{"id":4704,"date":"2015-10-29T16:27:58","date_gmt":"2015-10-29T11:27:58","guid":{"rendered":"http:\/\/www.cawater-info.net\/all_about_water\/en\/?p=4704"},"modified":"2015-10-31T16:28:32","modified_gmt":"2015-10-31T11:28:32","slug":"theres-no-way-to-stop-the-water-and-no-time-to-waste-the-science-behind-rising-sea-tides","status":"publish","type":"post","link":"http:\/\/www.cawater-info.net\/all_about_water\/en\/?p=4704","title":{"rendered":"\u201cThere\u2019s no way to stop the water, and no time to waste\u201d: The science behind rising sea tides"},"content":{"rendered":"

A hundred years after it spawned the iceberg that sank the Titanic in the North Atlantic, the Jakobshavn Glacier is now a major contributor to global sea-level rise, this time threatening the homes and lives not of 2,200 passengers and crew but of a billion people across the world.<\/em><\/p>\n

\"01\"<\/a>As climate-watchers and coastal-dwellers keep a weather eye out for signals of irreversible changes in the environment, the world\u2019s fastest-moving glacier has already begun self-destruction.<\/p>\n

Jakobshavn is now shedding ice nearly three times as quickly as it was 20 years ago, dumping enormous and growing quantities into the ocean. It\u2019s contributed 0.1 millimeters per year to worldwide sea-level rise \u2014 more than 3 percent of the 3 mm produced globally \u2014 for the past decade.<\/p>\n

The glacier \u201chas been retreating for the last 100 years,\u201d according to Ian Joughin, senior principal engineer at the Polar Science Center, part of the University of Washington\u2019s Applied Physics Laboratory. \u201cRetreat\u201d means a glacier is shrinking in length, losing more ice from its face that meets the water than accumulates from higher up.<\/p>\n

But it was only in recent decades that the retreat reached extreme levels.<\/p>\n

Jakobshavn\u2019s story isn\u2019t unique. For decades now, more ice has been melting into the ocean than is falling from the sky in the world\u2019s mountain and polar regions, where ice sheets store two-thirds of the planet\u2019s fresh water \u2014\u00a0and the science shows us the situation won\u2019t reverse any time soon.<\/p>\n

To understand exactly what\u2019s happened and what\u2019s likely to come, it\u2019s critical to understand the topography underneath each glacier.<\/p>\n

First, a note on how to think about glacial ice: it\u2019s not as simple as frozen water. Scientists consider glaciers to be \u201cnonlinear viscous fluids,\u201d which behave like both solids and liquids. Think of a glacier as a frozen river, always flowing at some speed from source to outlet, but growing and receding with the seasons. Because ice is heavy and not a perfect solid like rock, it flows under gravitational pull and pressure from above. Sometimes big chunks become unstable and fall into the sea. When Earth\u2019s climate is in balance, about the same amount of water flows into the oceans from glaciers as is evaporated and then precipitated as snow onto ice sheets from which those glaciers are made.<\/p>\n

The Jakobshavn Glacier, known in Danish as Jakobshavn Isbr\u00e6, has its origins in large areas of land well above sea level, from which it flows down toward the ocean (in this case, Ilulissat Icefjord), where it calves icebergs into the water.<\/p>\n

As physicist Joughin describes it, Jakobshavn Glacier flows off the land and into the 1,600-meter-deep fjord, filling it entirely with ice for a distance of about 50 kilometers, ultimately climbing up a slope in the sea floor that peaks at about 600 meters of depth. The narrow sheet of ice coming off the edge of the glacier at that peak \u2014\u00a0called a \u201cglacier tongue\u201d \u2014 once served as a sort of \u201ccork\u201d for the glacier, holding it back significantly and preventing quicker loss of ice.<\/p>\n

You can see the rise in sea-bed elevation just to the left of center inthis graphic.<\/p>\n

In 1992, Jakobshavn was melting at a rate of about 6 kilometers a year. It was \u201cabout in balance\u201d with the natural rhythm \u2014 gaining and losing roughly the same amount of ice over the course of a year\u2019s winter accumulation and summer melting, Joughin said.<\/p>\n

But in the late 1990s, the glacier\u2019s tongue broke off, and the \u201cuncorked\u201d Jakobshavn began to calve and lose mass in ever-deeper water.<\/p>\n

By 2000, the glacier was losing 11 kilometers in length every year, nearly twice the stable speed. As of last summer, according to a paper Joughin and others published recently in academic journal The Cryosphere, it was losing nearly 17 kilometers a year, retreating up the fjord into increasingly deep water that could cause it to melt even faster in the coming decades.<\/p>\n

\"02\"<\/a>This graphic shows ice flow velocity as color over SAR amplitude imagery of Jakobshavn Isbr\u00e6 in a) February 1992 b) October 2000. In addition to color, speed is contoured with thin black lines at 1000 m\/yr intervals and with thin white lines at 200, 400, 600, and 800 m\/yr. Note how the ice front has calved back several kilometers from 1992 to 2000. Further retreat in subsequent years caused the glaciers speed to increase to 12,600 km\/yr near the front. The town of Illulisat is just off the edge of the image on the north side of the fjord. (Courtesy of PSC\/APL\/UW)<\/p>\n

Jakobshavn\u2019s dramatic change was recorded in the 2012 film \u201cChasing Ice,\u201d in a compelling scene that captured the calving of a kilometer of ice in a single event. That happens throughout the summer, Joughin said, though not always in such significant individual moments. (When it does, though, global seismic monitors have been known to register them as 4 or 5 on the Richter scale, he said.)<\/p>\n

Here\u2019s the clip from the documentary:<\/p>\n

Eventually \u2014 perhaps in about 100 years \u2014 the glacier will have retreated far enough that it will no longer feed directly into the fjord. At that point, essentially landlocked, the glacier will only shrink through melting, which happens much slower than calving. With that slowing will come more stability in terms of the glacier\u2019s size. \u201cThe next stable condition could be a regionally smaller ice sheet,\u201d Joughin said.<\/p>\n

Jakobshavn would still contribute a significant amount of global sea-level rise before then. But the real danger lies at the other end of the Earth, in the West Antarctic Ice Sheet (WAIS), which holds enough water to raise the ocean between three and six meters.<\/p>\n

The WAIS is also melting, but it\u2019s doing it in open water; once the process starts, the sheet will never stop calving.<\/p>\n

\u201cThe [WAIS] glaciers are going to keep retreating. At this point there is nothing we can do but watch,\u201d said Eric Rignot, a glaciologist at the University of California Irvine who published his latest paper about the WAIS in December\u2019s Geophysical Research Letters. \u201cJust how fast they can flow, we don\u2019t know,\u201d he said.<\/p>\n

It could take hundreds or thousands of years, but as Joughin puts it, the next stable point for WAIS is \u201cno ice sheet.\u201d By then huge areas of land, home to massive proportions of the world\u2019s population, would be under water.<\/p>\n

The question facing scientists and coastal dwellers is akin to the one facing the Titanic\u2019s passengers: The water is rising, and we don\u2019t quite know how fast it\u2019s coming, or how quickly it will accelerate. But we need to plan, move, and adapt if we are to survive. There\u2019s no way to stop the water, and no time to waste.<\/p>\n

Source: http:\/\/www.salon.com\/<\/a><\/p>\n

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