Last month we reported on the inability of climate models to predict the severity of rapid climate change events. One of these events, the collapse of the Atlantic meridional overturning circulation (AMOC), has been linked to a series of what are termed Heinrich events. These occur during glacial eras, when large amounts of ice break off glaciers and enter the Atlantic Ocean. New data and simulation results published in the Proceedings of the National Academies of Science suggest that Heinrich events occurred during the AMOC decline, which forced warmer water beneath the surface of the ice sheets, causing the edges to break off—similar to recent ice loss after the collapse of Antarctic ice shelves.

Six known Heinrich events took place in the late Pleistocene (120,000-10,000 years ago). During these events, large icebergs broke off the Hudson Strait Ice Stream (part of the Laurentide ice sheet, which covered most of Canada and the northern United States) and entered the North Atlantic Ocean.

Evidence for these events was first discovered in 1988 by the marine geologist for which they are named, Helmut Heinrich. He found that ocean sediments contained periodic layers rich in continental rock—and in sizes too large to be carried by ocean currents. The accepted explanation for this is that icebergs broke off the ice sheets, then released the rock into the ocean as they melted.

It hasn’t been clear, however, just how the ice broke off. One common explanation is internal instability of the ice sheet, while another is that subsurface warming (due to higher ocean water temperatures) weakened the sheet, though evidence for this hypothesis has not been very conclusive—until now.

The authors of the new paper measured the ratio of magnesium to calcium (Mg/Ca) in the calcium carbonate shells of benthic foraminifera, organisms that live on and near the seabed. Magnesium incorporates into the shells more easily at higher temperatures, so higher Mg/Ca ratios suggest formation at higher water temperatures. By measuring this ratio in ocean sediments with established dates, changes in the temperature of the water can be determined. Other studies used the ratio of oxygen-18 to oxygen-16 (δ18O) in the shells, so Mg/Ca provides a parallel measure.

Their results indicate that subsurface warming caused Heinrich events, and help provide a timeline leading up to them. Temperatures began rising about 1,000-2,000 years before each Heinrich event, coinciding with a decrease in the AMOC. This decrease, in turn, was caused by freshwater flux from ice sheets due to small changes in climate, and led to warmer ocean waters (the AMOC helps cool the ocean through convective mixing).

The most significant result of this study was that the Heinrich events, rather than causing or occurring at the beginning of the AMOC collapse, happened when the AMOC was at a minimum. Considering recent ice shelf collapses, this research is relevant to current events. The authors don’t suggest that we are in the middle of Heinrich event, but understanding the causes of past ice loss will allow us to better understand and predict similar events in the future.

PNAS, 2011. DOI: 10.1073/pnas.1104772108 (About DOIs)