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 CO2, accounting for the occurrences of D-O type events under intermediate glacial conditions in the late Pleistocene.