still unclear. The pattern in the Southern Hemisphere is different, with slow warming and much smaller temperature fluctuations. Indeed, the Vostok ice core was drilled before the Greenland cores, and the existence of Dansgaard–Oeschger events was not widely recognised until the Greenland (GRIP/GISP2) cores were done; after which there was some reexamination of the Vostok core to see if these events had somehow been "missed". The events appear to reflect changes in the North Atlantic Ocean circulation, perhaps triggered by an influx of fresh water or rain. The events may be caused by an amplification of solar forcings, or by a cause internal to the earth system – either a "binge-purge" cycle of ice sheets accumulating so much mass they become unstable, as postulated for Heinrich events, or an oscillation in deep ocean currents (Maslin et al.. 2001, p25). More recently, these events have been attributed to changes in the size of the ice sheets and atmospheric carbon dioxide. The former determines the strength of the Atlantic Ocean circulation via altering the northern hemisphere westerly winds, gulf stream, and sea-ice systems. The latter modulates atmospheric inter-basin freshwater transport across Central America, which changes the freshwater budget in the North Atlantic and thus the circulation. These studies corroborate the previously suggested existence of a "D-O window" of AMOC bistability ('sweet spot' for abrupt climate changes) associated with ice volume and atmospheric , accounting for the occurrences of D-O type events under intermediate glacial conditions in the late Pleistocene. Timing Although the effects of the Dansgaard–Oeschger events are largely constrained to ice cores taken from Greenland, there is evidence to suggest that D-O events have been globally synchronous. A spectral analysis of the American GISP2 isotope record showed a peak of [18O:16O] abundance around 1500 years. This was proposed by Schulz (2002) to be a regular periodicity of 1470 years. This finding was supported by Rahmstorf (2003); if only the most recent 50,000 years from the GISP2 core are examined, the variation of the trigger is ±12% (±2% in the 5 most recent events, whose dates are probably most precise). However, the older parts of the GISP2 core do not show this regularity, nor do the same events in the GRIP core. This may be because the first 50 kyr of the GISP2 core are most accurately dated, by layer counting. The climate system response to the trigger is varying within 8%