Rapid changes of glacial climate simulated in a coupled climate model

Abrupt changes in climate, termed Dansgaard-Oeschger and Heinrich events, h ave punctuated the last glacial period (similar to 100- 10 kyr ago) but not the Holocene (the past 10 kyr). Here we use an intermediate-complexity cli mate model to investigate the stability of glacial climate, and we rnd that only one mode of Atlantic Ocean circulation is stable: a cold mode with de ep water formation in the Atlantic Ocean south of Iceland. However, a `warm ' circulation mode similar to the present-day Atlantic Ocean is only margin ally unstable, and temporary transitions to this warm mode can easily be tr iggered. This leads to abrupt warm events in the model which share many cha racteristics of the observed Dansgaard-Oeschger events. For a large freshwa ter input (such as a large release of icebergs), the model's deep water for mation is temporarily switched off, causing no strong cooling in Greenland but warming in Antarctica, as is observed for Heinrich events. Our stabilit y analysis provides an explanation why glacial climate is much more variabl e than Holocene climate.