A 1565 painting by Pieter Brueghel depicts icy conditions in Flanders

The Little Ice Age -- an average cooling of just a few degrees -- caused widespread disruption, particularly to agriculture. In the seventeenth and eighteenth centuries there was famine in Europe, and great hardship and mass migrations among Native Americans. In recent years, scientists studying deep ocean sediments and Greenland ice cores have discovered that there have been several abrupt cooling events like this in the 12,000 years since the end of the last Ice Age.

For a mechanism to explain these past cooling events, the focus is now on large freshwater intrusions into the region of the Nordic Seas, leading to the shutdown of the Ocean Conveyor which brings warmth to the north. One such event, which happened about 8,200 years ago, was probably triggered when a huge lake of freshwater burst through the remains of ice sheets bordering Hudson Bay. The surge flooded down Hudson Strait and out into the path of the Gulf Stream. Ocean sediments show a cooling at the same time, all around the north Atlantic. Today, the Ocean Conveyor could slow or stop either as a result of the steady freshening of the Arctic Ocean, or of a sudden massive influx of ice from the Greenland ice sheet.