Nearly 12,900 years ago, as the last ice age was fading away, the world was suddenly plunged back into the freezer for another 1400 years. Temperatures dropped by as much as 12 degrees Celsius, and ice sheets advanced from the polar regions toward the equator. The sudden changes were thought to be global in scale, but new research suggests that the Southern Hemisphere remained relatively balmy as the Northern Hemisphere froze. The results could shed light on what might happen if deep-sea currents change because of global warming.
The cooling episode, known as the Younger Dryas event, was likely linked to currents in the Atlantic Ocean. Today, cold water sinks near the Arctic and flows deep below the surface of the Atlantic toward the southern oceans, where it rises up. It then flows back along the surface northward. Eventually, the water approaches the Arctic again, where it cools and sinks. This "global conveyor belt" transports heat from the tropics into the Northern Hemisphere, which would otherwise be much colder. If this cycle were disrupted, as some say it might be if the Greenland ice sheet were to slide into the ocean, it could have big implications for the climate. But how big?
The geological history of the Younger Dryas might help answer that question, as a breakdown of the Atlantic conveyor may have triggered the cooling. The chill definitely hit the Northern Hemisphere, but researchers debate whether it was global in extent. In New Zealand, wood has been found under glacial debris dating to the Younger Dryas, suggesting that glaciers responded to the event by growing.
But Timothy Barrows, a paleoclimatologist at the Australian National University near Canberra, and colleagues have found evidence that the Younger Dryas had little chilling effect down under. First, Barrows and colleagues severed the connection between the glacier data and the cooling event. They studied boulders from the New Zealand site where the glacial wood had been found, measuring the concentrations in the rocks of radioactive isotopes beryllium-10 and chlorine-36, which are produced by nuclear reactions between minerals and cosmic rays. This reveals how long it's been since the rocks were last exposed to cosmic rays. The boulders, they report in the 5 October issue of Science, were deposited by glaciers 1000 years after the end of the Younger Dryas. That means that the Southern Hemisphere glaciers kept flowing after the end of the cold snap, implying that the Younger Dryas did not hold sway there.
The researchers found other hints that the Southern Hemisphere didn't cool by examining a deep-sea core drilled off the coast of New Zealand. They analyzed compounds in the rock produced by algae to track ancient ocean temperatures. The data suggested that temperatures in the region warmed during the Younger Dryas, which Barrows says makes sense: "Heat accumulating in the Southern Hemisphere as the north cooled is what we expected to see if the conveyor belt were shut down."
The results undermine previous theories that temperatures in the two hemispheres changed in synchrony, says Derek Fabel, a geomorphologist at the University of Glasgow in the U.K. The next step, Fabel says, is to use climate models to see whether the events would replay themselves if global warming shuts down the Atlantic conveyor once again.