History of sea ice in the Arctic
2010, Polyak et al., Quaternary Science Reviews
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Abstract
Arctic sea-ice extent and volume are declining rapidly. Several studies project that the Arctic Ocean may become seasonally ice-free by the year 2040 or even earlier. Putting this into perspective requires infor- mation on the history of Arctic sea-ice conditions through the geologic past. This information can be provided by proxy records from the Arctic Ocean floor and from the surrounding coasts. Although existing records are far from complete, they indicate that sea ice became a feature of the Arctic by 47 Ma, following a pronounced decline in atmospheric pCO2 after the Paleocene–Eocene Thermal Optimum, and consistently covered at least part of the Arctic Ocean for no less than the last 13–14 million years. Ice was apparently most widespread during the last 2–3 million years, in accordance with Earth’s overall cooler climate. Nevertheless, episodes of considerably reduced sea ice or even seasonally ice-free conditions occurred during warmer periods linked to orbital variations. The last low-ice event related to orbital forcing (high insolation) was in the early Holocene, after which the northern high latitudes cooled overall, with some superimposed shorter-term (multidecadal to millennial-scale) and lower-magnitude variability. The current reduction in Arctic ice cover started in the late 19th century, consistent with the rapidly warming climate, and became very pronounced over the last three decades. This ice loss appears to be unmatched over at least the last few thousand years and unexplainable by any of the known natural variabilities.
Excerpts
The thickness of sea ice, which varies markedly in both space and time, can be described by a probability distribution. For the Arctic Ocean as a whole, the peak of this distribution has been typically cited at about 3 m (Williams et al., 1975; Wadhams, 1980), but there is growing evidence (discussed below) that shrinking ice extent over recent decades has been attended by substantial thinning.
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Undeformed first-year ice can reach as much as 1.5–2 m in thickness. Although multi-year ice is generally thicker, first-year ice that undergoes convergence and/or shear can produce ridges as thick as 20–30 m.
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About 20% of the total ice area of the Arctic Ocean and nearly all of the annual ice export is discharged each year through Fram Strait, the majority of which is multi-year ice.
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Patterns of ice-margin retreat may differ between different periods and regions of the Arctic, but the overall retreat trend is clearly larger than decadal-scale variability, consistent with observations and modeling of the 20th-century ice concentrations and water temperatures (Polyakov et al., 2005; Kauker et al., 2008; Steele et al., 2008). The severity of present ice loss can be highlighted by the breakup of ice shelves at the northern coast of Ellesmere Island (Mueller et al., 2008), which have been stable until recently for at least several thousand years based on geological data (England et al., 2008).
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Strong evidence for a thinning ice cover comes from an ice-tracking algorithm applied to satellite and buoy data, which suggests that the amount of the oldest and thickest ice within the multi-year pack has declined significantly (Maslanik et al., 2007b). The area of the Arctic Ocean covered by predominantly older ice (5 or more years old) decreased by 56% between 1982 and 2007. Within the central Arctic Ocean, the coverage of old ice has declined by 88%, and ice that is at least 9 years old (ice that tends to be sequestered in the Beaufort Gyre) has essentially disappeared. Examination of the distribution of ice of various thicknesses suggests that this loss of older ice translates to a decrease in mean thickness for the Arctic from 2.6 m in March 1987–2.0 m in 2007 (Maslanik et al., 2007b).
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Recent modeling studies have discussed the possibility of rapid change in future Arctic summer ice conditions. Simulations based on the Community Climate System Model, version 3 (CCSM3) (Holland et al., 2006a) indicate that the end-of-summer ice extent is sensitive to ice thickness in spring. If the ice thins to a more vulnerable state, a "kick" associated with natural climate variability can result in rapid summer ice loss enhanced by the ice-albedo feedback. In the CCSM3 events, anomalous ocean heat transport acts as this trigger.
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Models consistently simulate amplification of Arctic surface warming in response to rising CO2 levels (e.g., Manabe and Stouffer, 1980; Holland and Bitz, 2003). While a number of processes cause
this Arctic amplification, much of the signal can be attributed to the loss of sea ice and surface albedo change.
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It is important to understand that although all of the above proxies have a potential for identifying the former presence or the seasonal duration of sea ice, each has limitations that complicate interpretations based on a single proxy. Identification of the sea-ice signal can be obscured by other hydrographic controls such as temperature and salinity, as well as nutrient-related changes in biological productivity. For instance, by use of a dinocyst transfer function from East Greenland, it was estimated that the sea-ice duration is about 2–3 months (Solignac et al., 2006), when in reality it is closer to 9 months (Hastings, 1960). A multi-proxy approach is desirable for making confident inferences about sea-ice variations. A thorough understanding of sea-ice history depends on refining sea-ice proxies in sediment taken from strategically selected sites in the Arctic Ocean and along its continental margins.
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