first interval made normally dry regions wet, and wetter regions, dry, and then the situation reversed 2,000 years later, said Putnam. In the U.S., lake levels in the mid-latitudes swelled as the jet stream pushed south bringing more rain. Lake Lohantan in Nevada and Lake Estancia in New Mexico reached their highest levels about 16,000 years ago, research by Lamont’s Wally Broecker suggests. At the southern edge of the tropical rain belts, Lake Tauca in Bolivia reached its maximum extent at the same time. Meanwhile, the monsoon rains in Asia were failing, leaving evidence of drought in Hulu Cave near Nanjing, China, and Venezuela’s Cariaco Basin. Antarctic ice cores also show evidence of less vigorous vegetation growth in the northern forests. “These are massive changes that are happening,” said Putnam.
The rapid retreat of glaciers in New Zealand suggest that the Southern Hemisphere warmed quickly once the Southern Ocean started to release carbon dioxide. Moraine dating by Putnam and his Lamont colleagues, Joerg Schaefer and Michael Kaplan, show that glaciers were biggest at 17,800 years ago. In just 2,000 years, the ice retreated close to where it is today and temperatures warmed 3 degrees Celsius, their research shows. (Another degree of warming would happen by the onset of the Holocene 12,000 years ago.)
Today, with the North Atlantic now warming, Putnam and his colleagues expect the chain of events to reverse, with wind and rain belts shifting north. “We should anticipate that the dry lands and deserts of the Northern Hemisphere will become drier, which has implications for water resources,” he said. “Monsoons could pick up in South Asia and Venezuela.”
Aaron Putnam’s account of trekking through the Bhutan Himalaya in search of glacial moraines New York Times, November 2012