Academic Reading — Test 191

Reading passage

The Larsen Ice Shelf lies along the eastern coast of the Antarctic Peninsula and is conventionally divided into three sections — Larsen A, Larsen B and Larsen C — named in order from north to south. Unlike sea ice, which forms each winter from frozen ocean water and melts back again in summer, an ice shelf is a thick, floating extension of a land-based glacier that has flowed out over the sea while remaining attached to the ice sheet behind it. Some ice shelves are hundreds of metres thick and extend for tens of kilometres beyond the coastline before their outer edge finally gives way to open water. Because ice shelves act as a brace, slowing the seaward movement of the glaciers that feed them, scientists regard their stability as an important indicator of the wider health of the Antarctic ice sheet, and consequently as a useful early signal of broader change in the polar climate system. Calving — the breaking away of a mass of ice from the edge of a glacier or ice shelf — is a normal and continuous process, but the Larsen shelves have experienced calving events of an unusual scale in recent decades. Larsen A collapsed abruptly in 1995, losing the greater part of its area within a few weeks, and Larsen B underwent a far more dramatic disintegration in 2002, when roughly 3,200 square kilometres of shelf fragmented into thousands of icebergs within little more than a month. Glaciologists who studied the 2002 event concluded that meltwater pooling on the surface of the shelf during an unusually warm summer had trickled into existing crevasses, prising them apart through a mechanism now generally known as hydrofracturing. Once enough crevasses had been driven through the full thickness of the shelf, the remaining ice could no longer hold together under its own weight, and the structure broke apart with remarkable speed. In July 2017, Larsen C, the largest of the three remaining sections, released an iceberg subsequently named A68, with a surface area of nearly 5,800 square kilometres, among the biggest ever documented at the time. The rift that produced A68 had been monitored for several years beforehand, its slow widening tracked by researchers using satellite radar, an instrument capable of imaging the ice shelf regardless of cloud cover or the darkness of the polar winter, when months pass without sunlight. Although the calving of A68 itself was not unexpected, given how closely the rift had been watched, its sheer scale renewed concern among scientists about the structural integrity of Larsen C and the inland glaciers it currently restrains. Satellites equipped with radar altimeters and synthetic aperture radar instruments now allow scientists to observe the Antarctic coastline at a frequency that would have been unimaginable a generation ago, when researchers relied chiefly on occasional aerial photography and infrequent ship-based surveys. A single rift can now be measured every few days rather than once a season, allowing subtle changes in width and depth to be charted almost in real time. This continuous record has revealed that icebergs calved from the Larsen shelves do not simply drift away intact; many run aground on shallower seabed, rotate slowly under the influence of currents, or break into smaller fragments long before they melt completely, occasionally posing a hazard to shipping in the Southern Ocean. Tracking the resulting debris field also helps researchers estimate how much freshwater is being added to the surrounding sea, since meltwater released by drifting icebergs can alter local salinity and, in turn, ocean circulation patterns. A central question for glaciologists is whether the recent calving episodes reflect a natural cycle of advance and retreat or are instead being driven primarily by a warming climate. The Antarctic Peninsula warmed markedly during the latter half of the twentieth century, faster than the global average over the same period, and several studies have linked this pronounced regional warming to the destabilisation of the northern Larsen sections. However, attributing any single calving event to climate change is difficult, since ice shelves periodically reach a critical size and shed material as part of an ordinary growth-and-loss rhythm that predates significant human influence on the climate. Most researchers therefore treat each new calving event as one data point within a much longer record, to be interpreted cautiously rather than in isolation, and resist drawing firm conclusions from any single occurrence. Continued monitoring matters because the consequences of shelf loss extend well beyond the immediate area of the Antarctic Peninsula. Should Larsen C eventually disintegrate in the manner of Larsen B, the glaciers it currently holds back would be free to flow into the ocean considerably faster than at present, contributing to global sea level rise over subsequent decades. For this reason, agencies responsible for polar research have prioritised sustained satellite coverage of the region, ensuring that any future acceleration in calving can be detected and analysed promptly rather than discovered only well after the event has taken place.