An area of ice thought to weigh almost 500 billion tonnes has broken off the Antarctic continent and shattered into thousands of icebergs in one of the most dramatic examples yet of the effects of climate change.
Scientists from the British Antarctic Survey said yesterday the speed of the complete disintegration of the 200-metre thick, 3,250 square kilometre Larsen B iceshelf was "staggering".
They had predicted the collapse of the continent's northernmost iceshelf four years ago following evidence of the retreat of many glaciers on the Antarctic Peninsula, but the final break up of the whole shelf took just 31 days and has shocked glaciologists with its scale and speed. The shattered ice has formed a plume of thousands of icebergs, adrift in the Weddell Sea, east of the peninsula.
"We knew what was left of the Larsen B iceshelf would collapse eventually, but this is staggering," said Dr David Vaughan, a glaciologist at the BAS in Cambridge. "It's just broken apart. It fell over like a wall and has broken as if into hundreds of thousands of bricks".
The collapse is believed to have dumped more ice into the Southern Ocean than all of the previous half century's icebergs combined. "This is the largest single event in a series of retreats by ice shelves in the peninsula over the last 30 years. The retreats are attributed to a strong climate warming in the region," said the US government's Ice Center.
The Ice Centre added to concerns over global warming by reporting on Monday that a monster iceberg broke off in the past weeks from the Thwaites ice tongue into the southern Amundsen Sea. Iceberg B22, as it is now known, is more than 64 kilometres (40 miles) wide and 85 kilometres (53 miles) long, and covers an area of about 5,500 square kilometres.
Ice shelves are floating plates of ice up to 800 metres thick that are still attached to the bedrock of continents and which form when large glaciers flow toward the ocean in polar areas.
Glaciologists do not fully understand why the Antarctica peninsula has warmed almost 2.5C in just 50 years, a rate at least five times that of warming elsewhere. Temperatures in the region are believed to be the warmest now for more than 1,800 years. However temperatures in the bulk of Antarctica are not consistently warming and in some places may be actually decreasing.
British and US scientists using satellite pictures, together with evidence collected by Greenpeace, discovered that the ice shelves on either side of the 1,300 kilometre (800 mile) peninsula were in full retreat in 1999.
Until the last month, it is thought some 7,700 sq km of coastal ice shelves had disintegrated since 1945 with several major collapses. Two smaller ice shelves, called the Larsen A and the Prince Gustav disintegrated in 1995 after years of shrinking. A smaller ice shelf, the Wordie, disappeared in the late 1980s. Two other ice shelves, the Wilkins and the George VI, are also thought to be on the point of collapse.
But nothing, says the British Antarctic Survey, compares with the final collapse of the Larsen B. "It could well be regional amplification of global warming", said Dr Vaughan. "All we know is that the climate has changed significantly in this area. The Larsen B is only a tiny part of the whole Antarctic shelf, but it shows us how climate change will affect the world in the future. Some areas will warm much more than others."
Yesterday the environment minister Michael Meacher said the collapse of the Larsen B shelf was "a great cause of concern and a wake-up signal to the whole world". "When an ice-shelf of such enormous proportions can break up, that shows the effect that we are having. The rapid warming at the Antarctic Peninsula is broadly consistent with global warming but it is not understood why its rate of warming is so much greater than the global average".
Because the Larsen B iceshelf and the Thwaites ice tongue were already floating, their disintegration will have no impact on sea levels which will only rise if the ice held back on land by the ice shelf flows more quickly onto the sea. Over the next months and years the icebergs formed will drift north under the action of ocean currents and wind and will melt rapidly.
A spokesman for Greenpeace international said yesterday that the current melting trend was an acutely disturbing sign of global warming, and it may be the precursor for more dangerous events.
"This area and that of the western Arctic off Alaska are the two most rapidly warming places on the globe. The trends of melting ice shelves is now clear," said Steve Sawyer, a climate change scientist.
The real worry is what may happen to the much larger areas of ice which are grounded on land. It has been predicted that if temperatures across Antarctica continue to rise the whole West Antarctic Ice Sheet could eventually collapse leading to a rise in sea levels of 5-6 metres.
The Western Antarctic Ice Sheet is grounded, not on rock, but on a muddy ocean floor, and is buttressed by floating ice shelves. Though it appears to be solid, the ice is actually in a constant state of flux, with melting ice at the margins being replaced by glacial movements from the interior of the continent.
A further fear of international climate scientists is that the disappearance of large quantities of ice means the Antarctic could warm up more quickly because 80% of the heat and light of the sun is reflected back into space from ice or snow. With a loss of sea ice and ice shelves, reduced albedo, or reflective power, causes a change in the absorption of heat and carbon dioxide, which means more warming.
Melting signals change
· Ice shelves cover 50% of the Antarctic coast. The surface area of all ice shelves together is more than one-tenth the size of the continent.
· The largest individual shelf is the Ross ice shelf in West Antarctica, also called the Great Ice Barrier. It is as big as France.
· Ice shelves float up and down with the tides, grating against the rocks and eventually breaking apart.
· Every year, the edges of ice shelves break off, or calve, into icebergs as a result of seasonal warming.
· During normal years, the total mass of calvings is an extremely small percentage of the ice cap, and the ice lost through calving equals the mass of snowfall on the continent.
· During the past few years, ice shelves on the Antarctic peninsula have been quickly melting.
· Some scientists fear that the region has now entered a vicious cycle of polar warming, with rising average temperatures, fewer cold years and longer summer melting, resulting in the warming of Antarctica's waters.