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Arctic Sea-Ice

Source:
The Oxford Companion to Global Change
Author(s):

Julienne Stroeve

Arctic Sea-Ice 

In the Northern Hemisphere, sea ice covers most of the Arctic Ocean and in winter can extend as far south as 40°N (e.g., Bo Hai Bay, China; Chesapeake Bay, United States).

Table 1. Hydrological and Salinity Characteristics of the Aral Sea, 1960–2025

Year (part of Aral Sea)

Level (m asl)

Area (km2)

% 1960 area

Volume (km3)

% 1960 Volume

Avg. Salinity (g/l)

% 1960 Salinity

1960 (Whole)a

53.4

67,499

100

1,089

100

10

100

Large

53.4

61,381

100

1,007

100

10

100

Small

53.4

6,118

100

82

100

10

100

1971 (Whole)

51.1

60,200

89

925

85

12

120

1976 (Whole)

48.3

55,700

83

763

70

14

140

1989 (Whole)

39,734

59

364

33

Large

39.1

36,930

60

341

34

30

300

Small

40.2

2,804

46

23

28

30

300

2007 (Whole)b

13,958

21

102

9

Large

29.4

10,700

17

75

8

East >100

>1000

West 75–85

750–850

Small

42.0

3,258

53

27

33

12?

120

2025 (Whole)b

9,658

14

68

6

Largec

21–28.3

6,400

10

41

4

>100 to >200

>1000 to >2000

Small

42.0

3,258

53

27

33

10?

100

aAnnual average. bAs of January 1. cThe sea will consist of a western and eastern part with the west basin at 21 meters and the east at 28. Compiled from a variety of Russian and English sources as well as data and information gathered by the author.

Remote sensing employing instruments in the microwave portion of the electromagnetic spectrum are well suited for studying changes in the Arctic sea-ice cover. These instruments are able to view the surface under cloud cover or at night, and because there is a large emissivity difference between ice and water, the presence of sea ice is easily distinguished from open water. Data from satellite-borne passive microwave instruments have been available since the late 1970s and provide the most accurate and consistent estimates of Arctic sea-ice cover.

Current Changes in Arctic Ice Cover

Both the extent and the age (and thickness) of the Arctic ice cover are affected by climate change.

Ice extent

Analysis of passive microwave-measured sea-ice extent (defined as the area of the ocean having at least 15% ice cover) reveals a rapid retreat in the extent of Arctic sea ice since the late 1970s. Based on data through 2006 the estimated rate of decline in annual ice extent is approximately 3.8% per decade (Figure 2). However, changes in the summer ice cover are much larger (e.g., Serreze et al., 2003; Stroeve et al., 2005). For example, Figure 4 shows the spatial extent of September sea ice in 2005, together with the median ice extent (dark gray line, based on data from 1979 to 2000). In 2005, the September ice cover was the smallest since 1979, with an average September ice extent of only 5.56 × 106 km2, compared to the median ice extent of 7.04 × 106 km2. Thus, in 2005 we saw a 21% reduction from normal in the area covered by sea ice. However, every year since 2002 has seen remarkable ice losses during the month of September, especially off the shores of Alaska and Siberia (e.g., Serreze et al., 2003; Stroeve et al., 2005). Before 2002, the decline in sea-ice extent at the end of the summer melt season was 6.5% per decade. In 2002 it jumped to 7.3% per decade and by 2006 it increased to 9.1% per decade (or at an ice loss rate of about 100,000 km2 per year) (Stroeve et al., 2007).

Arctic sea-ice extent in September 2007 surpassed all previous September records for the lowest absolute minimum extent. Sea-ice extent retreated significantly in the East Siberian side of the Arctic (e.g., the Chukchi and East Siberian seas) and also in the Beaufort Sea north of Alaska. Ice conditions were also anomalously low in the Canadian Archipelago, resulting in an opening of the Northwest Passage (NWP) during the third week of August 2007. The 2007 summer ice cover shrank dramatically in relation to the previous record minimum year.

