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Low probability, but high impact scenarios: Introducing the backgrounds of a “Dangerous Climate Change”

The balance of scientific evidence now suggests that anthropogenic greenhouse gas emissions are having a significant effect on the earth’s system and especially on the earth’s climate. Since the 1970s, there has been a significant increase of extreme weather events. Tropical and extra-tropical storm frequency and magnitude have considerably increased and so have the flood risks and heatwave occurrences along with very severe socio-economic and ecological impacts all over the globe. Even though the natural science of extreme weather events has progressed over the last decades, modelling climate scenarios still remains pretty speculative. However, it is now scientific consensus that if we continue to follow our “Business as usual”-path and if the greenhouse gas emissions weren’t cut drastically within the next decades, the impacts of a changing climate will intensify throughout the 21st century, with dangerous high impact scenarios becoming more likely to happen. A “dangerous climate change” with raising temperatures especially above 2°C (above pre-industrial levels) could tip certain ecological thresholds and trigger non-linear processes and feedback loops within the earth’s system, forcing the system rapidly into a totally new equilibrium. Dramatic changes within the carbon cycle, the eco- and hydrosphere and most obviously within the kryosphere would exceed our society’s ability to adapt to these changes. Unmanageable and most likely irreversible consequences could put our mankind on the edge of extinction. (1)

Fig. 1: Reasons for concern about projected climate change impacts.
Source: http://www.grida.no/climate/ipcc_tar/wg2/008.htm

Although rising greenhouse gas emissions are increasing the probability of extreme weather events, extreme climate scenarios still have to be characterised as low probability, high impact scenarios, involving most likely non-linear processes. Therefore there is a certain need to think of them in a risk-and uncertainty-based manner, rather than looking for deterministic prognosis’. This is especially important since both the probability of occurrence and the impacts are unknown, but the high uncertainty adds to the concern.
The need today to consider such extreme “high impact, low probability” or “worst case” – scenarios makes the society to face a possible grim and potentially dramatic future, with plenty of negative effects that many societal actors might rather want to avoid. Even though the consideration of these unpleasant scenarios might generate various levels of resistance it can also help the society to better avoid or adapt to some of the possible future events.
There have been extensive studies over the last decade, trying to draw an accurate picture of how a “dangerous climate change” would look like in the future. Even though a collapse of the thermohaline circulation (THC) has attracted most attention (2,3,4), there have been numerous studies on other very sensitive parts of the earth’s system of which every single one of them is characterized by its non-linear nature and with enough influence to lead the system into a new equilibrium. Among a possible die-off of the Amazonian Rainforest or the probability of a changing monsoon system in Asia, it is especially the likelihood of a collapse of the West Antarctic Ice Sheet (WAIS) that has caught many researchers’ interests over the last few years.

The collapse of the WAIS as a high impact, low probability scenario

Despite the entire Antarctic ice shield holds enough water to raise the global sea-level by about 57 metres, it is generally considered as being stable. However, latest studies suggest that the western part of the Antarctica (WAIS) might be more sensitive towards a global warming than previously assumed. The WAIS covers about 10 percent by volume of the entire Antarctic ice sheet, which is equivalent to a sea-level rise of about 6m. (5,6)

Previous state-of-the-art models aren’t sufficient to simulate current and future changes in the WAIS

Only a few years ago, in the last report of the UN’s Intergovernmental Panel on Climate Change (IPCC), it was commonly accepted that the WAIS was stable, and its ice mass balance was maintained by an equal amount of precipitation and iceberg calving on the one side and melting processes on the other side. Worries that the ice sheet could disintegrate in the future were firmly dismissed. As a matter of fact the numerical ice sheet models, as used in the IPCC report and elsewhere calculated the scenarios of a possible sea-level rise without considering the potential contribution melting polar ice sheets could have. The Antarctica had been characterized as a „slumbering giant” in terms of climate change.

The Awakened Giant

However, recent observations of unexpected melting processes particularly along the margins of Greenland and the West Antarctic have caused the scientific community to re-think their assumptions. Latest observations revealed that the air temperatures in the West Antarctic have risen approximately eight times faster than the global average during the last 50 years (7) and that ocean temperatures to the west of the Peninsula have also increased by over 1°C since 1955. (8)
However, recent satellite observations suggest that the local warming in the West Antarctic seems to have increased significantly particularly since 2002 and that it has triggered a notable increase in ice discharge, ever since then. During this specific period of time the Antarctica was losing 152 (+/- 80) cubic kilometres of ice annually, with the WAIS contributing the most to the Antarctica-caused sea-level rise of 0.4mm per year. (9)
The giant now seems to have awakened from his millennium lasting sleep.

