Southern Annular Mode

Today, glaciers in Patagonia and Antarctica are receding. In both cases, this has been attributed to changes in the Southern Westerly Winds and the Southern Annular Mode (SAM). The Southern Westerly Winds are an important wind belt that encircles the globe in the southern mid-latitudes1,2, with its core between 50-55°S1.

The Southern Annular Mode describes the north-south movement of this wind belt over a timescale of decades to centuries. The Southern Annular Mode is a key climatic component that will strongly affect how glaciers in the Southern Hemisphere respond to climate change. It explains the key drivers for glaciation in the Southern Hemisphere, and why glacier advances are asynchronous with those in the Northern Hemisphere3,4.

The Southern Annular Mode is currently in a positive phase5, and this is projected to continue due to increased greenhouse gas emissions. A positive phase of the Southern Annular Mode will continue to drive changes in the Southern Westerly Winds, causing warming and drying over Patagonia, and increased upwelling of warm Circumpolar Deep Water and glacier recession in western Antarctica and the Antarctic Peninsula.

The Southern Ocean climate system

The Southern Westerly Winds and Southern Ocean are an important coupled climate system that controls climate in the southern third of the world.  This system is closely connected to CO2 degassing from the deep ocean, and the position of the Intertropical Convergence Zone1.

Westerly Winds and ocean fronts around Antarctica

The Westerly Winds and Ocean Currents

The Southern Westerly Winds drive the Antarctic Circumpolar Current6. This current encircles Antarctica. The northern boundary of the Antarctic Circumpolar Current is the Subantarctic Front (SAF), and the southerly boundary is the Polar Front (PF).

The Antarctic Circumpolar Current (ACC) is the World’s strongest current. It is able to flow unimpeded through the Drake Passage and around the continent of Antarctica. The winds over the channel and the flow of the ACC are aligned for the length of the channel. The ACC is getting stronger as the Westerly Winds have contracted poleward during the current postive phase of the Southern Annular Mode, as the winds are more directly aligned with the current26.

Simplified schematic map of ocean currents of the Southern Ocean.

Beneath the cooler surface waters is a warmer layer of water. This is Circumpolar Deep Water (CDW), centered at a depth of ~500 m. It is dense and warm, with a high salt content27.

The Antarctic continental shelf is around 600 m deep, and deepens as it heads inland (a ‘retrograde’, or negative, slope). The ability of Circumpolar Deep Water to access the continental shelf is determined by the height of the CDW in the water column and the local bathymetry. The higher in the water column the CDW sits, the more it will be able to cross the continental shelf27.

Strengthening of the regional westerly winds has increased the circulation of CDW onto the continental shelf, where it can reach ice-shelf cavities and grounding lines28. If more CDW can penetrate onto the continental shelf, it may be able to reach the base of ice shelves on the Antarctic continent, melting them and risking ice-shelf collapse and marine ice sheet instability.

The Southern Annular Mode

The belt of the Southern Westerly Winds moves north and south over timescales of 10s to 100s of years. The Southern Annular Mode (SAM) (also known as the Antarctic Oscillation; AO) describes the north-south movement of these winds.

The Southern Annular Mode is usually defined as the difference in the zonal mean sea level pressure at 40°S (mid-latitudes) and 65°S (Antarctica)7,8. Changes in air pressure distribution causes changes in the strength and position of the westerly winds7. The SAM-index is effectively a measure of the strength of the Southern Westerly Winds, and as it increases, the westerlies have been moving south and increasing27.

Positive Southern Annular Mode

In a Positive phase of the Southern Annular Mode, there is lower anomalous air pressure over Antarctica, and higher anomalous air pressure over the mid-latitudes7.

In a Positive Southern Annular Mode (the situation today), the belt of strong westerly winds strengthens and contracts towards Antarctica. It weakens at the northern boundary in the mid-latitudes (40-50°S). It is drier over Patagonia, driving glacier recession. In Antarctica, increased Circumpolar Deep Water upwells onto the continental shelf, driving glacier and ice sheet recession9.

Negative Southern Annular Mode

In a Negative Southern Annular Mode, the belt of strong Southern Westerly Winds expands northwards towards the equator, bringing cold, wet weather to Patagonia and glacier advance, and decreased Circumpolar Deep Water upwelling on the Antarctic Continental Shelf. The winds are weaker in this phase.

This was the situation during Holocene neoglaciations in Patagonia and during the Last Glacial Maximum4,10–12.

Positive and Negative phases of the Southern Annular Mode

Changes in the Southern Annular Mode

We can track the past behaviour of the Southern Annular Mode in numerous proxies such as ice cores8, lake sediment cores in Patagonia13 and glacier behaviour10. The Southern Westerly Winds in Patagonia can be reconstructed through changes in precipitation.

Over recent decades, the Southern Annular Mode has been consistently positive8. There have been decreases in surface pressure over the Antarctic and corresponding increases in the mid-latitudes. This has resulted in an increase in the Westerlies over the Southern Ocean.

The positive trend in the Southern Annular Mode since ~AD 1940 has been attributed to rising greenhouse gas levels and ozone depletion14, and the long-term average SAM index is now at its highest level for the last 1000 years8.

Numerical models suggest that increased greenhouse gas emissions will drive further increases in the SAM-index over the next 100 years, driving further glacier recession in Patagonia and Antarctica8,15. Increasing CO2 leads to a poleward shift and a strengthening of the Southern Westerly Winds7.

Oceanographic implications of the increasing strength of the Westerlies ove rthe Southern Ocean include an intensification of the eddy field and a reduction in the efficiency of the Southern Ocean CO2 sink due to changes in upwelling and mixing23.

The Southern Westerly Winds in Patagonia

The Southern Westerly Winds surround the Antarctic continent. The moisture within these winds sustains the Patagonian icefields today, leading to large temperate ice masses16. At latitudes within the core of the wind belt, precipitation is high and temperature seasonality is low17.

The Andes mean that there is a strong precipitation gradient across Patagonia, with the western Andes receiving vastly more precipitation than the drier lands to the east.

Present-day climate in Patagonia (From Davies et al., 2020). A: mean annual air temperature. B: Mean annual precipitation. Note large precipitation gradient. C: Mean annual wind speed. D: Ocean fronts around the Southern Hemisphere.

In Patagonia, changes in the Southern Westerly Winds have meant that it is warmer and drier. This has driven rapid glacier recession in Patagonia. Past periods of rapid glacier change in Patagonia have also been attributed to changes in these winds. When the southern westerly wind belt moved north (for example, during the Antarctic Cold Reversal17), there was a strong precipitation increase in Patagonia, and the glaciers advanced. This has been related to negative phases of the Southern Annular Mode10,11.

Conversely, when the Southern Westerly Wind belt contracts, during positive phases of the Southern Annular Mode, it is warm and dry over Patagonia and glaciers shrink11. Over the last 10,000 years, several periods of glacier advance and retreat have been related to the changes in the Southern Annular Mode over a centennial timescale1.

