Melting Glaciers: Clues to Climate Change

Rapid Change in a Warming World

Another significant effect is the rapid melting of glacier ice. The overwhelming majority of it—including ice on tropical peaks like the Andes and the Himalayas, Antarctic ice shelves, and portions of the Greenland ice sheet—is diminishing. In many places, the ice is melting at an increasingly faster clip each year.

Similar mass melting occurred during the interglacial period preceding the Younger Dryas. Melt water from the ice sheet covering central and eastern Canada collected in a gargantuan lake that dammed behind the ice. As the ice melted further, the dam burst. The water rapidly drained eastward to the sea via the St. Lawrence River. This rapid flooding covered the North Atlantic Ocean with a layer of fresh water.

These freshwater floodgates appear to have been the trigger that set off a complex chain of events that directly resulted in the Younger Dryas’s cold and dry plunge. The first step was the fresh water’s influence on the ocean: specifically, its thermohaline circulation. This large-scale circulation between oceans is intimately tied to water temperatures and saltiness, two parameters that affect water density.

In this conveyer-belt-like circulation, warm, relatively salty tropical Atlantic Ocean water flows northward. Upon arrival in the North Atlantic, the water warms the atmosphere and keeps northern Europe comfortable. As this salty water cools in the winter, however, it sinks. Now denser, this water flows nearer to the ocean floor back down to the South Atlantic. There, it is joined by sinking cold water from Antarctica and flows as a deep current into the Pacific. The Pacific water eventually returns to the Atlantic in surface currents in a global circulation that takes perhaps a thousand years or more.

How is this connected to the Younger Dryas? Fresh water is less salty and hence less dense than seawater. The sudden influx of fresh glacial melt water into the North Atlantic floated atop the warm salty water already there. The top fresh layer wasn’t salty and dense enough to sink. Thus, it prevented the salty water underneath from cooling in the winds and sinking. This stopped the thermohaline circulation in its tracks. It’s as if you stuck a fork in the “down” end of that grocery conveyor to cease its descent.

With no cold water sinking, the warm surface water flowing northward in the Atlantic stopped arriving. Without warm replenishment, the temperature of the North Atlantic Ocean surface dropped drastically. Ice formed on the surface. The Europe-bound winds were now cooled by the frozen ocean.

This freeze-over is a major switch for global climate, according Richard Alley, a Penn State glaciologist whose ice-core and glacier work has contributed significantly to our understanding of Younger Dryas events and mechanisms. “When you put ice over the top of seawater and let it get really ridiculously cold, you’re turning the ocean into a continent in the winter time.” After about 1,300 years, the saltiness of the North Atlantic had increased enough to allow the conveyor to start up again. The Younger Dryas was finished.

Could today’s global warming cause a conveyor shutdown? Models by the Met Office and other climate-change groups don’t predict a total switch-off within the next century. “Now what [the scientific community] needs to do is put probabilities on these events,” says Alley. “But we’re simply not quite good enough at doing that yet.” He likens the gamble to that of buying insurance. “You sort of know how much to spend on car insurance because you sort of know what the odds are of someone running over you with an SUV,” says Alley. “But we don’t know how much to spend on North Atlantic shutdown insurance.”

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