Disintegrating ice-shelves as harbingers of a rapid collapse of the WAIS

Yet another observation could be of even greater concern. The changing mass balances of the WAIS indicate that the ice sheet looses most of its volume along the margins of the peninsula. Over the last few years the WAIS has suffered from a significant retreat or near-total loss of several ice shelves. An exceptional weather pattern in 2002 led to both unprecedented summer warming and intense, prolonged surface melting which culminated in the disintegration of the 500 billion tonne Larsen B ice shelf within less than a month. (10,11)

But even though warming ocean waters and thereby warming subshelf waters increase the basal melting of the ice shelves and increase the chances of a ice-shelf breakup, their collapse doesn’t directly contribute to a sea-level rise, since the shelves are already floating. Nevertheless ice-shelf collapses have been noteworthy, because apparently the backpressure on the area’s landlocked glaciers from the abutting ice shelves has helped to hold them in place. But with the collapse of the Larsen B ice shelf its tributary glaciers have sped up significantly, with more land ice falling into the sea, confirming the link between ice shelf stability and glacier force balance. (12, 13)

Recent satellite image analyses seem to support this assumption by showing that the ice streams in the Amundsen region are clearly retreating, thinning and accelerating, particularly the two Thwaites and Pine Island glaciers.
That means: if even bigger ice shelves, such as the Ross ice shelf to just name one of them, would collapse the probability of a total collapse of the whole WAIS would increase dramatically.
What further changes can we expect over the next 100 years?

What these recent studies have revealed are changes of greater scale, magnitude and speed than were considered possible before, with high impact scenarios becoming more probable in the future. Models used in previous projections apparently lack some of the physical processes that might explain the rapid dynamic changes and have to be improved if we want to find out what the future my hold.

Palaeoclimatic evidence for future Ice-sheet instability and rapid sea-level rise

Corals on tectonical stable coasts from the last interglacial period about 130.000 years ago and ice core records provide strong evidence that the WAIS responds very sensitive to a global temperature rise of about 2 degrees above pre-industrial levels. New research says that during the last interglacial there is strong evidence that sea-levels were about 12-18 feet higher than they are now. Even though most of the meltwater came from Greenland and other Arctic sources, these two sources couldn’t raise the sea-level up to its full amount. Consequently the Antarctic Ice Sheet melting must have produced the remainder of the sea-level rise and it could do the same if the Earth’s climate warms sufficiently in the future the study suggests. The study assumes that a possibly much reduced WAIS could have lowered the albedo and altered the circulation over a large area of Antarctica, resulting in a widespread melting along the margins of the whole Antarctic Ice Sheet. Assuming that Greenland and the WAIS both may have contributed to the sea-level high stand, 130.000 years ago, a latest state-of-the-art coupled atmosphere-ocean climate model suggests that mainly two factors may have led to a last interglacial collapse of the WAIS. The first factor may have been the high speed of sea-level rise due to the melting of Greenland, which could have helped destabilize the ice shelves surrounding the WAIS and may have even prevented the up lifted ice-shelves from regrounding, due to the reduced ability of isostatic rebound. A second factor may have been shallow ocean warming around and under the WAIS ice shelves that could have caused a further weakening and thinning from below. (14, 15)

Eemian conditions thanks to human-induced global warming

Given this palaeoclimatic evidence for the WAIS-instability and dramatic sea-level rise, the recently observed ice dynamics in the WAIS are a serious reason for concern. A comparison of the summer-season warmth sufficient to have caused much of the Greenland melting 130.000 years ago with present temperatures show that a greenhouse gas increase of 1 percent/ year could warm the Arctic by 3-5°C in summertime by 2130 (or sooner (16)), triggering the complete meltdown of Greenland and initiating the destabilization of the WAIS. Even more dramatic ocean warming along with surface air temperature increases (in all seasons) are very likely to continue throughout the next century, increasing the risk of a total collapse of the WAIS.