The Southern Westerly Winds in Antarctica

A strengthening of the circumpolar vortex leads to a deepening of the Amundsen Sea Low, cooling most of Antarctica but warming the Antarctic Peninsula, with drier conditions over West Antarctica and the Ross Ice Shelf and Lambert Glacier23. The deepening of the Amundsen Sea Low, adjacent to Thwaites Glacier, has influenced the amount of CDW beneath Thwaites Ice Shelf27.

The positive SAM index is driving surface cooling over East Antarctica. Winter cooling over East Antarctica during periods with a high SAM index is due to the greater thermal isolation of Antarctica23. This is due to increased zonal flow, decreased meridional flow, and an intensified temperature inversion on the ice sheet due to weaker near-surface winds23.

West Antarctica and Circumpolar Deep Water

Mass loss from Antarctica is dominated by the Amundsen Sea/Bellingshausen Sea sectors. In West Antarctica, mass loss from West Antarctica has averaged 6.9 mm per decade from 1979-20179. This mass loss is largely driven by ocean melt.

Stronger westerly winds in the Bellingshausen Sea and northern Amundsen Sea have changed ocean circulation, leading to more Circumpolar Deep Water (CDW) intruding onto the continental shelf18, towards the grounding zones of Thwaites Glacier19. The enhanced polar westerlies are pushing more Circumpolar Deep Water onto the continental shelf where it melts the base of ice shelves9,20.  Continental shelf waters in the Amundsen Sea have warmed, causing ice shelf thinning and retreat of grounding lines21.

The mass loss in the Amundsen Sea, Wilkes, Western Peninsula and Bellingshausen Sea sectors of Antarctica has been increasing since the 1970s, and this is consistent with the polar contraction of the westerlies that force more CDW onto the continental shelf9. The Circumpolar Deep Water reaches glaciers through the deep bathymetric troughs on the sea floor that were carved by former ice streams. This water can then melt ice shelves and destabilise glaciers9.

Warm Circumpolar Deep Water is penetrating beneath the ice shelves of Pine Island Glacier and Thwaites Glacier.

Antarctic Peninsula

Around the Antarctic Peninsula, glacier advances and recessions have been related to a weakening or strengthening of the band of Southern Westerly Winds10. Around the Western Antarctic Peninsula, glaciers that terminate in warm Circumpolar Deep Waster have undergone considerable retreat 22.

The positive phase of the SAM also causes warming on the Antarctic Peninsula8. The greatest warming has been observed during the summer months, and is associated with the strengthening of the Southern Westerly Winds and the positive phase of the Southern Annular Mode. Stronger winds have resulted in more warm, maritime air masses crossing the peninsula and reaching the low-lying ice shelves. The spine of the Antarctic Peninsula mountains also causes the adiabatic descent and warming of the winds as they cross the topography23.

Increased sea ice and an equatorward expansion of the Westerlies favour larger glaciers in Patagonia and the eastern Antarctic Peninsula. Reduced glacier extents in Patagonia and the eastern Antarctic Peninsula occur when conditions resemble a persistent positive Southern Annular Mode10.

Ice cores on the Antarctic Peninsula, on the Landsat Image Mosaic of Antarctica.

Future projections

Under a limited warming scenario (~1.5°C), with the gradual repair of the ozone hole and stabilisation of greenhouse gas emissions, it is likely that the Southern Westerly Winds will return to values typical of the Twentieth Century24. Ozone recovery in particular will encourage a weakening of the circumpolar westerlies25.

Under a higher emissions scenario, increases in greenhouse gases would cause a continued shift in the Southern Westerly Winds poleward, with a strengthening in all seasons24. Wind-driven changes in ocean currents will increase ocean heat transport to the Amundsen Sea, entering the cavities beneath floating ice shelves and driving higher basal melt, and a reduction in backstress on grounded ice upstream24.

Further reading

References

1.           Moreno, P. I. et al. Onset and evolution of southern annular mode-like changes at centennial timescale. Sci. Rep. 8, 3458 (2018).

2.           Toggweiler, J. R. Shifting Westerlies. Science (80-. ). 323, 1434–1435 (2009).

3.           Darvill, C. M., Bentley, M. J., Stokes, C. R. & Shulmeister, J. The timing and cause of glacial advances in the southern mid-latitudes during the last glacial cycle based on a synthesis of exposure ages from Patagonia and New Zealand. Quat. Sci. Rev. 149, 200–214 (2016).

4.           Davies, B. J. et al. The evolution of the Patagonian Ice Sheet from 35 ka to the present day (PATICE). Earth-Science Rev. 204, 103152 (2020).

5.           Marshall, G. J. Trends in the Southern Annular Mode from observations and reanalyses. J. Clim. 16, 4134–4143 (2003).

6.           Carter, L., McCave, I. N. & Williams, M. J. M. Chapter 4 Circulation and Water Masses of the Southern Ocean: A Review. in Antarctic Climate Evolution (eds. Florindo, F. & Siegert, M. B. T.-D. in E. and E. S.) 8, 85–114 (Elsevier, 2008).

7.           Lee, D. Y., Petersen, M. R. & Lin, W. The Southern Annular Mode and Southern Ocean Surface Westerly Winds in E3SM. Earth Sp. Sci. 6, 2624–2643 (2019).

8.           Abram, N. J. et al. Evolution of the Southern Annular Mode during the past millennium. Nat. Clim. Chang. 4, 564–569 (2014).

9.           Rignot, E. et al. Four decades of Antarctic Ice Sheet mass balance from 1979–2017. Proc. Natl. Acad. Sci. 116, 1095–1103 (2019).

10.        Kaplan, M. R. et al. Holocene glacier behavior around the northern Antarctic Peninsula and possible causes. Earth Planet. Sci. Lett. 534, 116077 (2020).

11.        Reynhout, S. et al. Holocene glacier fluctuations in Patagonia are modulated by summer insolation intensity and paced by Southern Annular Mode-like variability. Quat. Sci. Rev. in press, (2019).

12.        Sagredo, E. A. et al. Trans-pacific glacial response to the Antarctic Cold Reversal in the southern mid-latitudes. Quat. Sci. Rev. 188, 160–166 (2018).

13.        Moreno, P. I. et al. Southern Annular Mode-like changes in southwestern Patagonia at centennial timescales over the last three millennia. Nat. Commun. 5, 1–7 (2014).

14.        Thompson, D. W. J. et al. Signatures of the Antarctic ozone hole in Southern Hemisphere surface climate change. Nat. Geosci. 4, 741–749 (2011).

15.        Bracegirdle, T. J. et al. Back to the Future: Using Long-Term Observational and Paleo-Proxy Reconstructions to Improve Model Projections of Antarctic Climate. Geosciences  9, (2019).

