The World in 2050_ Four Forces Shaping Civilization's Northern Future - Laurence C. Smith [120]
But by morning the winds would die down and we went to work. Jason installed time-lapse cameras to track the speed of the glacier’s sliding snout; I submerged electronic sensors in its outgoing torrent of meltwater to monitor how much was flowing off to the sea. We were studying these things to help answer a burning scientific question that should worry us all. Chapter 4 showed that we are facing decimeters of sea-level rise by century’s end. Many scientists wonder if even these estimates might be too low. Could climate warming cause the Greenland and West Antarctic ice sheets to accelerate their dumpage of ice and water into the sea, thus cranking up its rise even faster than is happening already? Could the world’s oceans go even higher, say a couple of meters by the end of this century?
The short answer is maybe. The geological record tells us sea levels are certainly capable of responding quickly to shrinking glaciers. And over the long haul—meaning several thousands of years—it looks like the Greenland Ice Sheet is in trouble and could well disappear completely.500 Glaciers and ice sheets are nourished on their tops by snow. They are removed at their margins by melting and—if they float out into an ocean or lake—by calving off icebergs into the water. When nourishment exceeds removal, glaciers grow, storing water up on land, so sea level falls. When removal exceeds nourishment, glaciers retreat and their stored water returns to the ocean. In this way sea levels have danced in a tight waltz with glaciers, falling and rising anywhere from about 130 meters lower to 4-6 meters higher than today over the past few ice ages. Other things—especially thermal expansion of ocean water as it warms—also drive sea level, but the waxing and waning of land ice is a huge driver.
As the last ice age unraveled, sea levels commonly rose 1 meter per century, and sometimes as fast as 4 meters per century during intervals of very rapid glacier melting.501 Looking forward, if average air temperatures over Greenland rise by another +3°C or so, its huge ice sheet, too, must eventually disappear. Depending on how hot we allow the greenhouse effect to become, this will take anywhere from one thousand to several thousand years, raising global average sea level by another 7 meters or so.
Based on the emissions scenarios currently being bandied about by policy makers, the temperature threshold to begin this process will indeed be crossed in this century, and the long, slow decline of Greenland’s ice sheet will begin.502 It is already something of a stubborn relic of the last ice age; if it magically disappeared off the island tomorrow, it’s doubtful this ice sheet could grow back.503 One thousand years from now, eighteen of the twenty-seven megacities of 2025 listed in Chapter 2 will lie partially or wholly beneath ocean water that might once have been blue ice in Greenland.504
But over the shorter term, meaning between now and the next century or two, the scary genie of Greenland and Antarctica isn’t from their ice sheets melting per se (indeed, it will never become warm enough at the South Pole for widespread melting to occur there) but from their giant frozen rumbling ribbons of ice that slide over hundreds of miles of land to dump icebergs into the sea. Already, there are many such ice streams in Antarctica and Greenland moving tens of meters to more than ten thousand meters per year. They empty out the deep frozen hearts of these ice sheets, where temperatures are so cold the surface never melts at all.
Of grave concern is collapse of the West Antarctic Ice Sheet. This vast area is like a miniature continent of ice towering out of the ocean, much of it frozen to bedrock lying below sea level. If it became unstuck, a great many Antarctic glaciers would start lumbering toward the water, eventually raising average global sea level by around five meters. There is geological evidence