A fighter plane lost in Greenland in 1942 was found under 81 metres of ice
When a P-38 Lightning crash-landed on Greenland's ice in 1942, nobody expected it to vanish. Fifty years later it was dug out from beneath 81 metres of new ice. Ice sheets are still growing at their centres even as they flow, crack and melt at their edges.
An ice sheet, or continental glacier, is a body of ice covering more than 50,000 square kilometres of land. Only two exist now. Greenland's covers about 1.7 million square kilometres and Antarctica's about 14 million, each averaging roughly 2 kilometres thick, built up because most of the snow that lands on them never melts and is squeezed into ice by later falls. At the peak of the last glacial period, ice sheets also buried much of North America, northern Europe and southern South America; their retreat opened the current warm interval, the Holocene.
Even a stable sheet is always moving. Ice piles up on the high central plateau and spreads toward the steep margins, where melting and calving remove it. Geothermal heat warms the base, and meltwater there lubricates fast-flowing channels called ice streams. Remarkably, tides can nudge them: a 1-metre tidal swing can be felt up to 100 kilometres inland, and during spring tides an ice stream may sit nearly still for hours, then lurch about a foot in under an hour just after high tide.
Warming air takes around 10,000 years to conduct down through the ice, but it acts faster from above. Surface melt forms lakes, and those wider than about 300 metres can open a water-filled crack all the way to the bed, sending relatively warm water to the base. In the 1970s Johannes Weertman pointed out a further danger for ice resting on ground below sea level: as it thins, seawater can push underneath and lift it, a runaway known as marine ice sheet instability.
The entire West Antarctic Ice Sheet sits below sea level, and its Thwaites and Pine Island glaciers have been thinning and speeding up for decades, which could add tens of centimetres to sea level rise this century. A more extreme idea, that tall ice cliffs will collapse under their own weight in a chain reaction, remains highly controversial.
Source: Ice sheet