16.        Vilanova, I., Moreno, P. I., Miranda, C. G. & Villa-Martínez, R. P. The last glacial termination in the Coyhaique sector of central Patagonia. Quat. Sci. Rev. 224, 105976 (2019).

17.        Montade, V., Peyron, O., Favier, C., Francois, J. P. & Haberle, S. G. A pollen–climate calibration from western Patagonia for palaeoclimatic reconstructions. J. Quat. Sci. 34, 76–86 (2019).

18.        Steig, E. J., Ding, Q., Battisti, D. S. & Jenkins, A. Tropical forcing of Circumpolar Deep Water Inflow and outlet glacier thinning in the Amundsen Sea Embayment, West Antarctica. Ann. Glaciol. 53, 19–28 (2012).

19.        Scambos, T. A. et al. How much, how fast?: A science review and outlook for research on the instability of Antarctica’s Thwaites Glacier in the 21st century. Glob. Planet. Change 153, 16–34 (2017).

20.        team, I. et al. Mass balance of the Antarctic Ice Sheet from 1992 to 2017. Nature 558, 219–222 (2018).

21.        Schmidtko, S., Heywood, K. J., Thompson, A. F. & Aoki, S. Multidecadal warming of Antarctic waters. Science (80-. ). 346, 1227–1231 (2014).

22.        Cook, A. J. et al. Ocean forcing of glacier retreat in the western Antarctic Peninsula. Science (80-. ). 353, 283–286 (2016).

23.        Mayewski, P. A. et al. State of the Antarctic and Southern Ocean climate system. Rev. Geophys. 47, 1–38 (2009).

24.        Rintoul, S. R. et al. Choosing the future of Antarctica. Nature 558, 233–241 (2018).

25.        Siegert, M. et al. The Antarctic Peninsula Under a 1.5°C Global Warming Scenario. Front. Environ. Sci. 7, (2019).

26. Toggweiler, J. R., & Russell, J. (2008). Ocean circulation in a warming climate. Nature, 451(7176), 286–288.

27. Holland, D. M., Nicholls, K. W., & Basinski, A. (2020). The Southern Ocean and its interaction with the Antarctic Ice Sheet. Science, 367(6484), 1326 LP – 1330. https://doi.org/10.1126/science.aaz5491

28. Pattyn, F., & Morlighem, M. (2020). The uncertain future of the Antarctic Ice Sheet. Science, 367(6484), 1331–1335.

The westerly winds and the Patagonian Ice Sheet

The moisture-bearing Southern Westerly Winds

The Patagonian Ice Sheet, which formed during the Last Glacial Maximum (LGM) around 21,000 years ago, was strongly influenced by the Southern Westerly Winds. These winds blow around the Southern Hemisphere in the mid-latitudes (see map below) and deliver snow and rain to the western coast of southern South America[1], sustaining glaciers.

These strong winds also control the location of major ocean fronts (the boundary between water masses of different temperature) in the Southern Ocean and, as a result, the temperature of waters at the ocean surface[2,3].

Windswept Nothofagus antarctica tree, Ushuaia, Tierra del Fuego, Patagonia, Argentina. By Leonardo Pallotta, Wikimedia Commons.

Reconstructing past changes in the southern westerlies

As Patagonia covers a large latitudinal transect, extending from 40°S to 56°S, it is a critical region for investigating how the Southern Westerly Winds, and other climate systems, have changed over the last 21,000 years, and how these changes affected Patagonian glaciers.

Map of the Southern Hemisphere showing the Southern Westerly Wind belt (SWW) and Subtropical Front (STF) in the present day. The westerlies bring rain and snowfall to the west coast of Patagonia. The Subtropical Front sits at the northern limit of the westerly wind belt. Figure copyright Jacob Bendle.

Southward wind shifts driving glacier recession

Following the end of the Last Glacial Maximum (LGM) the Southern Westerly Winds abruptly shifted southward towards Antarctica[4,5], and pulled the warm Subtropical Front with them[2,3] (see left-hand side of diagram below). Records of former glacier extent show that the Patagonian Ice Sheet began to rapidly retreat and thin at about the same time (~18,000 years ago[6,7,8]), suggesting that as the winds moved south, the amount of snowfall feeding the ice sheet decreased.

The wind-driven shift of the Subtropical Front caused the coastal waters around Patagonia to warm[2]. With less accumulation (snowfall) and warmer temperatures, the Patagonian Ice Sheet started to retreat.

Diagram showing how the location of the Southern Westerly Winds (SWW) and Subtropical Front (STF) impact the mid-latitudes. Left: When the westerly winds and Subtropical Front contract (move south) cool, moist air stops flowing over Patagonia, and warm waters enter the mid-latitude oceans. This favours glacier retreat. Right: When the westerly winds and Subtropical Front expand (move north) strong winds bring precipitation to Patagonia, and cold Southern Ocean waters cool the mid-latitude oceans. This favours glacier advance. Figure copyright Jacob Bendle.

Northward wind shifts driving glacier advance

Whereas the southward shift of the Southern Westerly Winds triggered Patagonian Ice Sheet retreat at ~18,000 years ago, a northward wind shift between ~14,500 and 12,800 years ago, in the Antarctic Cold Reversal (a cool period recorded in Antarctic ice cores), revived glacier activity[9,10,11,12].

As the westerly winds moved north over Patagonia (see right-hand side of diagram above), increased snowfall led to glacier growth. Because the winds also pulled cool Southern Ocean waters into the mid-latitudes, ocean and air temperatures around Patagonia cooled, leading to less ice sheet melting. The combination of increased accumulation (snowfall) and decreased ablation (melting) led to glacier readvance.

Hemisphere-wide glacier response

Glaciers in the Southern Alps of New Zealand also readvanced in the Antarctic Cold Reversal, at the same time as glaciers in Patagonia[13]. This suggests that the shift in the position of the the westerly winds and ocean fronts were a major driver of climate and ice sheet behaviour across the entire mid-latitude belt below ~40°S.

SWW controls on climate

The Southern Westerly Wind system controls the climate of the Southern Hemisphere in other ways, and these are important for modern and past glaciers.

For example, when the westerlies move towards Antarctica, the warm waters they drag southwards causes the sea-ice around Antarctica to break up and retreat[14]. This causes the ocean around Antarctica to warm, and releases heat to the atmosphere.

Also, when the westerly winds are positioned over the Southern Ocean, they cause relatively warm water that is trapped at depth to rise to the ocean surface. This releases heat and CO2 from the ocean, and causes atmospheric warming[15].

Further reading

References

1. Garreaud, R.D., Vuille, M., Compagnucci, R. & Marengo, J. 2009. Present-day South American climate. Palaeogeography, Palaeoclimatology, Palaeoecology, 281, 180–195.