Although changes in winter have not been as dramatic as those in summer, the last three years have also witnessed large changes in the winter ice cover (e.g., Meier et al., 2005), with ice extents 6–8% below the long-term mean (1979–2000). These recent reductions have been observed in both the Pacific and Atlantic sectors, and winter 2006–2007 set record lows in ice extent over the satellite record. We are now in an era when statistically significant (at the 99% confidence level) downward trends in Arctic sea-ice extent occur in all calendar months.

Ice age and thickness

There is growing evidence for accompanying thinning of the ice pack. Rothrock et al. (1999) noted reductions of more than 1 m in late-summer ice-draft (the submerged depth of floating ice) over much of the Central Arctic Ocean from the period 1953–1976 to the period 1993–1997. The sparse sampling of the submarine sonar data used for making this assessment complicates interpretation, but there is other supporting evidence for a thinning Arctic ice pack. Thickness of the Arctic ice pack can be reduced through enhanced melt, reduced ice growth, weaker ridging processes or export of sea ice from the Arctic Basin into the North Altantic. Changes in the thickness distribution of the Arctic ice pack may also occur via transport of ice into other regions of the Arctic Basin, leaving some areas thinner and others thicker. Ice that has survived through at least one summer (multiyear ice) is typically thicker than first-year ice, and ice that has survived several summers is assumed to be thicker than second-year ice. Using satellite data and drifting buoys from 1979 on, Fowler et al. (2004) were able to assess the formation, movement, and melt of the ice, which in turn was used to estimate ice age. Results from this study show that the area of oldest ice (i.e., ice older than 4 years) is decreasing in the Arctic Basin, and being replaced by younger, and therefore thinner, ice. The oldest (and thus thickest) ice is now confined to a relatively small area north of the Canadian Archipelago.

Future Arctic Ice Conditions?

Zhang and Walsh (2006) note that all models used in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) show declining ice over the period of observations. These IPCC AR4 model simulations (under the A1B emissions scenario) suggest a seasonally ice-free Arctic may occur as early as 2050. The subsequent study of Stroeve et al. (2007), however, found that over the period 1953–2006 (using a sea-ice record extended back in time with earlier satellite data and other sources, such as aircraft and ship observations), none of 13 IPCC AR4 models examined shows a summer ice-loss trend as large as is observed (Figure 4). Considered as a group, the models simulate a loss of 2.5% per decade, whereas the observations indicate a loss of 7.8% per decade. This suggests that the transition to an ice-free summer Arctic Ocean may occur much earlier than many of these models predict, perhaps well within the twenty-first century.

Holland et al. (2006) suggest the transition to a seasonally ice-free Arctic might occur rather abruptly. Simulations based on one of the IPCC AR4 climate models (the Community Climate System Model, version 3 [CCSM3]) indicate that if the ice thins to a vulnerable state, natural climate variability may provide the extra push that would cause rapid summer ice loss through the ice-albedo feedback. [See Albedo.] In these simulations, abrupt transitions to a nearly ice-free summer Arctic were seen to occur in as little as 10 years.

Explanations for the Observed Ice Losses

Almost all climate model simulations depict ice loss from the late twentieth through the twenty-first century as associated with rising greenhouse gas concentrations. However, the rates of decline vary widely between models. Rising surface air temperatures (SATs) help to explain the observed loss of ice. Comiso (2003) reports on an overall increase in SATs from satellite observations while Stroeve et al. (2006) note that the length of the melt season has increased by about two weeks per decade.

Variability in the atmospheric circulation has also played a strong role, especially through altering the circulation of the ice cover. Links with the North Atlantic Oscillation (NAO) and its hemispheric-scale counterpart, the Arctic Oscillation (AO), are especially prominent (e.g., Deser et al., 2000; Rigor et al., 2002; Zhang et al., 2003). [See North Atlantic Oscillation.] Rigor et al. (2002) showed that when the AO is positive in winter, altered wind patterns result in more offshore ice motion and ice divergence along the Siberian and Alaskan coastal areas, which leads to the production of thinner first-year ice in spring that requires less energy to melt in summer. Zhang et al. (2003) explained reduced summer ice cover during positive AO phases as a result of the spread of warmer air temperatures from Eurasia into the central Arctic. While this may have been the case in the 1990s, these hypotheses do not neatly explain the large ice losses that have occurred in the 2000s, which have seen the AO generally regress from its high positive phase to a more neutral phase.

