At the peak of the last ice age around 21,000 years ago, the so-called Laurentide ice sheet covered a huge chunk of North America, including nearly all of present-day Canada. As the climate warmed, the ice sheet began to break up, sending myriad icebergs into the northern Atlantic Ocean. These icebergs would routinely drift as far south as South Carolina, but once there would melt in the warm waters of the Gulf Stream, according to the study, published online Sunday in the journal Nature Geoscience.

Every now and then, however, a huge glacial lake would drain into the ocean following the melting of the ice barriers holding it in place. Lake Agassiz, for example, which was once bigger than all of the Great Lakes combined, disappeared roughly 8,400 years ago. “It’s thought to be a catastrophic event, like the breach of a dam,” said Alan Condron, a climate modeler at the University of Massachusetts in Amherst, who co-authored the study.

Previously, it was believed that the freshwater from such lakes drifted east into the subpolar North Atlantic, where it disrupted the Gulf Stream and caused the climate to cool. But Condron, using a high-resolution numerical model, determined that much of the water actually flowed south along the coast down to Florida. Confined to the surface, this near-freezing water passed above the Gulf Stream, Condron said, and insulated the icebergs traveling along with it.

Iceberg scars left along the continental shelf. (Credit: Jenna C. Hill)

In order to confirm his model’s findings, Condron partnered with Jenna C. Hill, an associate professor of marine science at Coastal Carolina University, whom he met last year at an Arctic research meeting. In 2006, Hill was part of a team that discovered iceberg keel marks off the coast of South Carolina. She has continued looking for them ever since, using high-resolution images to identify about 400 from North Carolina to Florida, including some near Miami and possible ones around the Florida Keys.

Many of these keel marks, or grooves, on the seafloor are 30 feet wide, indicating the presence of icebergs up to 300-meters (984-feet) thick, Hill explained. “Since the general rule is that nine-tenths of an iceberg is underwater, a 300-meter-thick iceberg would probably be about 10 stories tall above the water,” she said via email from a research vessel currently at sea. These days, she added, icebergs that large are found only near Greenland.

Drifting along at approximately 2.2 miles per hour, the icebergs could make it from northern Canada to Florida in as little as four months, Condron explained. “It would have been very interesting to have been in Florida for one of these events,” he said. “It would be nice and warm along the coast and then, on the horizon, you’d start to see these big icebergs. It would be quite abrupt and quite scary, probably.” He speculated that during the last deglaciation, from about 21,000 to 6,000 years ago, “you might see icebergs for a month” straight in Florida, “and then not again for 2,000 years.”

If the past is any indication, then meltwater from the shrinking Greenland ice sheet may likewise follow the coast south into the subtropics, according to the study. Condron pointed out that gradual melting is expected in Greenland, without the sudden draining of huge glacial lakes, and that in no way will icebergs appear again in Florida any time soon. Nonetheless, with the notable exception of sea-level rise, the consequences of melting ice sheets remain hard to pin down. “That’s the big question,” Condron said. “How sensitive is ocean circulation and climate to increases in freshwater to the ocean? We don’t have a firm handle on that right now.”