2. Lamy, F., Kaiser, J., Arz, H.W., Hebbeln, D., Ninnemann, U., Timm, O., Timmermann, A. & Toggweiler, J.R. 2007. Modulation of the bipolar seesaw in the Southeast Pacific during Termination 1. Earth and Planetary Science Letters, 259, 400–413.

3. Barker, S., Diz, P., Vautravers, M.J., Pike, J., Knorr, G., Hall, I.R. & Broecker, W.S. 2009. Interhemispheric Atlantic seesaw response during the last deglaciation. Nature, 457, 1097–1102.

4. Denton, G.H., Anderson, R.F., Toggweiler, J.R., Edwards, R.L., Schaefer, J.M. & Putnam, A.E. 2010. The last glacial termination. Science, 328, 1652–1656.

5. Moreno, P.I., Villa-Martínez, R., Cárdenas, M.L.& Sagredo, E.A. 2012. Deglacial changes of the southern margin of the southern westerly winds revealed by terrestrial records from SW Patagonia (52°S). Quaternary Science Reviews, 41, 1–21.

6. Boex, J., Fogwill, C., Harrison, S., Glasser, N., Hein, A., Schnabel, C. & Xu, S. 2013. Rapid thinning of the Late Pleistocene Patagonian Ice Sheet followed migration of the Southern Westerlies. Scientific Reports 3, 2118.

7. Moreno, P.I., Denton, G.H., Moreno, H., Lowell, T.V., Putnam, A.E. & Kaplan, M.R. 2015. Radiocarbon chronology of the last glacial maximum and its termination in northwestern Patagonia. Quaternary Science Reviews, 122, 233–249.

8. Bendle, J.M., Palmer, A.P., Thorndycraft, V.R. and Matthews, I.P., 2019. Phased Patagonian Ice Sheet response to Southern Hemisphere atmospheric and oceanic warming between 18 and 17 ka. Scientific Reports, 9,.

9.  Moreno, P.I., Kaplan, M.R., François, J.P., Villa-Martínez, R., Moy, C.M., Stern, C.R. and Kubik, P.W., 2009. Renewed glacial activity during the Antarctic cold reversal and persistence of cold conditions until 11.5 ka in southwestern Patagonia. Geology, 37(4), 375-378.

10. García, J.L., Kaplan, M.R., Hall, B.L., Schaefer, J.M., Vega, R.M., Schwartz, R. and Finkel, R., 2012. Glacier expansion in southern Patagonia throughout the Antarctic cold reversal. Geology, 40, 859-862.

11.  Sagredo, E.A., Kaplan, M.R., Araya, P.S., Lowell, T.V., Aravena, J.C., Moreno, P.I., Kelly, M.A. and Schaefer, J.M., 2018. Trans-pacific glacial response to the Antarctic Cold Reversal in the southern mid-latitudes. Quaternary Science Reviews, 188, 160-166.

12. Davies, B.J., Thorndycraft, V.R., Fabel, D. and Martin, J.R.V., 2018. Asynchronous glacier dynamics during the Antarctic Cold Reversal in central Patagonia. Quaternary Science Reviews, 200, 287-312.

13. Putnam, A.E., Denton, G.H., Schaefer, J.M., Barrell, D.J., Andersen, B.G., Finkel, R.C., Schwartz, R., Doughty, A.M., Kaplan, M.R. and Schlüchter, C., 2010. Glacier advance in southern middle-latitudes during the Antarctic Cold Reversal. Nature Geoscience, 3, 700-704.

14. Pedro, J.B., Jochum, M., Buizert, C., He, F., Barker, S. and Rasmussen, S.O., 2018. Beyond the bipolar seesaw: Toward a process understanding of interhemispheric coupling. Quaternary Science Reviews, 192, 27-46.

15. Toggweiler, J.R., Russell, J.L. and Carson, S.R., 2006. Midlatitude westerlies, atmospheric CO2, and climate change during the ice ages. Paleoceanography, 21(2).

Changes to Circumpolar Deep Water

What is Circumpolar Deep Water?

Circumpolar Deep Water is derived from a mixture of all the World’s oceans[1]. It is a relatively salty, warm current, >3.5°C above freezing point, which flows onto the continental shelf at depths of more than 300 m[2]. It is overlain by colder, fresher surface waters. Circumpolar Deep Water is critical because it rapidly melts the base of ice shelves. Further, changes in the frequency, duration and extent of cross-continental shelf intrusions of Circumpolar Deep Water may alter the rate at which basal melting occurs[1].

Antarctic Bottom Water and upwelling Circumpolar Deep Water. From Wikimedia.

Antarctic Bottom Water and upwelling Circumpolar Deep Water. From Wikimedia.

Circumpolar Deep Water (CDW) is a key component of the Antarctic Circumpolar Current[3]. In the Amundsen Sea, CDW is found near to the shelf break, where the continental shelf acts as a large topographic barrier for the majority of the deep water. The height of the offshore CDW in the water column and the height of the continental shelf are therefore crucial in determining whether CDW can flow onto the continental shelf. A significant depression on the continental shelf, such as a glacial trough, may allow a thicker, undiluted layer of CDW onto the continental shelf.

Simplified schematic map of ocean currents of the Southern Ocean.

Simplified schematic map of ocean currents of the Southern Ocean.

How is Circumpolar Deep Water changing?

Outlet glaciers in the Amundsen Sea Embayment have accelerated[4-6], receded[7] and thinned in recent years, which has largely been attributed to increased melting of their ice shelves by Circumpolar Deep Water[8, 9]. This has resulted in grounding line recession[7], which may result in marine ice sheet instability in West Antarctica.

In Pine Island Bay, the ice shelf buttressing Pine Island Glacier is melting rapidly. Observations show that the temperature and volume of CDW in Pine Island Bay have increased[2], which is able to reach the glacier through a deep submarine glacial trough[3]. Meltwater production has increased by 50% since 1994, resulting in stronger sub-ice-shelf circulation, leading to the formation and enlargement of an inner cavity, where sea water at 4°C can readily access the grounding line[2].

Warm Circumpolar Deep Water is penetrating beneath Pine Island Glacier's ice shelf

Warm Circumpolar Deep Water is penetrating beneath Pine Island Glacier’s ice shelf

Why is Circumpolar Deep Water changing?

CDW may be increasingly driven onto the continental shelf in response to changing wind patterns in the Bellingshausen Sea[10], Amundsen Sea[11] and western Antarctic Peninsula[1, 12]. However, the exact mechanisms involved are complex and poorly understood.

Ice streams of Antarctica with Pine Island Glacier and Thwaites glacier highlighted.

Ice streams of Antarctica with Pine Island Glacier and Thwaites glacier highlighted.

