Antarctic Sea Ice

Guest post by Dr Jonathan Day, Department of Meteorology, University of Reading

What is going on with the Antarctic sea ice?

March 2017 was an interesting month for sea ice. Both northern and southern hemispheres experienced record breaking low extents for the time of year. The extent of Arctic sea ice reached the maximum area of its seasonal cycle on March 7th coming in at 14.42 million km2. This was a fraction below the previous record, set in 2015 and is in line with what we expect to see in a warming climate. Meanwhile the other side of the planet Antarctic sea ice continues to confound expectations. Continue reading

Increasing Antarctic Sea Ice

Characteristics of Antarctic sea ice | Increasing Antarctic sea ice | Wind and movement | Changes to sea surface temperatures | Increased precipitation | Difficulty in measurement | Climate model simulations of sea-ice trends | Summary | References | Comments |

Characteristics of Antarctic sea ice

The Antarctic continent is surrounded by seasonal, floating sea ice. This sea ice, which comprises mainly frozen sea water, with occasional icebergs from glaciers and ice shelves, covers a minimum of 3×106 km2 in February to a maximum of 18×106 km2 in September. This effectively doubles the size of Antarctica in the winter. Most of the summer sea ice stays in the Weddell Sea, where it is relatively protected from the ocean currents.

Map of seasonal sea ice extents in the Arctic and Antarctic. From the NSIDC.

Map of seasonal sea ice extents in the Arctic and Antarctic. From the NSIDC.

The Arctic regularly reaches ever smaller extents of end-of-summer minimum extents of sea ice. This changing sea ice extent is cited by the IPCC as an indicator of a warming world. However, sea ice extent is growing in Antarctica [1]. In fact, it’s recently broken a record for maximum extent.

However, aside from the fact that many people confuse land ice, sea ice and ice shelves, it’s important to note that there are huge differences between the Arctic and the Antarctic. The Arctic is an ocean surrounded by land. The Antarctic is land, covered by ice, surrounded by ocean. Sea ice in the Arctic is generally thick, multi-year sea ice that survives several seasons, whereas the sea ice in Antarctica largely melts away each summer. Antarctic sea ice is mostly thin (~0.6 m thick [2]), single-year sea ice. It’s also warmer, more saline and more mobile than Arctic sea ice [3].

Changes in sea ice extent in the Arctic and Antarctic. From Vaughan et al., 2013.

Changes in sea ice extent in the Arctic and Antarctic. From Vaughan et al., 2013. Glaciers are highlighted in yellow. The yellow line indicates the sea ice winter maximum extent (30 year average).

Clearly, these two different regions will have very different responses to climate and oceanic change, and these differences will affect sea ice response. And it’s also important to remember that, while sea ice is increasing in Antarctica, glaciers and ice shelves are all melting rapidly, producing large volumes of fresh water.

Increasing Antarctic sea ice

The figure below (from Ref. [4]) shows the total variability of Antarctic sea ice extent over the last 34 years. Decadal monthly averages almost overlap, and there is little change in seasonal variability.

The trend maps in the figure below show changes in sea ice extent in winter, spring, summer and autumn. The trends are mostly significant near the ice edge. Positive trends are evident in the Ross Sea, with autumn and summer negative trends mainly confined to the Bellingshausen and Amundsen seas. These trends are showing very small increases in total sea ice area and extent; the trend magnitude is approximately one third as large as the trend in decreases in the Arctic[5]. Further, the increases are strongly controlled by region (some regions are showing ever smaller sea ice extents); the Ross Sea has shown the greatest increase in sea ice extent in Antarctica.

(a) Plots of decadal averages of daily sea ice extent in the Antarctic (1979–1988 in red, 1989–1998 in blue, 1999– 2008 in gold) and a 4-year average daily ice extent from 2009 to 2012 in black. Maps indicate ice concentration trends (1979–2012) in (b) winter, (c) spring, (d) summer and (e) autumn (updated from Comiso, 2010). From Vaughan et al., 2013.

(a) Plots of decadal averages of daily sea ice extent in the Antarctic
(1979–1988 in red, 1989–1998 in blue, 1999– 2008 in gold) and a 4-year average
daily ice extent from 2009 to 2012 in black. Maps indicate ice concentration trends
(1979–2012) in (b) winter, (c) spring, (d) summer and (e) autumn (updated from
Comiso, 2010). From Vaughan et al., 2013.

