HELHEIM GLACIER, Greenland — Suddenly and without warning, the gigantic river of ice sped up, causing it to spit icebergs ever faster into the ocean off southeastern Greenland. The Helheim Glacier nearly doubled its speed in just a few years, flowing through a rift in the barren coastal mountains at a stunning 100 feet per day. Alarm bells rang as the pattern was repeated by glaciers across Greenland: Was the island’s vast ice sheet, a frozen water reservoir that could raise the sea level 20 feet if disgorged, in danger of collapse? Half a decade later, there’s a little bit of good news - and a lot of uncertainty. ‘It does seem that the very rapid speeds were only sustained for a short period of time, although none of these glaciers have returned to the ‘normal’ flow speeds yet, says Gordon Hamilton, a glaciologist from the University of Maine who’s clocked Helheim’s rapid advance using GPS receivers on site since 2005. Understanding why Greenland’s glaciers accelerated so abruptly in the first half of the decade - and whether they are now slowing down - is crucial to the larger question of how fast sea levels […]