Ice-age temperature swings may have been caused by fluctuating Atlantic Ocean currents

Science : During the last ice age, temperatures on Earth’s surface seesawed several times between the Northern and Southern Hemispheres: As the north got colder, the south grew warmer, and vice versa. Now researchers have found that the abrupt temperature changes in the two hemispheres may have been caused by a slowdown of the Atlantic Meridional Overturning Circulation (AMOC), an ocean current that drives shallower, warmer waters north and deeper, colder waters south. Studying a sediment core drilled from the Bermuda Rise in the Atlantic Ocean, Jerry McManus of Columbia University and colleagues say they saw at least four instances when the ratio of two radioactive daughter isotopes changed sharply, which indicated a weakening of the AMOC. Such slowdowns could have caused the north to receive less warm water and temperatures there to drop while the south heated up due to the backlog of warm water. What caused the slowdowns is still unknown, but one explanation is the breaking off of Canadian icebergs, whose melting would have increased the amount of freshwater in the North Atlantic and possibly disrupted ocean flow. Over some 1500 years, the freshwater would have dissipated, allowing the AMOC to increase once again in strength.