The major late Quaternary ice sheets are the Laurentide (LIS), Cordilleran (CIS), Scandinavian (SIS), Barents-Kara (BKIS), Greenland (GIS) and East (EAIS) and West (WAIS) Antarctic Ice Sheets (Figure 1), which can be divided into predominately terrestrial and marine-based ice sheets. Terrestrial ice sheets, such as the LIS, CIS, SIS, GIS, and EAIS, have their margins and ice-base resting and terminating on land. That said, portions of these ice sheets, and the entire GIS and EAIS margins, did terminate in the ocean. Marine-based ice sheets like the BKIS and WAIS have much of their ice-base resting on ground below sea level, with their ice margin terminating in the ocean. This type of ice-sheet may be unstable because a rise in sea level, or thinning of the ice sheet, could trigger a destabilization where parts of the ice sheet could float, break up and rapidly retreat, which is often referred to as a collapse.

The combined behavior of global ice volume is recorded by fluctuations in sea level. The rise and fall of sea-level can be reconstructed by radiometrically dating corals who's habitat is near the sea surface and using their present elevations above or below current sea level as indicators of past sea-level fluctuations (Figure 2d) (e.g., Lambeck & Chappell 2001). Another method is to document changes in the salinity of evaporative basins only marginally connected to the open ocean, such as the Red Sea. A fall in sea level increases the influence of evaporation on the basin and the isotope ratio of the heavy 18O to light 16O increases in surface dwelling (planktonic) foraminifera shells (Figure 2d), because the lighter 16O is preferentially evaporated and not replenished by the reduced in low of ocean water (Siddall et al. 2003, Rohling et al. 2009). A third means of reconstructing sea level-usually applicable to time scales longer than discussed here-is the use of the 18O to 16O relationship in bottom dwelling (benthic) foraminifera (amoeboid animals) shells (e.g., Hays et al. 1976, Imbrie et al. 1993). Due to preferential evaporation of lighter 16O from the global ocean, this isotope accumulates in ice sheets and thus the ratio of 18O to 16O increases in the ocean during periods of ice growth. Similarly, the retreat of ice releases the sequestered 16O back to the ocean, reducing the 18O/16O ratio. This ratio is commonly expressed against a known standard of 18O/16O and denote using the symbol δ (i.e., δ18O).

The history of individual ice sheets can be determined directly by dating ice margin retreat and ice surface thinning, and indirectly from inferences of ice sheet runoff to the ocean. Radiocarbon (the radioactive isotope of C; 14C) dates on organic matter or calcite shells indicate the absence of ice, and with a significant number of ages bracketing glacial deposits, the advance and retreat of ice can be reconstructed (e.g., Landvik et al. 1998, Dyke 2004, Clark et al. 2009). Ice-sheet retreat can be directly dated using cosmogenic isotope ages of boulders deposited by the ice. Retreat exposes these boulders to the bombardment of cosmic radiation from outer space (fast protons, neutrons, and electrons) that impact upon elements in the rock's crystals, splitting or attaching to the atoms to make new isotopes (i.e., 3He, 10Be, 14C, 21Ne, 26Al, 36Cl) (Gosse & Phillips 2001). With a known production rate of these isotopes, one can calculate how long a boulder has been exposed to this bombardment by measuring the concentration of the isotope in the boulder surface. The melting of ice during retreat increases the discharge of 16O water to the ocean where the δ18O will decrease in planktonic foraminifera, allowing inference of greater ice retreat (Jones & Keigwin 1988). The runoff from ice-sheet melting will also transport more sediment to the ocean, which can be tracked by analyzing marine sediments for the abundance of terrestrially-derived elements such as Ti and Fe (Carlson et al. 2008b).

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The retreat of ice sheets can be compared with ice core O and H isotope records of the ice (HO). The δO of precipitation decreases as atmospheric temperatures cool because the colder air causes more of theO to be precipitated out of the cloud, leaving behind a lowerO toO ratio that is recorded in the ice core. Warming causes the opposite effect. TheH isotope called Deuterium behaves in a similar manner with the more commonH isotope. Their ratio relative to a standard is denoted as δD. With a known linear relationship between δO and δD (e.g., Cuffey1995, Jouzel2007), ice core records can be converted into changes in temperature.