One cause may be related to changes in the Southern Annular Mode (SAM). The SAM is the main mode of variability in atmospheric circulation in the southern high latitudes. The SAM has changed towards positive polarity, meaning decreased pressure over the Antarctic and increased pressure at the mid-latitudes in recent years, likely driven by changes in greenhouse gases and the ozone hole[1]. Continued movement of the SAM towards positive polarity will result in strengthening and poleward displacement of the circumpolar westerlies. Across the Antarctic Peninsula, a positive SAM is associated with strong warming, strengthening of the westerlies, and a shift in storm tracks[13].

Stronger westerlies, associated with a positive SAM, will result in more Circumpolar Deep Water moving onto the Antarctic continental shelf, leading to increased heat transport and more melting at the grounding line and beneath ice shelves[1].

Steig et al., (2012) found that both interannual variability and long-term changes in westerly wind stress in the Amundsen Sea Embayment was relevant to forcing more CDW onto the continental shelf[8]. They found that this was strongly influenced by changes in the tropics, which may be an even greater influence than changes in the Southern Annular Mode.

Summary

The ice, the ocean and the atmosphere are all intrinsically linked, and in Antarctica we now see how complex changes in atmospheric circulation, driven by climate change and the ozone hole, are changing ocean circulation. Increased upwelling of warm, salty Circumpolar Deep Water is melting away the base of the ice shelves and the grounding lines of some of the largest, most vulnerable glaciers and ice streams in Antarctica, resulting in rapid, far-reaching and irreversible changes.

Further Reading

References

  1. Dinniman, M.S., J.M. Klinck, and E.E. Hofmann, Sensitivity of Circumpolar Deep Water Transport and Ice Shelf Basal Melt along the West Antarctic Peninsula to Changes in the Winds. Journal of Climate, 2012. 25(14): p. 4799-4816.
  2. Jacobs, S.S., et al., Stronger ocean circulation and increased melting under Pine Island Glacier ice shelf. Nature Geoscience, 2011. 4(8): p. 519-523.
  3. Walker, D.P., et al., Oceanic heat transport onto the Amundsen Sea shelf through a submarine glacial trough. Geophysical Research Letters, 2007. 34(2): p. L02602.
  4. Pritchard, H.D., et al., Extensive dynamic thinning on the margins of the Greenland and Antarctic ice sheets. Nature, 2009. 461(7266): p. 971-975.
  5. Pritchard, H.D. and D.G. Vaughan, Widespread acceleration of tidewater glaciers on the Antarctic Peninsula. Journal of Geophysical Research-Earth Surface, 2007. 112(F3): p. F03S29, 1-10.
  6. Mouginot, J., E. Rignot, and B. Scheuchl, Sustained increase in ice discharge from the Amundsen Sea Embayment, West Antarctica, from 1973 to 2013. Geophysical Research Letters, 2014: p. 2013GL059069.
  7. Rignot, E., et al., Widespread, rapid grounding line retreat of Pine Island, Thwaites, Smith, and Kohler glaciers, West Antarctica, from 1992 to 2011. Geophysical Research Letters, 2014: p. n/a-n/a.
  8. Steig, E.J., et al., Tropical forcing of Circumpolar Deep Water Inflow and outlet glacier thinning in the Amundsen Sea Embayment, West Antarctica. Annals of Glaciology, 2012. 53(60): p. 19-28.
  9. Shepherd, A., D. Wingham, and E. Rignot, Warm ocean is eroding West Antarctic Ice Sheet. Geophysical Research Letters, 2004. 31(23): p. L23402.
  10. Dinniman, M.S., J.M. Klinck, and W.O. Smith Jr, A model study of Circumpolar Deep Water on the West Antarctic Peninsula and Ross Sea continental shelves. Deep Sea Research Part II: Topical Studies in Oceanography, 2011. 58(13): p. 1508-1523.
  11. Thoma, M., et al., Modelling circumpolar deep water intrusions on the Amundsen Sea continental shelf, Antarctica. Geophysical Research Letters, 2008. 35(18).
  12. Wåhlin, A.K., et al., Inflow of Warm Circumpolar Deep Water in the Central Amundsen Shelf*. Journal of Physical Oceanography, 2010. 40(6): p. 1427-1434.
  13. Kwok, R. and J.C. Comiso, Southern Ocean Climate and Sea Ice Anomalies Associated with the Southern Oscillation. Journal of Climate, 2002. 15(5): p. 487-501.

Glaciers and Climate

This section of the website highlights how glaciers interact with climate, and how changing climate is changing glaciers around the world today.

Globally, glaciers are receding and shrinking in response to atmospheric warming. The signal is remarkably consistent across different continents and mountain ranges.

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:

Patagonian Ice Sheet thinning during a changing climate

J.Boex, C. Fogwill, S. Harrison, N.F. Glasser, A. Hein, C. Schnabel and S. Xu.  Rapid thinning of the Late Pleistocene Patagonian Ice Sheet followed migration of the Southern Westerlies. Scientific Reports 3: 2118, p. 1-6

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The Patagonian Ice Sheet

Patagonian mountains east of the North Patagonian Icefield. Credit: Stephen Harrison

Patagonian mountains east of the North Patagonian Icefield. Credit: Stephan Harrison

This recent open-access paper in the new journal Science Communications, which is part of the Nature group, has demonstrated that the during the deglacial period (~19,000 years ago), the Patagonian Ice Sheet in South America responded rapidly in response to changing precipitation patterns and warming during the last deglaciation. The fact that the Patagonian Ice Sheet responded so quickly to changes in precipitation and temperature has vivid implications for the current, and future, behaviour of the current North Patagonian Icefield  and South Patagonian Icefield. We already know that the shrinkage of the North and South Patagonian ice fields was faster over the last decade or so than at any point in the last couple of centuries. Understanding on a broader scale how these sensitive, high-latitude ice masses are dependent on small changes in atmospheric circulation means that we will better be able to predict the future behaviour of these ice sheets. Reconstructing rates of ice-sheet decay since the Last Glacial Maximum means that we can better assess the mechanisms of climate change (including changing wind patterns) during a major climate transition. Continue reading

Climate Change

What is climate? | Quaternary climates | Is our climate changing? | The Role of Carbon Dioxide | Climate change in Antarctica | Observed impacts of climate change | References | Comments |

What is climate?

First, it is important here to note the differences between weather and climate. Weather is local and what you can see out of your window; weather is the cold British winter snows in 2011 and our 2012 wet and rainy summer (these are to do with short term variations in winds and atmospheric pressures), or heat waves in the USA in July 2012.

Climate is much broader scale; we are looking at long term statistical patterns in weather. An individual flood or storm is not caused by climate change, but climate change may make extreme weather events (like floods) occur more frequently. “Climate” encompasses data such as temperature, humidity, pressure, wind, precipitation (snow or rain), and other meteorological measurements.

This page is intended to be a short introduction, and by no means covers all of the vast topic of climate change. For more information, see: New Scientist; IPCC; Met Office.

