The role of dust in glacial cycles

Dust records show up to 27–30 times larger fluxes of dust during the last glacial
maximum (LGM) relative to present day. As is well known from recent studies on the effects of large volcanic eruptions, sudden large emissions of atmospheric aerosol particles that persist in the atmosphere for a period of months to years can certainly influence global climate, causing a negative climate forcing (cooling) during that time period. To investigate the role of dust during full glacial–interglacial cycles we conduct million-year simulations using a relatively simple two-dimensional model that has been used in the past to elucidate the role of many processes in the climate system on glacial–interglacial time scales (LLN 2D NH model). We demonstrate that dust likely took part in shaping the structure of glacial cycles (emphasizing the asymmetry of the glacial cycles, especially in the cycles that were symmetric in the reference simulation) and in determining glacial–interglacial temperature differences.



Glacial-interglacial dynamics

Earth's climate undergoes variations on a wide range of time scales, from seasonal and interannual to glacial-interglacial and beyond. Glacial-interglacial cycles are among the most striking phenomena in climate dynamics. While we are now at an interglacial, only about 20,000 years ago was the last ice age at its peak, with ice sheets covering much of North America and Europe, global average temperature about 6o colder than it is today and sea level about 120 m than now. These dramatic transitions between glacial and interglacial conditions have occurred some 8-9 times over the past 1 millions years or so.

A mechanism for the glacial-interglacial oscillations was proposed which we call  the "sea-ice switch mechanism": a rapid growth or melting of sea ice cover in the northern polar oceans is proposed to shift the global climate system from a growing land-glacier mode (glaciation) to a withdrawing glacier mode (deglaciation).

The land ice volume (upper panel) and sea ice area (lower) as function of month and time during the glacial cycle from a model of the coupled ocean, atmosphere, land ice and sea ice components of the climate system.


The Red Sea during the Last Glacial Maximum
 
The reconstruction of past sea level is crucial for understanding the mechanisms responsible for both glacial-interglacial cycles as well as climate variability on shorter time scales. Present estimates for the ice equivalent eustatic sea level reduction for the Last Glacial Maximum (LGM) interval range between  120 and 135 m below the present-day global sea level. The amount by which sea level is reduced at a given location will not normally be equal to the globally averaged reduction. Estimates of local relative sea level change can be employed to validate and refine models of the global variation of sea level, and can be used to distinguish between competing models. Encircled by arid land masses with low precipitation and undergoing one of the highest evaporation rates that have been recorded globally, the properties of the RS are largely controlled by the exchange flow through the Strait of Bab el Mandab and therefore are extremely sensitive to sea level reduction, as previously noted.

We have employed an Ocean General Circulation Model (OGCM), and recent developments in the theory of hydraulic control  which take into account the effects of mixing processes, to estimate the relative sea level reduction at the Strait of Bab el Mandeb which connects the Red Sea to the Indian Ocean. Our estimate is thus independent of reconstructions based on the theory of glacial-isostatic adjustment. The model salinity shows high sensitivity to sea level reduction with a mild atmospheric impact as well. Sea level reduction affects the stratification, and alters the circulation pattern at the Strait of Bab el Mandab, which experiences a transition from a submaximal flow to a maximal flow.  The best correlation to reconstructed conditions during LGM exists when the depth of the Hanish Sill (the shallowest part in the Strait Bab el Mandab) is 33±10.75 m. Thus, our model results suggest a relative sea level reduction of approximately 105 m, in agreement with the inference of the LGM low stand of the sea at the location of the sill based on the ICE-5G (VM2) model. Our model results also point to the need for an accurate estimate of mixing intensity in order to reconstruct sea level based on sediment records.