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　　The world’s mountain glaciers contain just 1 percent of all the ice on Earth. But their rapid melting accounts for about a third of recent sea level rise. Most of these glaciers—that start and end on land—shrink away through gradual surface melting. But others, known as tidewater glaciers, flow out into the sea. And by calving off massive icebergs, these glaciers can collapse in the blink of an eye, geologically speaking.

　　For example, the Columbia Glacier in Alaska’s Chugach Mountains, is a tidewater glacier that once filled a long fjord. It’s retreated 12 miles in just the last 25 years.

　　Shad O’Neel: “So this tiny little glacier in Alaska is capable of producing 1 percent of the sea level rise budget. And so we know that individual calving glaciers can do crazy things and can act really dramatically and can have this rapid response.”

　　Shad O’Neel, a glaciologist with the U.S. Geological Survey in Anchorage and co-author of a new study in the journal Geophysical Research Letters. [C. F. Larsen et al, Surface melt dominates Alaska glacier mass balance]

　　Scientists thought that tidewater glaciers, because of their unstable behavior, might be an important source of the torrent of meltwater flowing out of Alaska and northwest Canada, where ice has been disappearing at especially alarming rates. But all mountain glaciers, tidewater and otherwise, are notoriously hard to quantify.

　　So O’Neel and his colleagues undertook the daunting task of assessing the health of every glacier in the region. Using data collected from airplanes, they tracked how the masses of 116 glaciers, making up 41 percent of Alaska’s ice-covered area, changed during two decades. Then they used similarities in the shape, size and location of the remaining 27,000 glaciers to estimate how those changed, too.

　　The results surprised them: Tidewater glaciers accounted for just 6 percent of the region’s total ice loss. The vast majority came from surface melting of glaciers that sit completely on land.

　　[O'Neel:] “Your initial response is, 'Wow, this is great, the tidewater glaciers actually aren’t doing that much in Alaska.'”

　　That sounds like good news, because tidewater glaciers can shed a lot of ice really quickly. But scientists expect the sea-level contributions of tidewater glaciers to wind down in the near future, as they retreat up fjords and onto land.

　　[O'Neel:] “But the surface melt part isn’t going to decrease.”

　　Which we now know drives most of the region’s ice loss.

　　[O'Neel:] “It means that while we might have more certainty about what’s happening, that means we have more certainty that Alaska’s contribution is going to stay the same or grow if climate continues to warm.”

　　—Julia Rosen

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