The increase in total Antarctic sea ice extent 1978-2012 is slightly positive at 1.5 ± 0.3% per decade[4]. On a seasonal basis, the trends in ice extent and ice area per decade are:

Season Sea ice maximum extent Sea ice-covered area
Winter 1.2 ± 0.5% 1.9 ± 0.7%
Spring 1.0 ± 0.5% 1.6 ± 0.5%
Summer 2.5 ± 2.0% 3.0 ± 2.1%
Autumn 3.0 ± 2.0% 4.4 ± 2.3%

So, the largest trends in sea ice extent and in ice-covered area are in autumn. The trends are higher for ice area than for ice extent, indicating less open water, which may be related to changes in ice drift and wind patterns.

While changes in Antarctic sea ice extent remains an exciting topic for further research, there are a number of reasons put forward that explain these trends.

Wind and movement

Changes in atmospheric dynamics and winds are an important driver of regional sea-ice trends. Ozone and greenhouse forcings cool the Antarctic stratosphere, which increases the stratospheric vortex and tropospheric zonal winds. This results in an increase in the Southern Annular Mode [6]. Increases in the Southern Annular Mode (SAM) signify increased westerly winds [7] and a rigorous isolation and cooling of parts of the Antarctic continent [6].

Because the Arctic is a semi-enclosed ocean, there is little scope for sea ice movement. Ice in the Arctic is thicker as a result of collisions, which means that the ice will last longer. This means that much of the Arctic sea ice lasts for several seasons, leading to permanent ice cover at the pole. However, in the Antarctic, there are far fewer such constraints. The sea ice is able to move around far more freely. It floats northwards to warmer waters, where it melts away almost entirely. Changes in the winds around Antarctica therefore change ice-concentration trends around Antarctica [8] by influencing sea-ice production and melt rates [9]. The pattern of wind change is complex, but variations in winds can help to explain some of the regional patterns in sea-ice formation [8]. Where the wind blows to the north, the sea ice is blown north where it melts, resulting in increased sea-ice extent. Where the winds blow south, the sea ice is blown towards the continent, resulting in decreased sea-ice concentrations.

Polynyas are areas of persistently open water in regions where sea ice is usual. The water remains unfrozen as a result of processes that either prevent ice from forming or that move ice out of the area. Polynyas are therefore an important part of sea-ice production. An increase in the extent of polynyas in the Ross Sea from 1978 to 2008 contributed to sea ice production [2]. The resulting increased ice export accounts for a large proportion of the increased trend in ice production. Changes in wind circulation alter ice production and export in and from these polynyas.

Changes to sea surface temperatures

As the glaciers and ice shelves melt on the Antarctic continent, freshwater is added to the oceans. This layer of cold, fresh water on the ocean surface freezes easily [10]. When combined with increased ocean stratification due to this enhanced run off [11], sea-surface temperatures are depressed, encouraging sea-ice formation.

A recent modelling study has shown that increases in fresh meltwater flux from melting glaciers and ice caps on Antarctica under various IPCC standardised global warming scenarios offsets the decline in sea-ice area and to even further encourage the increases in sea-ice extent, especially in winter (in summer, air temperatures are too high to support significant sea-ice growth) [12].

Increased stratification has further implications. Suppression of ocean circulation overturning decreases the ocean heat flux available to melt ice, leading to an increase in net ice production[13].

Increased precipitation

Warmer air holds more moisture, and so precipitation is increasing around Antarctica [13]. Strong warming in the middle latitudes of the Southern Ocean can lead to an enhanced hydrological cycle, with enhanced evaporation and moisture content in the lower troposphere [14]. This additional moisture is transported poleward, where it results in increased precipitation. Increases in snow and rain falling onto the ocean contribute to the freshening of the ocean surface in the high latitudes of the Southern Ocean. Fresher, colder water freezes more easily, so this mechanism may contribute to the growth in area of Antarctic sea ice.

Furthermore, the increased weight of snow on the sea ice may force it deeper into the water, forming thicker sea ice when the snow refreezes. Deeper snow also insulates the ice, protecting it from melting [15].

