Ice Shelves

The break-up of the Larsen B ice shelf occured in early 2002. This event has been attributed to the effects of global warming. That it occurred is beyond dispute and that it is a result of the warming of the Antarctic Peninsula where it is situated is also beyond dispute. What remains unclear is whether or not this is a taste of things to come and an indicator of an Antarctic-wide phenomena or simply a localized result of the localized warming of the Antarctic Peninsula region alone.

An ice shelf is a thick layer of ice that is floating on the sea. They are fed from the land by glaciers. Where the ice leaves the land and starts to float on the sea is a region known as the “hinge zone” where the ice is particularly chaotic, broken-up and a nightmare to try and travel over. Ice shelves surround much of Antarctica.
The Larsen B ice shelf was about 220m thick (720 feet) and during a 35 day period in early 2002 lost about 3,250 km2 of ice into the ocean. It is thought to have been in existence for at least 400 years prior to this and probably as long as 12,000 years since the end of the last ice age.

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Such disintegration in such a short time period is therefore an extremely significant event. What now remains of the Larsen B is about 40% of what was there in 1995. It had been breaking up at what was considered to be a rapid rate anyway before this major event. The break-up is thought to be a consequence of higher temperatures and large amounts of summer melt-water running down crevasses in the ice shelf so speeding the disintegration process. Overall in the Antarctic Peninsula, seven ice shelves have between them declined in area by about 13,500 km2 since 1974.

A more recently seen phenomenon that follows this ice shelf collapse is that the glaciers that fed the ice shelf seem to now be speeding up their flow down to the sea. This will certainly deposit more water in the oceans, and as this was previously on the land it will add to an increase in sea-level. The Antarctic peninsula doesn’t have enough ice to make much of a difference to sea level in itself even if it were all to melt, but it is best seen as an indicator region that can be observed to enhance understandings of the mechanisms in other parts of the world.

Biological

Antarctica’s only two flowering plant species that grow only on the Peninsula have spread considerably in the last few decades in both geographic distribution and also abundance in the areas where they are found. In some areas they are becoming the dominant species. Adelie penguins (Pygoscelis adeliae) have also been suffering a steady decline in parts of the Antarctic Peninsula region for the last 20 years. Adelie are reducing in number and abandoning certain nesting sites while Chinstrap penguins (Pygoscelis antarctica) are taking their place. Adelie require pack ice for most of the year and feed almost exclusively on Krill, Chinstrap penguins will eat a wider variety of foods and prefer open water. The sea ice has declined over the last 20 years with the rise in temperature in the Peninsula region. Gentoo penguins (Pygoscelis papua) have also started to nest on the Peninsula in recent years, for the first time in living memory ( and it needs to be noted that any memory of Antarctica doesn’t stretch much beyond only a hundred years). Studies of the bones and remains found in abandoned colonies indicate that prior to 1950, no Gentoo penguins nested in these sites at all.

Krill shortages

Studies (November 2004) have shown that stocks of krill in Antarctica have declined dramatically in recent years. The reason for this is likely to be a fall in the amount of sea ice in the winter months particularly in the Antarctic Peninsula region. Krill numbers may have dropped by as much as 80% since the 1970’s - so today’s stocks are a mere 1/5th of what they were only 30 years ago. The decline in krill may in turn account for the decline in the numbers of some penguin species. Dr Angus Atkinson from British Antarctic Survey, says:”This is the first time that we have understood the full scale of this decline. Krill feed on the algae found under the surface of the sea-ice, which acts as a kind of ‘nursery’. The Antarctic Peninsula, a key breeding ground for the krill, is one of the places in the world where there has been the greatest rise in temperatures due to global warming. This region has warmed by 2.5C in the last 50 years (much more than the mean global rate), with a striking consequential decrease in winter sea ice cover. “We don’t fully understand how the loss of sea-ice here is connected to the warming, but we believe that it could be behind the decline in krill.”

There are commercial implications as well as scientific ones. The Southern Ocean is a valuable fisheries resource, many of the species caught feed on krill. Thousands of tourists are also attracted to Antarctica to enjoy the spectacular wildlife, most of which feed on krill.