TEMPERATURE records from Greenland ice cores1,2 suggest that large and abrupt changes of North Atlantic climate occurred frequently during both glacial and postglacial periods; one example is the Younger Dryas cold event. Broecker3 speculated that these changes result from rapid changes in the thermohaline circulation of the Atlantic Ocean, which were caused by the release of large amounts of melt water from continental ice sheets. Here we describe an attempt to explore this intriguing phenomenon using a coupled ocean-atmosphere model. In response to a massive surface flux of fresh water to the northern North Atlantic of the model, the thermohaline circulation weakens abruptly, intensifies and weakens again, followed by a gradual recovery, generating episodes that resemble the abrupt changes of the ocean-atmosphere system recorded in ice and deep-sea cores4. The associated change of surface air temperature is particularly large in the northern North Atlantic Ocean and its neighbourhood, but is relatively small in the rest of the world.