Difficulty in measurement

Sea ice is measured by repeated images taken by satellites orbiting the Earth [16]. Passive Microwave Sensors were developed in the late 1960s, with the ability to measure sea-ice edge, surface composition and soil moisture. These measurements have been taken with approximately a daily resolution from the late 1970s onwards.

The microwave emissivity of sea ice is higher than the ocean, which means that ice-covered areas have a higher brightness temperature than the ocean [5]. However, warmer surfaces also have a high brightness temperature, so it is difficult to distinguish between cold sea ice and a warm ice-free ocean. Scientists therefore use simultaneous measurements at multiple frequencies and polarisations (the difference in emissivity between sea ice and ocean varies with frequency and polarisation) [5]. Other factors complicating the measurement of sea ice include weather interference, cloud, thin ice, and so on.

Two algorithms for estimating sea ice extent from these measurements were developed in the 1980s. The ‘Bootstrap’ algorithm is one of the most widely used ice-concentration products, and forms the basis of the observations of sea ice made in the IPCC reports.

A change in the inter-calibration across two different sensors on successive satellites caused a substantial change in the long-term trend in sea-ice extent[5]. There is apparently an error in either the current dataset or the one used prior to the mid-2000s. The authors of this particular study suggest that observations should be re-examined to determine the sensitivity of observations to this change in the dataset.

Climate model simulations of sea-ice trends

Climate models simulate a decline in ice extent, thickness and volume in Antarctica. Equilibrium models cannot currently reproduce trends in Antarctic sea ice variability [17]. Virtually all equilibrium climate models simulate a strong decrease in the area of sea ice [18]. This may be because global climate models do not currently incorporate ice-shelf / -sheet/ -climate interactions. Basal melt from ice shelves is therefore disregarded. These equilbruim models may give an idea of what may eventually happen. Simulations with models that do include these interactions, particularly simulating the effect of extra freshwater from melting glaciers and ice caps, do simulate growths in Antarctic sea ice [10, 12].

Transient climate models are more able to capture the transient response of sea ice to changes in the winds. A modelling study by Marshall et al. [19] showed that changes in the winds tend to push the ice edge northwards, increasing ice extent. These winds also push the ocean surface northwards too, which effectively brings warmer water to the surface and eventually counters the increasing sea ice trend after a few decades. These transient models show that not enough time has elapsed for the equilibrium response to be achieved. We may well see the trends reversing in a few decades.

Summary

Antarctica is a unique environment, and the complex interactions between ice, ocean and atmosphere have led to a unique set of circumstances that have resulted in sea ice growth. It may be explained by many factors, or most probably by a combination of several. Climate change is a complex process governed by multiple feedbacks between different parts of the system; complex interactions between the melting land ice and ice shelves fringing the continent and changes in wind stress are all implicated in controlling Antarctic sea ice extent. Further, more work is required to ascertain the reliability of observations of sea ice increase given the recent discovery of an error in the algorithm used to quantify and map sea ice over the last few decades.

