Volcanoes and Little Ice Age

For years, researchers have tried to pinpoint the cause of the Little Ice Age, which began sometime after the Middle Ages and lasted into the late 19th century.

During the cool spell, advancing glaciers in mountain valleys in northern Europe destroyed whole towns. Famous paintings from the period depict people ice-skating on the Thames River in London and on canals in the Netherlands, places that remained ice-free before and after the Little Ice Age. And the New York Times describes that:

In the winter of 1780, New Yorkers could walk the five miles from Manhattan to Staten Island on ice as thick as eight feet.

Now geologists have discovered the cause for this cooling period, and determined the exact start date of Little Ice Age. Their results were published this week in Geophysical Research Letters.

The University of Colorado’s Gifford Miller and his colleagues went to Baffin Island in the Canadian Arctic and radiocarbon-dated 150 samples of dead plant material with roots intact collected from beneath receding ice caps. There was a large cluster of “kill dates” between 1275 and 1300, indicating the plants had been frozen and engulfed by ice during a relatively sudden event.

Samples in Iceland revealed similar dates pointing to the precise cause: four large tropical volcanoes that erupted in that same period. The Bad Astronomer explains it in his Discover blog:

The ash would have darkened the atmosphere, letting slightly less sunlight down. Some of the gases emitted by volcanoes also cool the air. It seems clear these volcanoes are what triggered the Little Ice Age. But why did it last so long?

To answer that question, the researchers used models to simulate the conditions. The simulations showed sustained cooling from volcanoes would have sent some of the expanding Arctic sea ice down along the eastern coast of Greenland until it eventually melted in the North Atlantic. Since sea ice contains almost no salt, when it melted the surface water became less dense, preventing it from mixing with deeper North Atlantic water. This weakened heat transport back to the Arctic and created a feedback system that preserved the sea ice long after the effects of the volcanic aerosols subsided.

“Our simulations showed that the volcanic eruptions may have had a profound cooling effect,” says co-author Bette Otto-Bliesner. “The eruptions could have triggered a chain reaction, affecting sea ice and ocean currents in a way that lowered temperatures for centuries.”

Twenty-five years of eruptions dropped the global temperature by about one degree Celsius for over 500 years! It’s astounding when we think of how much we are altering the climate now, reports the New York Times:

Dr. Miller emphasized that the importance of the findings lay not so much in pinpointing the likely start date for the Little Ice Age but rather in discovering just how rapidly the global climate can be altered, and for how long.

Image of Gifford Miller on Baffin Island, courtesy of Miller