Fig. 3: Simulated shallow (200 m) annual mean ocean potential temperatures for each of four time periods, from left to right: present day, 130,000 years ago (anomalies from present day), 2100 A.D. (time atmosphere reaches three times preindustrial CO2 levels, climate anomalies from present day), and 2130 A.D. (four times preindustrial CO2 levels, climate anomalies from present day). (14)

Fig. 4: Simulated climate for each of four time periods, from left to right: present day (Modern), 130,000 years ago (anomalies from present day, LIG), 2100 A.D. (the time atmosphere reaches three times preindustrial CO2 levels, climate anomalies from present day, D AD 2100), and 2130 A.D. (four times preindustrial CO2 levels, climate anomalies from present day, D AD 2130). (14)

Even if the greenhouse gas emissions were to be stabilized by 2130 due to a substantial stabilisation effort we still would be “committed” to a sea-level rise of several meters and regardless of any negative feedbacks such as changing ocean circulations or an increase of precipitation over either one of the ice-sheets. (17) This illustrates the long time scales associated with sea-level rise and underlines the urgent need to act now by cutting down our greenhouse gas emissions drastically and developing adaptation strategies for a possibly warmer environment in the near future.

References:

(1) SCHNEIDER S.H. AND J. LANE, (2006): An Overview of "dangerous" climate change.
In: Avoiding Dangerous Climate Change, Schellnhuber H.J. (editor), Camebridge. pp 18.

(2) Link, P. M. and R.S.J. Tol (2004): Possible economic impacts of a shutdown of the thermohaline circulation: an application of FUND. In: Portuguese Economic Journal, 3, 99-114.

(3) Rahmstorf, S. (2000): The Thermohaline Ocean Circulation: A System with Dangerous Thresholds? An Editorial Comment. In: Climatic Change, 46 (3), pp. 247-256.

(4) Vellinga, N. and R.A. Wood (2002): Global climatic impacts of a collapse of the Atlantic thermohaline circulation. In: Climatic Change, 54, pp. 251-267.

(5) Oppenheimer, M. and R.B. Alley (2004): The west antarctic ice sheet and long term climate policy. In: Climatic Change, 64, pp. 1-10.

(6) Vaughan, D. G. and J. R. Spouge (2002): Risk estimation of collapse of the West Antarctic Sheet. In: Climatic Change (52), pp. 65-91.

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

(8) Meredith, M. P. and J. C. King (2005): Rapid climate change in the ocean west of the Antarctic Peninsula during the second half of the 20th century. In: Geophysical Research Letters, 32, L19604, doi:10.1029/2005GL024042.

(9) Velicogna, I., and J. Wahr (2006): Measurements of Time-Variable Gravity Show Mass Loss in Antarctica. In: Science 311, pp. 1754-1756.

(10) Van den Broeke, M. (2005): Strong surface melting preceded collapse of Antarctic Peninsula ice shelf. In: Geophysical Research Letters 32, L12815, doi:10.1029/2005GL023247.

(11) Rott, H. and W. Rack (2005): Pattern of retreat and disintegration of the Larsen B ice shelf, Antarctic Peninsula. In: Annals of Glaciology 39, 2004, pp. 505 - 510.

(12) Scambos, T. J. Bohlander, C. Shuman, and P. Skvarca. (2004): Glacier acceleration and thinning after ice shelf collapse in the Larsen B embayment, Antarctica. In: Geophysical Research Letters 31, L18402, doi:10.1029/2004GL020670.)

(13) Rignot, E., G. Casassa, P. Gogineni, W. Krabill, A. Rivera, and R. Thomas (2004):
Accelerated ice discharge from the Antarctic Peninsula following the collapse of Larsen B
ice shelf. In: Geophysical Research Letters 31, L18401, doi:10.1029/2004GL020697.

(14) Overpeck, J. T. et al.(2006): Paleoclimatic Evidence for Future Ice-Sheet Instability and Rapid Sea-Level Rise, In: Science, Vol. 311, Seite 1747.

(15) Rapley, C. (2006): The Antarctic Ice Sheet and Sea Level Rise. In: Avoiding Dangerous Climate Change, Schellnhuber H.J. (editor.), Camebridge, pp 25.

(16) Oppenheimer, M. and R.B. Alley (2004): The West Antarctic Ice Sheet and Long Term Climate Policy. In: Climate Change, Vol. 64, pp. 1-10.

(17) B. L. Otto-Bliesner et al. (2006): Paleoclimatic Evidence for Future Ice-Sheet Instability and Rapid Sea-Level Rise. In: Science. 311. no. 5768, pp. 1747 – 1750.