Topography's crucial role in Heinrich Events

Significance Heinrich Events are frequently promoted as examples of an abruptly forced climate change. Although the evidence for changes in the ice sheets, which cause these events, is clear, the pathway by which these changes can affect the climate is not. The most common explanation is that the melting icebergs released by the ice sheet cause a change in the ocean’s circulation. Here we propose that the change in the height of the ice sheet, which changes the atmospheric circulation, is just as important. We must therefore reassess the simple paradigm that the climate changes that occurred during Heinrich Events are solely the result of a change in the ocean’s circulation. Heinrich Events, the abrupt changes in the Laurentide Ice Sheet that cause the appearance of the well-observed Heinrich Layers, are thought to have a strong effect on the global climate. The focus of most studies that have looked at the climate’s response to these events has been the freshwater flux that results from melting icebergs. However, there is the possibility that the varying height of the ice sheet could force a change in the climate. In this study, we present results from a newly developed coupled climate/ice sheet model to show what effect this topographic change has both on its own and in concert with the flux of freshwater from melting icebergs. We show that the topographic forcing can explain a number of the climate changes that are observed during Heinrich Events, such as the warming and wettening in Florida and the warm sea surface temperatures in the central North Atlantic, which freshwater forcing alone cannot. We also find regions, for example the tropical Atlantic, where the response is a mixture of the two: Here observations may help disentangle the relative importance of each mechanism. These results suggest that the simple paradigm of a Heinrich Event causing climate change via freshwater inputs into the North Atlantic needs to be revised.