There is evidence that the strongly positive phase of the winter AO in the early to mid 1990s led to the export of thick, multiyear ice out of the Arctic basin, leaving behind thinner ice that is more easily melted (Rigor and Wallace, 2004). Support for this view comes from tracking of ice age in the Arctic using satellite data that suggest the ice in the Arctic is younger now than it was in the 1980s (Fowler et al., 2004). However, Rigor and Wallace (2004) estimate that combined winter and summer AO-indices can explain less than 20% of the variance in summer sea-ice extent in the western Arctic, where most of the recent reduction in sea-ice cover has occurred. Maslanik et al. (2007) argue that atmospheric circulation patterns other than the AO have contributed to this variance. A separate analysis of the combined effect of winds, radiative fluxes, and advected heat shows that atmospheric forcing can account for about half of the total variance in summer sea-ice extent in the western Arctic (Francis et al., 2005).

There are also potential impacts of changes in ocean heat transport. Maslowski et al. (2004) explored the role of oceanic forcing on sea-ice cover in the East Greenland Sea and found that oceanic heat fluxes associated with Atlantic water recirculating near Fram Strait can explain over 60% of the total variance in the summer ice cover over the Greenland shelf on an annual basis. There are recent observations showing warming of the Atlantic water flowing north into the Arctic through the eastern Fram Strait and the Barents Sea (Walczowski and Piechura, 2006), and showing changes in the transport of warm Pacific water that enters the Arctic Ocean through the Bering Strait (e.g., Shimada et al., 2006). The study by Holland et al., (2006) showed that a change in ocean heat transport could provide the trigger to initiate an abrupt loss of summer ice cover.

In the summer of 2007, a combination of high summer temperatures, clear skies, thinner ice and warming ocean temperatures contributed to the record ice losses. Air temperatures in June and July showed strong positive temperature anomalies over much of the Arctic Ocean, accelerating ice melt, with the largest temperature anomalies (3–5°C) occurring on the Siberian side where the largest ice losses were observed. However, at the same time, high pressure dominated the central Arctic Ocean, promoting very sunny conditions just at the time when the sun was highest in the sky over the far north. This led to unusually strong solar heating of the Arctic ice surface, further accelerating the melting process. Over Siberia, low pressure dominated during the same time period, which, when combined with the high pressure over the central Arctic Ocean, generated strong southerly winds over coastal Siberia that not only brought warm air into the region but also acted to push ice away from the coast and into the central Arctic Ocean, further reducing ice extent in the coastal areas. These atmospheric conditions, combined with an already thinner Arctic ice pack and increasingly warmer ocean temperatures were enough to result in record-breaking daily rates of sea-ice loss in June, July, and August 2007.

Outlook

The Arctic is currently experiencing a dramatic reduction in its sea-ice cover, not only in its spatial extent, but also apparently in its volume. However, the causes of Arctic sea-ice decline are still not completely understood. Natural variability such as that associated with the AO and other circulation patterns will certainly continue to impact the Arctic sea-ice cover. However, results from the recent study by Stroeve et al. (2007) suggest the current summer ice losses are now too strong and persistent to be explained solely through natural processes and that the effects of greenhouse-gas loading are starting to emerge. It could also be that oceanic thermodynamic control of sea ice through under-ice ablation and lateral melt along marginal ice zones is playing a larger role than previously suspected and may be one of the reasons that current climate models underestimate how quickly the Arctic is currently losing its summer ice cover. Nevertheless, given the basic agreement between models and observations regarding the trajectory of ice loss, transition to a seasonally ice-free Arctic Ocean in summer seems increasingly certain.

Bibliography

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                                          Julienne Stroeve

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