A 0.5-million-year record of millennial-scale climate variability in the North Atlantic

Long, continuous, marine sediment records from the subpolar North Atlantic document the glacial modulation of regional climate instability throughout the past 0.5 million years. Whenever ice sheet size surpasses a critical th reshold indicated by the benthic oxygen isotope (delta(18)O) value of 3.5 p er mil during each of the past five glaciation cycles, indicators of iceber g discharge and sea-surface temperature display dramatically Larger amplitu des of millennial-scale variability than when ice sheets are small. Sea-sur face temperature oscillations of 1 degrees to 2 degrees C increase in size to approximately 4 degrees to 6 degrees C, and catastrophic iceberg dischar ges begin alternating repeatedly with brief quiescent intervals. The glacia l growth associated with this amplification threshold represents a relative ly small departure from the modern ice sheet configuration and sea level. I nstability characterizes nearly all observed climate states, with the excep tion of a Limited range of baseline conditions that includes the current Ho locene interglacial.