JUNEAU, Alaska — The idea that glaciers change at a glacial speed is increasingly false. They are melting and retreating rapidly all over the world. But the unpredictable flood surges at the Mendenhall Glacier, about 14 miles from downtown Juneau, Alaska’s capital, are turning a jog into a sprint as global temperatures and climate variability increase.

Starting in July 2011, and each year since, sudden torrents of water shooting out from beneath the glacier have become a new facet of Juneau’s brief, shimmering high summer season. In that first, and so far biggest, measured flood burst, an estimated 10 billion gallons gushed out in three days, threatening homes and property along the Mendenhall River that winds through part of the city. There have been at least two smaller bursts this year.

“That first one caught us by surprise,” said Tom Mattice, the emergency programs manager and avalanche forecaster for the City and Borough of Juneau.