Quaternary Climates

Throughout the last 2.6 million years (the “Quaternary Period”), the earth’s climate has oscillated many times, swinging between glacial and interglacial states (Figure 1). Over the last ~1 million years, we have experienced large ice ages and interglacials with a periodicity of around 100,000 years.

Figure 1. 5 million years of climate change – Global Warming Art Project. Wikimedia Commons

We are currently in an interglacial state, which began at the start of the Holocene, ~11,500 years ago. About 104 stages of these cold and temperate cycles have been recognised in deep ocean marine sediment cores (Figure 1) (Lowe and Walker, 1997). During glacials, large ice sheets developed in mid- to high-latitudes, including over Britain and North America.

Repeated climate oscillations

These large changes are driven by changes in the earth’s orbit around the sun – see The Quaternary Period (Table 1) (Jouzel et al., 2007). Glacials and interglacials can be further divided into stadials and interstadials, and within these we have smaller scale Dansgaard-Oeschger cycles, and then even smaller cycles, such as El-Nino and ENSO.

Climate data is therefore very noisy, and climate scientists must determine patterns in this data using complex statistical techniques. Throughout this time, carbon dioxide has mirrored temperature variations, which have formed a regular pattern.

Table 1. Geological timescale

Climate variations are a natural part of the earth system. It is therefore important, using instrumental records and proxies (such as ice cores, or microfossils in marine sediment cores), to compare current trends with those in the past (Bentley, 2010).

Is our climate changing?

Recent Change

The Earth warmed by an average of 0.6 ± 0.2°C during the twentieth century (Houghton et al., 2001). Rapid warming has been measured with global instrumental data since the 1800s (Figure 2). In 2019, the global average temperature was 0.95°C above the twentieth Century average, making it the second warmest year on record. The five warmest years have all occurred since 2015, and nine of the 10 warmest years since 2010 (Climate.Gov, accessed August 19, 2020).

Figure 2. This image shows the instrumental record of global average w:temperatures as compiled by the w:NASA’s w:Goddard Institute for Space Studies. (2006) “Global temperature change”. Proc. Natl. Acad. Sci. 103: 14288-14293. Following the common practice of the w:IPCC, the zero on this figure is the mean temperature from 1961-1990. This figure was originally prepared by Robert A. Rohde from publicly available data and is incorporated into the Global Warming Art project. Wikimedia Commons.

The 2001 IPCC report stated that most of this warming was likely to have been due to an increase in greenhouse gas emissions. Later IPCC reports have stated this with ever increasing confidence. However, this average rate hides considerable variations in the rate and magnitude of warming.

Regional Rapid Warming

Climate change varies seasonally, on decadal timescales, and is geographically patchy (Mann et al., 2008). Three areas in particular have been subject to recent regional rapid warming (sensu Vaughan et al., 2003), with rates of warming far faster than the average noted in the IPCC. These regions are: north-western North America, the Siberian Plateau in northeast Asia, and the Antarctic Peninsula and Bellingshausen Sea. These areas warmed by more than 1.5°C between 1950 and 2000 AD  (compared with a global mean of 0.5°C) (Mann et al., 2008).

More populated regions have atmospheric sulphate aerosols, that may mask warming. Urban meteorological stations may also record anomalous warming due to urban heat island effects. However, the Antarctic continent is free of these effects. Although direct meteorological observations are short (~60 years), trends in these areas are particularly important.

The Longer Term View

Although we have measured changing temperatures and carbon dioxide levels at short timescales (since the 1700s), surely this could just reflect natural variability over short timescales? In order to understand our climate, it is very important to look at the long-term view.

In many places, atmospheric temperatures are now warmer than they have been throughout the Holocene. Figure 3 shows that the temperature of the last 200 years is much higher than it was when Romans were making wine in southern England. However, what is really concerning is the rate of change. Temperatures have risen far more sharply in the last century than they have at any point in the last 2000 years.

2000 year temperature variations. Robert A. Rhode, Global Warming Art Project. Wikimedia Commons.
Figure 3. 2000 year temperature variations. Robert A. Rhode, Global Warming Art Project. Wikimedia Commons.

This conclusion is reached again and again, and a paper by Mann et al. (2008) in PNAS show that the Earth’s temperature is anomalous in a long-term time context. Mann et al. used stacked records from a variety of sources to create a global graph of temperature change. This famous ‘hockey stick’ graph (Figure 4) clearly demonstrates the rapid rate of change since the industrial revolution, with temperature rise sharply accelerating over the last 150 years.

From Mann et al., 2008. Original caption: Composite CPS and EIV NH land and land plus ocean temperature reconstructions and estimated 95% confidence intervals. Shown for comparison are published NH reconstructions, centered to have the same mean as the overlapping segment of the CRU instrumental NH land surface temperature record 1850–2006 that, with the exception of the borehole-based reconstructions, have been scaled to have the same decadal variance as the CRU series during the overlap interval (alternative scaling approaches for attempting to match the amplitude of signal in the reconstructed and instrumental series are examined in SI Text). All series have been smoothed with a 40-year low-pass filter as in ref 33. Confidence intervals have been reduced to account for smoothing.
Figure 4. From Mann et al., 2008. Temperature change over the last 2000 years. Composite land plus ocean temperature reconstructions and estimated 95% confidence intervals.

The Pages2k team have compiled global temperature records over the last 2000 years (Pages 2k Team, Nature Geoscience 12, 643-649). This reconstruction comes from many different kinds of proxy records, including tree rings, cave deposits, corals, etc.

Ed Hawkins (website: Climate Lab Book) has visualised the Pages2k climate record for the years 0 AD to 2019 AD using #warmingstripes (Figure 5). The colours highlight the recent very rapid warming.

Figure 5. PAGES 2k global temperature reconstruction, for the last 2019 years. By Ed Hawkins

The uncertainty estimates mean that you can also view the dataset in a more traditional way. The Medieval Warm Period and Little Ice Age are clearly shown, and the highly unusual recent rapid warming. The Medieval Warm Period and Little Ice Age are relatively small phenomena compared with the recent rapid warming.

Figure 6. PAGES 2k global temperature reconstruction. Image by Ed Hawkins.

Climate change over the last 11,300 years

A paper out in Science by Marcott et al. (2013) extends Figures 3 and 4 back to the last 11,300 years, with the Earth’s temperatures now warmer than the Earth was 4000 years ago (Figure 7). During the last 200 years, temperatures have rocketed up, confirming earlier reconstructions by Mann et al. 2008.

Figure 5. Climate change over the last 11,500 years from multiple proxies. From Marcott et al., 2013
Figure 7. Climate change over the last 11,500 years from multiple proxies. From Marcott et al., 2013

Although the Earth has not yet exceeded the temperatures of the Early Holocene (5000 to 11,000 years ago), global temperatures have risen from cooler than 95% of the Holocene at around 1900 to warmer than 72% of the Holocene in the last 100 years (Figure 7).