Further reading

References

  1. Turner, J. and J. Overland, Contrasting climate change in the two polar regions. Polar Research, 2009. 28(2): p. 146-164.
  2. Comiso, J.C., et al., Variability and trends in sea ice extent and ice production in the Ross Sea. Journal of Geophysical Research: Oceans, 2011. 116(C4): p. C04021.
  3. Wadhams, P. and J.C. Comiso, The Ice Thickness Distribution Inferred Using Remote Sensing Techniques, in Microwave Remote Sensing of Sea Ice. 2013, American Geophysical Union. p. 375-383.
  4. Vaughan, D.G., et al., Observations: Cryosphere, in Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, T.F. Stocker, et al., Editors. 2013, Cambridge University Press: Cambridge, UK. p. 317-382.
  5. Eisenman, I., W.N. Meier, and J.R. Norris, A spurious jump in the satellite record: has Antarctic sea ice expansion been overestimated? The Cryosphere, 2014. 8(4): p. 1289-1296.
  6. Thompson, D.W. and S. Solomon, Interpretation of recent Southern Hemisphere climate change. Science, 2002. 296(5569): p. 895-899.
  7. Spence, P., et al., Rapid subsurface warming and circulation changes of Antarctic coastal waters by poleward shifting winds. Geophysical Research Letters, 2014. 41(13): p. 2014GL060613.
  8. Holland, P.R. and R. Kwok, Wind-driven trends in Antarctic sea-ice drift. Nature Geosci, 2012. 5(12): p. 872-875.
  9. Goosse, H., et al., Consistent past half-century trends in the atmosphere, the sea ice and the ocean at high southern latitudes. Climate Dynamics, 2009. 33(7-8): p. 999-1016.
  10. Bintanja, R., et al., Important role for ocean warming and increased ice-shelf melt in Antarctic sea-ice expansion. Nature Geosci, 2013. advance online publication.
  11. Swingedouw, D., et al., Antarctic ice-sheet melting provides negative feedbacks on future climate warming. Geophysical Research Letters, 2008. 35(17): p. L17705.
  12. Bintanja, R., G. van Oldenborgh, and C. Katsman, The effect of increased fresh water from Antarctic ice shelves on future trends in Antarctic sea ice. Annals of Glaciology, 2015. 56: p. 69.
  13. Zhang, J., Increasing Antarctic Sea Ice under Warming Atmospheric and Oceanic Conditions. Journal of Climate, 2007. 20(11): p. 2515-2529.
  14. Liu, J. and J.A. Curry, Accelerated warming of the Southern Ocean and its impacts on the hydrological cycle and sea ice. Proceedings of the National Academy of Sciences, 2010. 107(34): p. 14987-14992.
  15. Powell, D.C., Markus, T., Stössel, A., 2005. Effects of snow depth forcing on Southern Ocean sea ice simulations. Journal of Geophysical Research: Oceans 110, C06001.
  16. Teleti, P.R. and A.J. Luis, Sea Ice Observations in Polar Regions: Evolution of Technologies in Remote Sensing. International Journal of Geosciences, 2013. 4: p. 1031-1050.
  17. Holland, P.R., The seasonality of Antarctic sea ice trends. Geophysical Research Letters, 2014.
  18. Collins, M., et al., Long-term climate change: projections, commitments and irreversibility, in Climate change 2013: the physical science basis, T.F. Stocker, et al., Editors. 2013, Cambridge University Press: Cambridge. p. 1029-1136.
  19. Marshall, J., Scott, J., Armour, K., Campin, J.M., Kelley, M., Romanou, A., 2014. The ocean’s role in the transient response of climate to abrupt greenhouse gas forcing. Climate Dynamics, 1-13.

Ice-Ocean Interactions

99% of Antarctica is ice-covered, and so most of the glaciers and ice streams here end in the ocean. It is in these oceanic margins that we see the most rapid changes: ice stream acceleration, thinning and grounding line migration. At the ice-ocean interface, the ice sheet is vulnerable to melting from below as well as from above, as warm ocean currents penetrate the continental shelf and melt the ice sheets at their grounding line.

This section of the website contains lots of information about how ice sheets interact with the ocean. For more information, please do see:

 

 

Ice shelves, icebergs and sea ice

Ice shelves | Icebergs | Sea ice | Further reading | References | Comments |

Ice shelves

Landsat Image Mosaic of Antarctica (LIMA) showing location of key ice shelves.

Landsat Image Mosaic of Antarctica (LIMA) showing location of key ice shelves.

An ice shelf is a floating extension of land ice. The Antarctic continent is surrounded by ice shelves. They cover >1.561 million km2 (an area the size of Greenland)[1], fringing 75% of Antarctica’s coastline, covering 11% of its total area and receiving 20% of its snow.

The difference between sea ice and ice shelves is that sea ice is free-floating; the sea freezes and unfreezes each year, whereas ice shelves are firmly attached to the land. Sea ice contains icebergs, thin sea ice and thicker multi-year sea ice (frozen sea water that has survived several summer melt seasons, getting thicker as more ice is added each winter).

In the photographs below, you can see the flat, floating ice shelf is almost featureless. The ice flows from the mainland into the sea, and when it becomes deep enough it floats.

Ice shelf flow

Simplified cartoon of a tributary glacier feeding into an ice shelf, showing the grounding line (where the glacier begins to float).

Simplified cartoon of a tributary glacier feeding into an ice shelf, showing the grounding line (where the glacier begins to float).

