Rapid Change in a Warming World

Climate change isn’t always slow, small, and imperceptible in a human lifetime. One of the most important lessons from ice core analysis is that Earth’s climate in some places can also change rapidly and dramatically, such as a 15-degree temperature change in a decade. This, you’d notice.

About 12,900 years ago, Earth was warming steadily, in resolute recovery from our most recent glaciation. Then, suddenly, things got a lot colder and drier. The ice sheets reversed direction. Much, if not all, of the North Atlantic Ocean froze over. The landmasses flanking it became colder. Elsewhere in the world, Asia and Africa were beset by dust storms, and tropical wetlands dried up. In some areas, flora and fauna--and likewise, early humans--responded to the climate change in as little as two human generations.

During periods of extensive glaciation, polar vegetation such as Dryas octopetala can grow much further south. The Younger Dryas, a period of abrupt climate change, was named after this species.

After about 1,300 years of odd behavior, this interruption, called the Younger Dryas, ended. Earth resumed its warming course. One important study of Greenland ice cores showed that in a single decade of the Younger Dryas’s departure, temperatures over Greenland shot up about 15 degrees C. That’s as if your local climate changed to one typical of that 1,500 kilometers south of you in the geologic blink of an eye.

Some mechanism, then, was overriding the gentle long-term astronomical rhythms, called Milankovitch cycles, that govern when glaciations come and go. (Learn about these cycles in “The Coming and Going of an Ice Age.”)

Could climate switch this abruptly again? It’s not impossible. But global warming complicates the scenario. Interestingly, human impact on the planet may have the power to both force such a rapid changeor prevent it entirely.

The Heat is On

According to the Met Office, the UK government’s meteorological branch, nine out of the ten warmest years on instrumental record (since 1861) were in the last decade. Today’s temperatures, however, aren’t unprecedented in recent geologic history. About 10,000-6,000 years ago, after the Younger Dryas left and the current interglacial period continued, temperatures in many northern locales were roughly 1 degree C warmer than recently. Epochs previous to the Pleistocene saw warmer periods as well.

Warm periods and high concentrations of the greenhouse gases methane and carbon dioxide usually occur together, although cause and effect are not clear. However, current-day levels of greenhouse gases may force the issue. Carbon dioxide is at its highest concentration in 260,000 years. And no mechanism other than our unprecedented burning of organic material such as fossil fuels and forest biomass can explain carbon dioxide’s increasingly rapid rise of late.

Computer models cited by the International Governmental Panel on Climate Change in 2001 predict that if greenhouse gases double their pre-industrial levels, average global temperatures will rise between 1 and 5 degrees C. A February 2005 modeling study in the journal Nature makes a more aggressive claim. Using nearly a hundred-thousand personal computers, the simulation produced a 2 to 11 degree C rise in global surface temperature in response to doubling of atmospheric carbon dioxide. In most IPCC model predictions, doubling will be reached before 2100.

Conveyed by the Ocean

We can already spot earthly changes from global warming. Arctic and subarctic permafrost is thawing. Winter ice on lakes and rivers breaks up earlier in the season. Trees flower earlier.

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

Global ocean circulation as a “conveyor belt.” Red indicates the flow of warm, shallow water. Blue is cold, salty, deep water.
Wallace Broecker, LDEO

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.”