Centuries from now, a large swath of the West Antarctic ice sheet is likely to be gone, its hundreds of trillions of tons of ice melted, causing a 4-foot rise in already swollen seas.
Scientists reported this month that the scenario may be inevitable, with new research concluding that some giant glaciers had passed the point of no return, possibly setting off a chain reaction that could doom the rest of the ice sheet.
For many, the research signaled that changes in the earth’s climate have reached a tipping point, even if global warming halted immediately.
But the mountain glaciers have been telling scientists what the West Antarctica glacier disintegration is now confirming: In the coming centuries, more land will be covered by water and more of nature will be disrupted. A full melt would cause sea level to rise 215 feet.
During recent ice ages, glaciers expanded from the poles and covered nearly a third of the continents. And in the distant past, there were episodes known as Snowball Earth, when the entire planet froze over. At the other extreme, a warm period near the end of the age of dinosaurs may have left the earth ice-free. Today the amount of ice is modest – 10 percent of land areas.
Glaciers are, simply, rivers of ice formed from snow in regions that are frozen year-round. The snow compacts over time into granular, porous ice, called firn. When firn compacts even more, it becomes glacier ice, which flows, usually slowly, down mountainsides. Depending on how fast new snow accumulates at the top, or melts at the bottom, a glacier grows or shrinks.
Not long ago, the only way to measure glaciers was to put stakes in the ice. Using surveying tools, glaciologists would mark the location and return later to see how far the ice had moved. Today, satellites provide a global view. Images show where the glaciers are and how areas change over the years. Most useful has been NASA’s Gravity Recovery and Climate Experiment, or GRACE. Two identical spacecraft have been measuring the earth’s gravity. When glaciers melt, the water flows elsewhere, and that part of the planet weighs less, slightly weakening its gravitational pull.
Another NASA satellite, IceSat, bounced lasers off the ice to precisely measure glaciers’ height. An analysis last year concluded that, on average, glaciers in all regions were withering away, dumping 260 billion metric tons of water into the ocean every year.