Ice shelves receive ice in several ways: flow of ice from the continent, surface accumulation (snow fall) and the freezing of marine ice to their undersides. Ice shelves lose ice by melting from below (from relatively warm ocean currents), melting above (from warm air temperatures) and from calving icebergs.  This is a normal part of their ablation.

Ice shelves can be up to 2000 m thick, with a cliff edge that’s up to 100 m high. They often show flow structures on their surface – a relic of structures formed on land[2-4].

Receding ice shelves

Prince Gustav Ice Shelf in 1988. It collapsed in 1995, and the glaciers which flowed into it subsequently accelerated and thinned, transmitting lots of ice into the ocean and resulting in measureable sea level rise.

Prince Gustav Ice Shelf in 1988. It collapsed in 1995, and the glaciers which flowed into it subsequently accelerated and thinned, transmitting lots of ice into the ocean and resulting in measureable sea level rise.

Ice shelves around the Antarctic Peninsula are retreating[5]. These ice shelves are warmed from below by changing ocean currents, thinning them and making them vulnerable. During warm summers, ice shelves calve large icebergs – and in some cases, can catastrophically collapse.

Icebergs

Bergy bits washed up at the high tide mark on James Ross Island

Bergy bits washed up at the high tide mark on James Ross Island

Icebergs are floating all around Antarctica. They calve off from tidewater glaciers or ice shelves. They can range in size from small chunks you could fit into a gin and tonic to huge floating behemoths that take decades to melt and that you can land a helicopter on. 90% of the mass of an iceberg is underwater. Small chunks of ice are called ‘bergy bits’, larger ones (fridge-sized) are called ‘growlers’, and chunks of ice greater than 5 m across are called ‘icebergs’. Ships navigating in polar waters must be careful to avoid icebergs and growlers, which can be hard to see, and will use radar to scan ahead, particularly in poor visibility or in the dark.

If you’re looking for ice to add to your drink, choose a bergy bit made from coarse clear crystal ice. See-through ice chunks are made from compressed glacier basal ice and are clean and pure enough to drink. The compressed air present in the ice bubbles away as it melts, making for the best G&T you ever had.

All shapes and sizes

Icebergs can have many colours. Blue icebergs are formed from basal ice from a glacier. The compressed crystals have a blue tint. Green and red icebergs are coloured by algae that lives on the ice. Stripy icebergs are coloured by basal dirt and rocks, ground up by the glacier and carried away within the glacier ice. Crevasses and other glacier structures may be preserved, giving yet more texture and beauty to the iceberg.

Iceberg tracks from 1999-2010, from The Antarctic Iceberg Tracking Database

Iceberg tracks from 1999-2010, from The Antarctic Iceberg Tracking Database

Icebergs are studied for a number of reasons. They are tracked with satellite images as they travel around the Southern Ocean. As they drift away from the Antarctic continent, they deliver cold, fresh water, dust and minerals to the surface ocean. The iceberg also may drag its keel on the continental shelf. Each of these processes has impacts for surface and deep-water animals[6]. The surface phytoplankton increases by up to one third in the wake of a large iceberg.

Tracking icebergs provides information on ocean currents. Scientists can assess whether the number of icebergs is increasing[7, 8]. The input of freshwater may affect surface water currents and even sea ice formation[9].

Sea ice

This image of Antarctic sea ice is from the NASA Scientific Visualisation Studio, showing the Earth on September 21st 2005. Source: Wikimedia Commons.

This image of Antarctic sea ice is from the NASA Scientific Visualisation Studio, showing the Earth on September 21st 2005. Source: Wikimedia Commons.

Sea ice surrounds the polar regions. On average, sea ice covers up to 25 million km2, an area 2.5 times the size of Canada. Sea ice is frozen ocean water. The sea freezes each winter around Antarctica.

Sea ice can modify climate change’s impact on terrestrial ice because it is highly reflective and because it has a strongly insulating nature. Each year, the extent of sea ice varies according to climate variability and long-term climate change.

In the Arctic, sea ice extent is steadily decreasing, with a trend of -5.3±00.6% per decade since 1985[10], as a result of long-term climate change. Year-on-year variations reflect normal variability. Because removal of sea ice changes the reflectivity of the Arctic, a diminishing sea-ice extent amplifies warming.

