If we polled climate scientists for the weirdest thing we learned by drilling into Greenland’s ice, Dansgaard-Oeschger cycles would be strong contenders. During the colder parts of Greenland’s ice age history, it has frequently experienced a wicked case of climate whiplash. Parts of Greenland could endure a warming of 10 degrees Celsius in the space of a couple of decades. That would be followed by centuries of cooling and, eventually, another abrupt warming.
Look at ocean sediment cores in the Atlantic and you’ll see something else happening at the same time: sand and stones appearing in the seafloor mud, carried there by dirty, slowly melting icebergs. (In fact, some icebergs may have made it as far as Florida.) These impressive launches of iceberg armadas are called Heinrich events—and there’s clearly some connection between Heinrich and Dansgaard-Oeschger.
We’re pretty sure these wild events relate to the downward flow of salty, dense surface water in the North Atlantic, which completes the conveyor belt that turns northward-flowing surface water into southward-flowing deep water. If the salty surface water loses its greater density, the conveyor belt—and the warmth it carries northward—seizes up. Conversely, switching the conveyor belt back on can rapidly deliver warmth northward, which may explain some of the sudden warmings.
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