This means that, in the last 100 years, the Earth’s temperature has reversed a long-term cooling trend that began around 5000 years ago to become near the warmest temperatures during the last 11,000 years. Furthermore, climate models predict that the Earth’s temperature will exceed the warmest temperatures of the Holocene by 2100, regardless of which greenhouse gas emission scenario is used (Marcott et al., 2013).

However, Marcott et al. do note in their paper that the ‘uptick’ shown in the graph is not statistically robust, as the median resolution of all data is 120 years. However, this has been shown by other authors, such as Anderson et al. 2013, who demonstrate rapid warming over the last century from geological data.

Kaufman et al. (2020) provide an updated global temperature reconstruction, dating back to 12,000 years ago (Figure 8). They used a median from five different reconstruction methods, generating a consensus global mean surface temperature reconstruction. It follows the same approach as the PAGES 2k Consortium.

Figure 8. From Kaufman et al. 2020. Global mean temperature composites. The fine black line is instrumental data, 1900-2010. Inset shows the last 2000 years. Coloured lines are the medians of the five different reconstruction methods. Temperature anomalies are relative to 1800-1900.

Kaufman et al. (2020) found that the warmest period in the Holocene was centred at 6,500 years ago, and was 0.6°C warmer than the 1800-1900 reference period.

The decade of 2011-2019 averaged 1 °C higher than 1850–1900 (Kaufman et al., 2020). For 80% of the ensemble members, no 200 year interval in the last 12,000 years was warmer than the last decade. Temperatures projected in the rest of this century, and beyond, are very likely to exceed 1 °C above pre-industrial temperature (IPCC, 2013).

The temperatures reconstructed vary in the different latitudes and different hemispheres; the palaeoclimate of the Northern and Southern hemispheres is very different (Figure 9).

Figure 9. From Kaufman et al., 2020. Reconstructed mean annual temperatures from the Temperature 12k database using different reconstruction methods from the different latitude bands. Temperature anomalies are relative to 1800-1900.

Below, you can see the Kaufman et al. 2020 reconstruction, compared with other published proxy reconstructions over this time period. There are more proxy datasets from the Northern Hemisphere than the Southern Hemisphere.

Figure 10. From Kaufman et al., 2020. (a) multi-method median global surface temperature reconstruction compared with previous reconstructions. (b) locations of proxy data sites.

Going back to the Last Glacial Maximum

However, it doesn’t end there. Using more proxies to extend the temperature graph back to the last glacial and modelling scenarios from the IPCC for the next 100 years, the graph below (Figure 11) has been created. Dubbed “The Wheelchair”, it shows the current and future alarming rate of temperature increase, compared with temperature fluctuations over the last 20,000 years. This figure was created by Jos Hagelaars.

Global average temperatures from 20,000 BC up to 100 years into the future under a middle-ranking emissions scenario.
Figure 11. Global average temperatures from 20,000 BC up to 100 years into the future under a middle-ranking emissions scenario.

The role of carbon dioxide

Carbon dioxide is a key factor in climate change. Carbon dioxide concentrations fluctuate annually, as seen in Figure 12 from the longest direct measurements of CO2 from Mauna Loa in Hawaii.

Atmospheric carbon dioxide measurements at Mauna Loa. Published by the Global Warming Art Project and sourced from Wikimedia Commons.
Figure 12. Atmospheric carbon dioxide measurements at Mauna Loa. Published by the Global Warming Art Project and sourced from Wikimedia Commons.

You can explore the changing global levels of carbon dioxide, methane, the global temperature record over the last 2000 years and global mean sea level here.

Figure 13. Climate Levels is an interactive website where you can explore the levels of carbon dioxide, methane, temperature and mean sea level over the last 1000 years.

This article from Bloomberg allows you to investigate the factors that may cause global temperature variations, including solar changes, volcanism, aerosols, deforestation, and carbon dioxide.

What’s really warming the world? Bloomberg, June 24, 2015

The seasonal fluctuation is caused by variations in uptake of carbon dioxide by land plants. However, looking at Figure 2 above, you can see that this is just part of a much longer record of carbon dioxide. The IPCC 2007 Synthesis Report stated:

“Global GHG emissions due to human activities have grown since pre-industrial times, with an increase of 70% between 1970 and 2004”.

IPCC 2007

These increases in carbon dioxide are primarily due to increased use of fossil fuels, but land use changes also provide a significant contribution. You can explore global energy production at the Global Electricity Review website.

Because humans are constantly adding more carbon dioxide to the atmosphere, we are changing its balance and affecting the Earth’s climate. The carbon dioxide and other atmospheric gasses in the atmosphere combine with changes in land cover and solar radiation to drive climate change. They affect the absorption, scattering and emission of radiation within the Earth’s atmosphere, resulting in a positive energy balance and warming influences on our climate.

The IPCC 2007 states that there is very high confidence that the world has warmed since 1750, and that the combined radiative forcing due to increases in carbon dioxide, methane and nitrous oxide is 2.3W/m2, and that the rate of increase during the industrial era is was unprecedented in the last 10,000 years.

Changes in solar irradiance since 1750 AD have caused a small radiative forcing of +0.12 W/m2.

But how has carbon dioxide changed over the last few thousand years?

Figure 14 shows the long-term temperature and carbon dioxide record from Antarctica. Carbon Dioxide (blue line) now approaches 400 parts per million, which is far higher than it has been at any point in the last 400,000 years.

420,000 years of ice core data from Vostok, Antarctica research station. Current period is at left. From bottom to top: * Solar variation at 65°N due to en:Milankovitch cycles (connected to 18O). * 18O isotope of oxygen. * Levels of methane (CH4). * Relative temperature. * Levels of carbon dioxide (CO2). From top to bottom: * Levels of carbon dioxide (CO2). * Relative temperature. * Levels of methane (CH4). * 18O isotope of oxygen. * Solar variation at 65°N due to en:Milankovitch cycles (connected to 18O). Wikimedia Commons.
Figure 14. 420,000 years of ice core data from Vostok, Antarctica research station. Current period is at left. From bottom to top: * Solar variation at 65°N due to en:Milankovitch cycles (connected to 18O). * 18O isotope of oxygen. * Levels of methane (CH4). * Relative temperature. * Levels of carbon dioxide (CO2). From top to bottom: * Levels of carbon dioxide (CO2). * Relative temperature. * Levels of methane (CH4). * 18O isotope of oxygen. * Solar variation at 65°N due to en:Milankovitch cycles (connected to 18O). Wikimedia Commons.

Methane (green line) is also higher than it has been before over this timescale. You can also see how closely temperature (red line) has tracked carbon dioxide and methane over this time. The temperature in Antarctica (red line) is expected to continue to rise with this increase in atmospheric greenhouse gases.

Climate change in Antarctica

Figure 15. Global climate change. Global climate change 2000-2009 and 1950-1980/Wikimedia Commons.

