John Metcalfe was CityLab’s Bay Area bureau chief, covering climate change and the science of cities.
Long ago, huge ice formations drifted down the East Coast to visit the Sunshine State.
Imagine walking to the beach in Miami only to see an iceberg passing by like a honkin' Carnival cruise ship. That sight would be enough to send most people back to bed, convinced they were still in the middle of a disturbed, crabby melt-induced dream.
But mammoth blocks of ice have floated down to southern Florida, though it was probably 21,000 years ago. Back then, a sprawling crystalline blanket known as the Laurentide Ice Sheet, two miles deep in places, covered almost all of what's now Canada. But as the climate began to warm, chunks would snap off in the Hudson Bay and possibly wander thousands of miles south in the Atlantic, sometimes making it as far as today's palm tree-dotted Bahamas.
We know this thanks to researchers who have simulated sea currents during the last ice age. Frigid meltwater and its icy passengers would have hugged the East Coast in a narrow ribbon, says oceanographer Alan Condron of the University of Massachusetts Amherst in Nature Geosciences. Here's the path its assumed they took:
Those red boxes highlight a second clue that 'bergs once chugged along here: Condron and his team found about 400 long grooves on the ocean floor, presumably left by the jagged bottoms of ice chunks. "The depth of the scours tells us that icebergs drifting to southern Florida were at least 1,000 feet, or 300 meters, thick," Condron says. "This is enormous. Such icebergs are only found off the coast of Greenland today."
Here's a closer view of some of these "iceberg scours" off of northern Florida (which look like some furious leviathan's fingernail scratches), as well as the final resting places of icebergs known as "terminal grounding pits":
There was probably not a constant parade of icebergs along the coastline. More likely, powerful but spaced-out floods around the Hudson gave some of them a forceful boost. Condron explains:
[D]uring these large meltwater flood events, the surface ocean current off the coast of Florida would have undergone a complete, 180-degree flip in direction, so that the warm, northward flowing Gulf Stream would have been replaced by a cold, southward flowing current, he adds.
As a result, waters off the coast of Florida would have been only a few degrees above freezing. Such events would have led to the sudden appearance of massive icebergs along the East Coast of the United States all the way to Florida Keys, Condron points out. These events would have been abrupt and short-lived, probably less than a year, he notes.