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TOPICAL SCIENCE Supplement July 2017 Special Feature: Birth of an Iceberg On the 12th of July 2017, news agencies around the world reported that a huge iceberg, weighing more than a trillion tons, had broken off from the Larsen ice shelf in Antarctica. With an area of 5,800 square kilometres, it was said to be comparable in size to County Galway, or about a quarter the size of Wales. Labelled A68, it is believed to be among the 10 biggest icebergs ever recorded, though not the biggest. Iceberg B-15, which split off from the Ross ice shelf in the year 2000, was twice as big.

The picture below shows the Larcen C Ice Shelf compared to the map of Wales.

Icebergs and the Hydrological Cycle. This so-called ‘calving’ of icebergs is a natural process and is part of the hydrological cycle, so this event was not unexpected. Scientists had been monitoring the situation for some time. Over the past few years, an enormous rift, or crack, had developed in the Larcen C ice shelf. But the calving reduced the area of the ice shelf by more than 12 percent, which has changed the landscape of the Antarctic peninsula.

The rift, in November 2016 The progress of the rift, and the loss of the iceberg, has been carefully followed by analysis of radar images from the European Space Agency’s Sentinel-1 mission, which provides data from the region every six days. The rift suddenly increased in length by 17 km during the last week in May. Then, towards the end of June, the process accelerated, as the movement of ice reached a rate of more than 10 metres per day, but it was not until the middle of July that the iceberg finally broke away. Journalists asked if this event could be linked to climate change, but scientists say there is no evidence for this, nor is it s sign that the ice shelf is disintegrating, as it is inevitable that ice shelves break up as they extend farther out from the land. Nevertheless, when such a large piece breaks off, it is a dramatic event.

Aerial photograph of the rift.

Satellite Image of the rift.

Ice Shelves and Ice Sheets.

Ice sheets occur on land, in polar regions. In the northern hemisphere, the Greenland ice sheet is the largest. In the southern hemisphere, the entire continent of Antarctica provides the foundation for enormous ice sheets. Ice shelves, on the other hand, do not have any land underneath them for support.

They are floating on the sea, but are attached to grounded ice sheets. These ice shelves may be hundreds of metres thick. They are fed by graciers, that slowly, but inexorably, flow towards the sea, just as rivers do, but at a much slower pace. The ice sheets hold back and tend to slow down the movement into the sea of the glaciers that feed them. So, once an iceberg breaks off from an ice sheet, the rate of flow of ice from the glacier to the sea initially speeds up, until a new ice shelf starts to develop.

Freshwater Storage There is no shortage of water on our planet Earth, but most of it occurs as salt water in the oceans. The Greenland and Antarctic ice sheets in fact store most of the earth’s fresh water. Almost 90 percent of Earth's ice mass is in Antarctica, while the Greenland ice cap contains 10 percent of the total.

The amount of water locked up in glaciers and ice caps is only a small percentage (1.7%) of all water on the Earth, but it accounts for 68.7% percentage of the world's total freshwater.

The Greenland ice cap ice cap contains 2.5 million cubic kilometers (km3) of ice because, over the millennia, more snow has fallen there than has melted. As the snow got deeper, it was compressed under its own weight and turned to ice. Greenland has many glaciers and it is estimated that 517 km3 of ice "calves" into the ocean each year. This is one of Greenland's contributions to the global water cycle. The icebergs are carried along by ocean currents, melting along the way. The iceberg that sank the Titanic in 1912 had originated from Greenland and has melted without trace.

Ice Caps, Glaciers and Climate Change.

Climate change has always been a feature of our planet. During the period when the dinosaurs lived, the earth was much warmer than it is now. But there have also been ice ages.and many cold periods, such as the last ice age of about 18,000 years ago. The polar ice caps become enlarged during ice ages, extending in the northern hemisphere over most of northern Eurpoe. Because they store water in solid form, ice ages are accompanied by a lowering of sea level.

Ice is very white, and since white reflects sunlight (and thus, heat), large ice fields can affect the weather. Air temperatures can be higher a couple of kilometres above ice caps than at the surface, and wind patterns, which affect weather systems, can be dramatic around ice-covered landscapes.

Away from polar regions, high altitude glaciers in mountain regions also store large amounts of water.

Glacial ice covers about 10 percent of all land. According to the National Snow and Ice Data Center (NSIDC), if all glaciers melted today, the seas would rise by about 70 meters. During the last ice age, the sea level was about 122 meters lower than it is today. At that time, glaciers covered almost onethird of the land

Bering Glacier in Alaska is the largest glacier in North America.

Credit: NASA Earth Observatory Conversely, during warm periods, glaciers recede, polar icecaps melt and there is a rise in sea level.

During the last warm spell, 125,000 years ago, the seas were about 5.5 meters higher than they are today. About three million years ago the seas could have been up to 50.3 meters higher.

So, while the calving of iceberg A68 from the Larcen ice shelf has atracted a great amout of media attention, it will not cause an immediate rise in sea level. The iceberg was already floating while it was part of the ice shelf and will continue to float, without any change in sea level. As it gradually melts, it will eventually increase the total volume of liquid water in the ocean, but not to a signifigant degree.

Author: Margaret Franklin