extended outside the North Atlantic and polar regions. Marine cores from the Santa Barbara basin reveal highly sensitive perturbations in the ocean circulation patterns of the East Pacific region 18 and ice-rafted debris events in the North Pacific that correlate with the Greenland ice core records. Abrupt changes in atmospheric circulation patterns and precipitation regime also are recorded over eastern Asia in a thick sequence of wind-deposited loess from central China. 19 Records of alpine glacier fluctuations, mountain snowlines, and paleovegetation in the Andes reveal climate fluctuations that are similar in regularity to events in the Greenland ice cores.20

While the exact phasing of rapid climate change events from region to region is still being examined, new advances in age-dating correlation techniques have provided insight into the bipolar phasing of major climate events close to the last glacial maximum. Measurements of the d18O of atmospheric O2 from the Byrd and Vostok ice cores in Antarctica and the GISP2 ice core suggest that the transition from glacial maximum to deglaciation began in Antarctica approximately 3,000 years before the onset of warming in Greenland.21 This view creates a more complex event phasing than that suggested by previous correlations of marine, coral reef, and ice extent records, which suggested that during the last termination nearly synchronous temperature changes affected ice masses from the poles to the equator. 22

New advances in paleoclimate reconstruction also come from the tropics. For example, a 30,000-year-long paleotemperature record from lowland Brazil, based on noble gas concentrations in groundwater23 and an Andean ice core24 suggests a cooling of 5 to 8 degress, contrasted with earlier estimates from marine cores that limit cooling to >3 degrees.25 Implications of this change in temperature to the hydrological cycle and consequently to climate are intriguing.26 These new findings have stimulated examination of other tropical paleoclimate records and renewed investigations into climate forcing that is tied to changes in the tropics.

Causal mechanisms for glacial-age climate fluctuations appear to be complex, and phasing of these events is not understood from region to region. However, evidence for the identification of regularity in the timing of some climate events is building. Studies ranging from the North Atlantic (GISP2) to the subtropics demonstrate 1,450-to 1,800-year periodicities for rapid climate change events.27 In addition, the cumulative effect of multiple climate forcings can now be demonstrated. As an example, ~90 percent of the variance in the GISP2 paleoatmospheric circulation series is related to insolation changes induced by the Earth's orbital cycles, which operate in concert with faster periodic climate forcings such as changes in ice sheet dynamics, thermohaline ocean circulation, and solar variability (see Figure 6.7).28 Additional climate forcing mechanisms are, undoubtedly, also involved, such as changes in CO2, CH4, water vapor, volcanism, biogenic source cloud condensation nuclei, and dusts.



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement