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Fact Corner – Melting ice-shelves

1st November 2008

In March 2008, a vast chunk of ice broke off from the West Antarctic, threatening the collapse of a much larger ice shelf behind it – shocking climate scientists worldwide.  The Wilkins shelf, an area of 14,500 square kilometres, has lost over 160 square miles since the end of February and is hanging by just a thin thread of ice.  The Antarctic Peninsula has experienced unparalleled warming over the past 50 years, with six other ice shelves already lost entirely.  As the shelf is already afloat, it will not affect sea level.  However, Antarctica’s ice shelves act as safeguards for land ice that could lead to dramatic changes in sea level if it melts.

Sea-level rise at the global level has three main components; thermal expansion due to warming of oceans, meltwater from glaciers and meltwater from polar ice-sheets.  The IPCC has observed a 17cm (approx) rise in sea-levels in the 20th century. 

In 2007, The IPCC provided six scenarios on rising sea levels for the 21st Century, with the most optimistic scenario sea level rising at 18 – 38 cm, and the most pessimistic at 26 – 59 cm.   Critics of the IPCC report claim that the sea-level rise projections do not include important feedbacks, particularly concerning polar ice-sheet dynamics, with new research indicating that sea-level rise could be much higher in the 21st Century. 

As early as the late 60’s, Mercer , a glaciologist, pointed out the problem in the West Antarctic Ice Sheet; this enormous mass of ice, separated by a mountain range from the majority of the continent, was held back in a delicate balance by the shelves of ice floating at its rims. He pointed out how a slight change in conditions could prompt an ice shelf to break up. Although the sea level would not change,  the larger mass of ice, the ice sheet, reinforced by the ice shelves would surge into the ocean, increasing the sea-level.  This melting could also encourage feedback mechanisms that intensify the effects of greenhouse gases in the atmosphere; with decreasing levels of ice and snow, which reflect the sun, there will be greater heat absorption by the Earth.

Critics argue that critical aspects of ice-sheets are excluded from the IPCC report because of uncertainty in ice-sheet dynamics. The question lies in the pace of the melting and surge of the ice sheet into the ocean once the ice shelves have split. Many models indicate that it is not possible for a large mass of Antarctic ice to collapse altogether in the 21st Century, and that if the Antarctic Ice Sheet were to diminish, it would happen over several centuries.  One critic, Hansen , scientist and administrator, argues that the slow melting of ice sheets that IPCC expects does not fit the data, claiming that ice at the poles do not melt in a gradual and linear fashion but change abruptly from one state to another.  Hansen argues that rapid increases are far more likely than linear changes and a multi-metre rise in sea-level this century is more likely.   Historically, there are numerous examples of ice-sheets ceding a sea level rise of several meters per century with temperature increases less than that of the scenarios for the 21st century. 200,000 years ago, during the last warm (inter-glacial) period when global temperature was several degrees higher, sea level was 5-6m higher than today due to melting of polar ice-sheets. And 3.5 million years ago, when temperatures increased 2-3 degrees above today’s level, sea levels rose by 25 metres.

Nonetheless, one thing is certain: if temperatures climb a few degrees, as is generally accepted by climate scientists, the sea level would rise simply because water expands when heated. Rahmstorf , oceanographer and climatologist, estimates that we could see a sea-level rise of 0.5m-1.4m by 2100 – and this number does not include significant disintegration of ice sheets.

Currently, about half the world’s population live in coastal areas; the potential impacts of sea-level rise are daunting.  If the sea level in China were to rise by 0.5m, this would inundate 40,000km2 of land and displace 30 million people.  In Bangladesh and Egypt, a sea-level rise of 1m could displace 15 and 7 million people, respectively.  Small island states would become uninhabitable. 

The additional effects of ice sheet melting remained highly uncertain. Even today, scientists are still debating how much of the 20thcentury’s sea level rise was due to thermal expansion and how much due to ice melting. The future of sea level depends crucially on what will happen to these ice sheets which is proving to be a daunting task to predict.

References:
Spencer Weart, Ice Sheets and Rising Seas , http://www.aip.org/history/climate/floods.htm
James Hansen, Business As Usual Will Cause Catastrophic Sea Level Rise, http://climateandcapitalism.com/?p=148

 

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