The Demise of the Last Ice Age A Role for Dust

The Earth has experienced a series of intense ice ages over the course of the last million years or so. Each ice age ended rather suddenly, with a rapid warming transition ('termination') from cold glacial conditions into a (relatively brief) mild interglacial period (Fig. 5a). Many different theories have been advanced for how these cycles might be driven. These have typically focused on the physical climate system, particularly interactions between ice sheets and underlying bedrock with external forcing provided by orbitally-driven variations in the seasonal intensity of sunlight received

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Fig. 5 Key indicators of climatic state contained within the Vostok ice core [Petit et al. (1999)]. (a) Isotopically-derived temperature change (relative to the present) at the surface. Cold glacial and warmer interglacial ('IG') intervals are indicated. (b) CO2 concentration in air bubbles contained within the ice. (c) Dust concentration in the ice. The correspondence between CO2 minima and prominent dust peaks are highlighted.

at the Earth's surface. However, such explanations fail to correctly predict the amplitude and timing of the observed cyclically in global ice volume, suggesting that additional factors might also be critical [Ridgwell et al. (1999)].

Records of past atmospheric composition, in the form of microscopic bubbles of ancient air trapped within the crystalline structure of ice sparked a revolution in understanding of what drives these ice age cycles. Cores of ice recovered from Antarctica and analysed for air bubble gas composition revealed that atmospheric CO2 varies cyclically between about ^280 ppm during interglacials and ^190 ppm during the most intense glacial periods (Fig. 5b).

What causes the observed variability in CO2? A possible clue comes from the changes in dust deposition, also recorded in the Vostok ice (Fig. 5c). The concentration of dust contained within the ice exhibits a

series of rather striking peaks against a background of relatively low values; a much greater dynamic range than can be accounted for by dilution effects arising from changes in snow accumulation rate alone. The occurrence of these peaks correlates with periods of particularly low atmospheric CO2 values. This is certainly consistent with increased dustiness during glacial times providing more iron to the surface and driving a more vigorous biological pump in the ocean [Martin (1990); Watson et al. (2000)]. However, investigations of the global carbon cycle using both numerical models [Archer et al. (2000); Bopp et al. (2003)] and observations [Bopp et al. (2003); Kohfeld et al. (2005)] suggest that an increase in the strength of the biological pump can only be part of the explanation for low glacial atmospheric CO2 concentrations. Other mechanisms must be at work.

If dust is responsible for some of the observed glacial-interglacial variability in atmospheric CO2, then we need a much better understanding of the factors that bring about changes in dust fluxes. Elevated glacial dust fluxes are not restricted to Antarctica. In fact, similar features are found in dust records from ice, marine, and terrestrial environments around the world. A colder, drier glacial climate, with a less vigorous hydrological cycle would result in decreased precipitation scavenging, more efficient transport of dust, and thus higher deposition rates. However, models of dust generation, transport, and deposition suggest that a reduction in the hydrological cycle alone is not sufficient to explain the increases in glacial dustiness. Greater source strengths of dust must also be invoked. The expansion of arid areas under cold, dry glacial climatic conditions, or even the exposure of continental shelves as the ice sheets grow and sea-level drops could result in new sources of dust. Furthermore, higher wind speeds during the glacial period could result in enhanced entrainment from existing source areas [e.g. Mahowald et al. (1999); Werner et al. (2003)]. Thus, changes in sea-level, aridity, and vegetation type and cover, as well as atmospheric circulation, and precipitation strength and patterns all combine to affect dust deposition, and with it, atmospheric CO2 and climate [Ridgwell and Watson (2002)].

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