Millennial variability

Even a cursory glance at the records in > Figure 12.4 reveals that climate variability on the timescale of millennia, particularly during glacial periods, is substantial, with larger oscillations reaching as much as half the size of the

D Figure 12.4

Climatic proxies from both hemispheres over four glacial cycles as recorded in a variety of paleoclimate records. (a) Summer insolation (21 June) at 65°N. (b) North Atlantic Sea surface temperature (SST) record from deep-sea sediment core at ODP site 980. (c) South Indian ocean SST derived from foraminiferal transfer functions, from deep-sea sediment core MD 94-101. (d). Atmospheric CO2 and temperature estimated from 8D from the Vostok ice core. (e) Summer insolation (December 21) at 65°S (Labeyrie et al. 2003)

glacial-interglacial signal itself. However, it was not until the mid-1980s that this variability was widely recognized in the paleoclimate community, initially due to the advent of high resolution ice core records from Greenland (Dansgaard et al. 1984) later corroborated by interpretation of alternating layers of "ice rafted debris'' in deep-sea sediment records from the North Atlantic ocean (Heinrich 1988). In honor of these early discoverers, the half a dozen or so larger climatic swings between 60 and 15 ka are commonly referred to as "Heinrich events'' while more than 20 smaller events are generally called "Dansgaard-Oeschger (D/O) events.'' Moreover, the events themselves seem to be grouped in cycles, sometimes called Bond Cycles, beginning with the most severe and gradually declining in amplitude until the next multiple cycle is initiated.

By combining the stratigraphic record of these rapid D/O oscillations with attempts to model the mechanisms responsible, the dominant hypothesis to emerge sees the events as a product of the instability of the ice sheets that girdled the North Atlantic. Rapid discharge of ice into the ocean is thought to have reduced surface seawater salinity to the point where North Atlantic Deep Water (NADW) formation was inhibited, thereby shutting down the related transport of heat into the region by the large scale overturning circulation of the North Atlantic ocean. This cut-off of NADW formation may then have so modified ocean circulation as to have influenced climate on a global scale. Such a hypothesis is consistent with the antiphase relationship between Greenland and much of Antarctica often referred to as a bipolar "see-saw."

It seems possible that the variations in amplitude of the cold "swings" reflect changes between iceberg surges reflecting instability of the Fennoscandian ice sheet, with a relatively more rapid response time, and the Laurentide Ice sheet with a longer response time. Surges of the latter are thought to have been responsible for the most extreme events, those that gave rise to the main "Heinrich" IRD layers. The periodicity of the D/O events is not regular, and no credible external forcing mechanism has been proposed. The bipolar antiphase relationship demonstrated for the major D/O events suggests that they are largely the product of cryosphere-ocean dynamics.

In the decades following the initial discoveries the same millennial events seen in the northern North Atlantic region have been found to be widespread, with clear evidence in such diverse locations as Chinese loess deposits and stalagmite isotope records, deep sea sediments from the Cariaco and Santa Barbara Basins, and even Antarctic ice cores. A compendium of a few such proxy records providing a flavor for nature and magnitude of climatic and environmental on millennial timescales in high and low latitudes and both hemispheres is presented in> Figure 12.5 (Labeyrie et al. 2003). The global nature of these events has lead to further questions and refinements regarding the possible atmospheric and oceanic mechanisms for transmitting these signals around the globe, for example, through influence on the Asian monsoon systems in the case of Chinese records, or the rate of ventilation of intermediate depth waters of the Pacific, in the case of the Santa Barbara Basin record. However, changing rates of overturning circulation driven by fresh water fluxes in the northern North Atlantic remain a widely accepted, plausible hypothesis for the primary driver of this variability.

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