Frozen winter sea ice trapping calved icebergs from the margin of a tidewater glacier

Frozen winter sea ice trapping calved icebergs from the margin of a tidewater glacier

Sea ice in the Antarctic is currently increasing[9]. This is associated with cooling sea surface temperatures in the Southern Ocean, in particular near the Ross Ice Shelf. Causes of this increasing Antarctic sea ice, which are contrasted with shrinking glaciers and ice shelves and warming deeper ocean current temperatures and atmospheric air temperatures, include changes to the Southern Annual Mode due to intensification and migration of the predominant Southern Ocean Westerlies, and cooler sea surface temperatures as a result of increased glacier and ice-shelf melting[9].

Further reading

References


1.            Rignot, E., S. Jacobs, J. Mouginot, and B. Scheuchl, 2013. Ice Shelf Melting Around Antarctica. Science.

2.            Glasser, N.F., B. Kulessa, A. Luckman, D. Jansen, E.C. King, P.R. Sammonds, T.A. Scambos, and K.C. Jezek, 2009. Surface structure and stability of the Larsen C Ice Shelf, Antarctic Peninsula. Journal of Glaciology, 55(191): 400-410.

3.            Glasser, N.F. and G.H. Gudmundsson, 2012. Longitudinal surface structures (flowstripes) on Antarctic glaciers. The Cryosphere, 6: 383-391.

4.            Glasser, N.F., T.A. Scambos, J.A. Bohlander, M. Truffer, E.C. Pettit, and B.J. Davies, 2011. From ice-shelf tributary to tidewater glacier: continued rapid glacier recession, acceleration and thinning of Röhss Glacier following the 1995 collapse of the Prince Gustav Ice Shelf on the Antarctic Peninsula. Journal of Glaciology, 57(203): 397-406.

5.            Cook, A.J. and D.G. Vaughan, 2010. Overview of areal changes of the ice shelves on the Antarctic Peninsula over the past 50 years. The Cryosphere, 4(1): 77-98.

6.            Schwarz, J.N. and M.P. Schodlok, 2009. Impact of drifting icebergs on surface phytoplankton biomass in the Southern Ocean: Ocean colour remote sensing and in situ iceberg tracking. Deep Sea Research Part I: Oceanographic Research Papers, 56(10): 1727-1741.

7.            Long, D.G., J. Ballantyn, and C. Bertoia, 2002. Is the number of Antarctic icebergs really increasing? Eos, Transactions American Geophysical Union, 83(42): 469-474.

8.            Ballantyne, J. and D.G. Long. A multidecadal study of the number of Antarctic icebergs using scatterometer data. in Geoscience and Remote Sensing Symposium, 2002. IGARSS ’02. 2002 IEEE International. 2002.

9.            Bintanja, R., G.J. van Oldenborgh, S.S. Drijfhout, B. Wouters, and C.A. Katsman, 2013. Important role for ocean warming and increased ice-shelf melt in Antarctic sea-ice expansion. Nature Geosci, advance online publication.

10.          Kwok, R. and D. Rothrock, 2009. Decline in Arctic sea ice thickness from submarine and ICESat records: 1958–2008. Geophysical Research Letters, 36(15).

Antarctic ice shelves – the hidden villain

Sea ice and ice shelves

What is sea ice? Sea ice is frozen sea water; it perennially expands and contracts during each year’s winter and summer. Amongst the sea ice are icebergs calved from tidewater glaciers and ice shelves. Melting sea ice does not contribute directly to sea level rise (ice floats and displaces the same volume of water), but sea ice is important because it enhances climate warming. It changes the reflectivity of the sea water, reflecting lots of sunlight back (it has a high albedo), and is therefore an important component of the climate and cryospheric (icey) system.
Continue reading

Arctic Sea Ice

The Arctic’s sea ice extent reached an all-time low in September 2012, with the smallest recorded extent since satellite observations began. At 3.42 million square kilometres, it may still sound large, but this small extent of Arctic sea ice could have profound long-term consequences, and it follows a long trend of low sea ice conditions. Sea ice extent has been decreasing over the past 4-5 decades (Kinnard et al., 2011), and sea ice extent is now about 2 million square kilometres less than it was during the late twentieth century. Continue reading