The western Antarctic Peninsula warmed by 2.5°C from 1950-2000 (Turner et al., 2005) (Figure 15). The mean annual temperature is rising, with the -9°C isotherm moving southwards, resulting in the collapse of several ice shelves (28,000 km (Turner et al., 2005) of ice shelf has been lost since the 1990s) (Morris and Vaughan, 2003). Climate records from the South Orkney islands suggest that warming began in earnest in the 1930s (Mann et al., 2008).

Southern Westerly Winds

This warming has been related to variations in the belts of upper atmosphere winds that encircle the Antarctic continent (the Circumpolar Vortex) (Figure 16).

Figure 16. Westerly Winds and ocean fronts around Antarctica

Periodic oscillations in this result in the periodic strengthening and weakening of this belt of winds. A strengthening of this vortex has been associated with changes in surface atmospheric pressures (van den Broeke et al., 2004).  This pressure pattern causes northerly flow anomalies, which in turn cause cooling over East Antarctica and warming over the Antarctic Peninsula.

Other factors contributing to the recent regional rapid warming over the Antarctic Peninsula include decreased sea ice in the Bellingshausen Sea, resulting in warmer air temperatures, and decreasing precipitation over the south western peninsula (van den Broeke and van Lipzig, 2004; van Lipzig et al 2004).

Finally, since the 1950s, the Antarctic Circumpolar Current has warmed by +0.2°C, with the warming greatest near the surface (Gille, 2008). Waters west of the Antarctic Peninsula have also warmed rapidly (Turner et al., 2005; Mayewski et al. 2009).

This video from NASA shows global warming since 1884, and you can really see the large and rapid warming around the Antarctic Peninsula over the last 50 years.

Observed impacts of climate change

The impacts of this recent regional rapid warming around the Antarctic Peninsula have been dramatic, with the collapse of ice shelves (Cook and Vaughan, 2010), and with 87% of glaciers in recession (Cook et al. 2005). The present-day ice loss from the Antarctic Peninsula is -41.5 giga-tonnes per year (Ivins et al. 2011). Ice-shelf tributary glaciers have become destabilised following ice-shelf collapse (Scambos et al. 2004; DeAngelis and Skvarca, 2003).

Other glaciers have thinned, accelerated and receded as a result of increased melting (see Recent Change) (Pritchard et al. 2009; Pritchard and Vaughan, 2007). The rapid shrinkage of glaciers around the Antarctic Peninsula, coupled with the potential for ice-shelf collapse and grounding line retreat, raises concerns for the future of the West Antarctic Ice Sheet, and this is an area of urgent current research (Bentley, 2010). This is covered in more detail under Marine Ice Sheet Instability.

Videos to watch

Abrupt Arctic Climate Change: Comparison of Today with Paleoclimate: Change Rates and Distribution. By Paul Beckworth

Further reading

Other websites

References


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Bentley, M.J., 2010. The Antarctic palaeo record and its role in improving predictions of future Antarctic Ice Sheet change. Journal of Quaternary Science, 2010. 25(1): p. 5-18.

Cook, A.J., Fox, A.J., Vaughan, D.G., and Ferrigno, J.G., 2005. Retreating glacier fronts on the Antarctic Peninsula over the past half-century. Science, 2005. 308(5721): p. 541-544.

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

De Angelis, H. and Skvarca, P., 2003. Glacier surge after ice shelf collapse. Science, 2003. 299: p. 1560-1562.

Gille, S.T., 2008. Decadal-scale temperature trends in the Southern Hemisphere Ocean. Journal of Climatology, 2008. 21: p. 4749-4765.

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Mann, M.E., Zhang, Z., Hughes, M.K., Bradley, R.S., Miller, S.K., Rutherford, S. & Ni, F. Proxy-based reconstructions of hemispheric and global surface temperature variations over the past two millennia. Proceedings of the National Academy of Sciences (2008).

Marcott, S.A., Shakun, J.D., Clark, P.U. & Mix, A.C. A Reconstruction of Regional and Global Temperature for the Past 11,300 Years. Science 339, 1198-1201 (2013).

Morris, E.M. and Vaughan, A.P.M., 2003. Spatial and temporal variation of surface temperature on the Antarctic Peninsula and the limit of viability of ice shelves, in Antarctic Peninsula climate variability: historical and palaeoenvironmental perspectives, E.W. Domack, et al., Editors. American Geophysical Union, Antarctic Research Series, Volume 79: Washington, D.C. p. 61-68.

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Mayewski, P.A., Meredith, M.P., Summerhayes, C.P., Turner, J., Worby, A., Barrett, P.J., Casassa, G., Bertler, N.A.N., Bracegirdle, T., Naveira Garabato, A.C., Bromwich, D., Campell, H., Hamilton, G.S., Lyons, W.B., Maasch, K.A., Aoki, S., Xiao, C., and van Ommen, T., 2009. State of the Antarctic and Southern Ocean climate system. Reviews of Geophysics, 2009. 47(RG1003): p. 1-38.

Pritchard, H.D., Arthern, R.J., Vaughan, D.G., and Edwards, L.A., 2009. Extensive dynamic thinning on the margins of the Greenland and Antarctic ice sheets. Nature, 2009. 461(7266): p. 971-975.

Pritchard, H.D. and Vaughan, D.G., 2007. Widespread acceleration of tidewater glaciers on the Antarctic Peninsula. Journal of Geophysical Research-Earth Surface, 2007. 112(F3): p. F03S29, 1-10.

Scambos, T.A., Bohlander, J.A., Shuman, C.A., and Skvarca, P., 2004. Glacier acceleration and thinning after ice shelf collapse in the Larsen B embayment, Antarctica. Geophysical Research Letters, 2004. 31: p. L18402.

Turner, J., Colwell, S.R., Marshall, G.J., Lachlan-Cope, T.A., Carelton, A.M., Jones, P.D., Lagun, V., Reid, P.A., and Iagovkina, S., 2005. Antarctic climate change during the last 50 years. International Journal of Climatology, 2005. 25: p. 279-294.

van den Broeke, M.R. and van Lipzig, N.P.M., 2004. Changes in Antarctic temperature, wind and precipitation in response to the Antarctic Oscillation. Annals of Glaciology, 2004. 39: p. 119-126.

van Lipzig, N.P.M., King, J.C., Lachlan-Cope, T.A., and van den Broeke, M.R., 2004. Precipitation, sublimation and snow drift in the Antarctic Peninsula region from a regional atmospheric model. Journal of Geophysical Research, 2004. 109: p. D24106.

Vaughan, D.G., Marshall, G.J., Connelly, W.M., Parkinson, C., Mulvaney, R., Hodgson, D.A., King, J.C., Pudsey, C.J., and Turner, J., 2003. Recent rapid regional climate warming on the Antarctic Peninsula. Climatic Change, 2003. 60: p. 243-274.